As per ruby-lang.org, Ruby is based on programming languages like Perl, Smalltalk, Eiffel, Ada, and Lisp. This book focuses on using Ruby from the command line, similar to Perl one-liners usage.
You'll learn about various command line options and Ruby features that make it possible to write compact CLI scripts. Learning to use Ruby from the command line will also allow you to construct solutions where Ruby is just another tool in the shell ecosystem.
You should be comfortable with programming basics and have prior experience working with Ruby. You should know concepts like blocks, be familiar with string/array/hash/enumerable methods, regular expressions etc. You can check out my free ebook Understanding Ruby Regexp if you wish to learn regular expressions in depth.
You should also be familiar with command line usage in a Unix-like environment. You should be comfortable with concepts like file redirection and command pipelines. Knowing the basics of the grep, sed and awk commands will come in handy as well.
- The examples presented here have been tested with Ruby version 3.3.0 and includes features not available in earlier versions.
- Code snippets are copy pasted from the
GNU bashshell and modified for presentation purposes. Some commands are preceded by comments to provide context and explanations. Blank lines to improve readability, onlyrealtime shown for speed comparisons, output skipped/modified for certain commands and so on. - External links are provided throughout the book for you to explore certain topics in more depth.
- The learn_ruby_oneliners repo has all the code snippets and files used in examples, exercises and other details related to the book. If you are not familiar with the
gitcommand, click the Code button on the webpage to get the files.
- ruby-lang documentation — manuals and tutorials
- stackoverflow — for getting answers to pertinent questions on Ruby and related commands
- tex.stackexchange — for help on pandoc and
texrelated questions - /r/ruby/ — helpful forum
- canva — cover image
- oxipng, pngquant and svgcleaner — optimizing images
- Warning and Info icons by Amada44 under public domain
- KOTP for spotting grammatical mistakes
A heartfelt thanks to all my readers. Your valuable support has significantly eased my financial concerns and allows me to continue working on programming ebooks.
I would highly appreciate it if you'd let me know how you felt about this book. It could be anything from a simple thank you, pointing out a typo, mistakes in code snippets, which aspects of the book worked for you (or didn't!) and so on. Reader feedback is essential and especially so for self-published authors.
You can reach me via:
- Issue Manager: https://github.com/learnbyexample/learn_ruby_oneliners/issues
- E-mail: learnbyexample.net@gmail.com
- Twitter: https://twitter.com/learn_byexample
Sundeep Agarwal is a lazy being who prefers to work just enough to support his modest lifestyle. He accumulated vast wealth working as a Design Engineer at Analog Devices and retired from the corporate world at the ripe age of twenty-eight. Unfortunately, he squandered his savings within a few years and had to scramble trying to earn a living. Against all odds, selling programming ebooks saved his lazy self from having to look for a job again. He can now afford all the fantasy ebooks he wants to read and spends unhealthy amount of time browsing the internet.
When the creative muse strikes, he can be found working on yet another programming ebook (which invariably ends up having at least one example with regular expressions). Researching materials for his ebooks and everyday social media usage drowned his bookmarks, so he maintains curated resource lists for sanity sake. He is thankful for free learning resources and open source tools. His own contributions can be found at https://github.com/learnbyexample.
List of books: https://learnbyexample.github.io/books/
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.
Code snippets are available under MIT License.
Resources mentioned in Acknowledgements section above are available under original licenses.
2.0
See Version_changes.md to track changes across book versions.
This chapter will give an overview of Ruby syntax for command line usage. You'll see examples to understand what kind of problems are typically suited for one-liners.
I'll assume that you are already familiar with use cases where the command line is more productive compared to GUI. See also this series of articles titled Unix as IDE.
A shell utility like Bash provides built-in commands and scripting features to easily solve and automate various tasks. External commands like grep, sed, awk, sort, find, parallel, etc help to solve a wide variety of text processing tasks. These tools are often combined to work together along with shell features like pipelines, wildcards and loops. You can use Ruby as an alternative to such external tools and also complement them for some use cases.
Here are some sample text processing tasks that you can solve using Ruby one-liners. Options and related details will be explained later.
# retain only the first copy of duplicated lines
ruby -e 'puts readlines.uniq' *.txt
# retain only the first copy of duplicated lines,
# using the second field as the comparison criteria
ruby -e 'puts readlines.uniq {_1.split[1]}' *.txt
# extract only URLs
# uses a third-party library CommonRegexRuby
ruby -rcommonregex -ne 'puts CommonRegex.get_links($_)' *.mdHere are some questions that I've answered with simpler Ruby solution compared to other CLI tools:
- stackoverflow: merge duplicate key values while preserving order
- unix.stackexchange: pair each line of file
The selling point of Ruby over tools like grep, sed and awk includes feature rich regular expression engine and standard/third-party modules. Another advantage is that Ruby is more portable, given the many differences between GNU, BSD and other such implementations. The main disadvantage is that Ruby is likely to be verbose and slower for features that are supported out of the box by those tools.
See ruby-lang.org for instructions on installing Ruby.
Visit ruby-doc.org for documentation.
Use ruby -h to get a list of command line options, along with a brief description.
| Option | Description |
|---|---|
-0[octal] |
specify record separator (\0, if no argument) |
-a |
autosplit mode with -n or -p (splits $_ into $F) |
-c |
check syntax only |
-Cdirectory |
cd to directory before executing your script |
-d |
set debugging flags (set $DEBUG to true) |
-e 'command' |
one line of script. Several -e's allowed. Omit [programfile] |
-Eex[:in] |
specify the default external and internal character encodings |
-Fpattern |
split() pattern for autosplit (-a) |
-i[extension] |
edit ARGV files in place (make backup if extension supplied) |
-Idirectory |
specify $LOAD_PATH directory (may be used more than once) |
-l |
enable line ending processing |
-n |
assume 'while gets(); ... end' loop around your script |
-p |
assume loop like -n but print line also like sed |
-rlibrary |
require the library before executing your script |
-s |
enable some switch parsing for switches after script name |
-S |
look for the script using PATH environment variable |
-v |
print the version number, then turn on verbose mode |
-w |
turn warnings on for your script |
-W[level=2|:category] |
set warning level; 0=silence, 1=medium, 2=verbose |
-x[directory] |
strip off text before #!ruby line and perhaps cd to directory |
--jit |
enable JIT for the platform, same as --rjit (experimental) |
--rjit |
enable pure-Ruby JIT compiler (experimental) |
-h |
show this message, --help for more info |
This chapter will show examples with -e, -n, -p and -a options. Some more options will be covered in later chapters, but not all of them are discussed in this book.
If you want to execute a Ruby program file, one way is to pass the filename as argument to the ruby command.
$ echo 'puts "Hello Ruby"' > hello.rb
$ ruby hello.rb
Hello RubyFor short programs, you can also directly pass the code as an argument to the -e option.
$ ruby -e 'puts "Hello Ruby"'
Hello Ruby
# multiple statements can be issued separated by ;
$ ruby -e 'x=25; y=12; puts x**y'
59604644775390625
# or use -e option multiple times
$ ruby -e 'x=25' -e 'y=12' -e 'puts x**y'
59604644775390625Ruby one-liners can be used for filtering lines matched by a regular expression (regexp), similar to the grep, sed and awk commands. And similar to many command line utilities, Ruby can accept input from both stdin and file arguments.
# sample stdin data
$ printf 'gate\napple\nwhat\nkite\n'
gate
apple
what
kite
# print lines containing 'at'
# same as: grep 'at' and sed -n '/at/p' and awk '/at/'
$ printf 'gate\napple\nwhat\nkite\n' | ruby -ne 'print if /at/'
gate
what
# print lines NOT containing 'e'
# same as: grep -v 'e' and sed -n '/e/!p' and awk '!/e/'
$ printf 'gate\napple\nwhat\nkite\n' | ruby -ne 'print if !/e/'
whatBy default, grep, sed and awk automatically loop over the input content line by line (with newline character as the default line separator). To do so with Ruby, you can use the -n and -p options. As seen before, the -e option accepts code as a command line argument. Many shortcuts are available to reduce the amount of typing needed.
In the above examples, a regular expression (defined by the pattern between a pair of forward slashes) has been used to filter the input. When the input string isn't specified in a conditional context (for example: if), the test is performed against the global variable $_, which has the contents of the current input line (the correct term would be input record, as discussed in the Record separators chapter). To summarize, in a conditional context:
/regexp/is a shortcut for$_ =~ /regexp/!/regexp/is a shortcut for$_ !~ /regexp/
$_ is also the default argument for the print method, which is why it is generally preferred in one-liners over the puts method. More such defaults that apply to the print method will be discussed later.
See ruby-doc: Pre-Defined Global Variables for documentation on
$_,$&, etc.
Here's an example with file input instead of stdin.
$ cat table.txt
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14
# same as: grep -oE '[0-9]+$' table.txt
# digits at the end of lines
$ ruby -ne 'puts $& if /\d+$/' table.txt
42
7
14
# digits at the end of lines that are not preceded by -
$ ruby -ne 'puts $& if /(?<!-)\d+$/' table.txt
42
14
table.txtin the above illustration).
Use the sub and gsub methods for search and replace requirements. By default, these methods operate on $_ when the input string isn't provided. For these examples, the -p option is used instead of -n, so that the value of $_ is automatically printed after processing each input line.
# for each input line, change only the first ':' to '-'
# same as: sed 's/:/-/' and awk '{sub(/:/, "-")} 1'
$ printf '1:2:3:4\na:b:c:d\n' | ruby -pe 'sub(/:/, "-")'
1-2:3:4
a-b:c:d
# for each input line, change all ':' to '-'
# same as: sed 's/:/-/g' and awk '{gsub(/:/, "-")} 1'
$ printf '1:2:3:4\na:b:c:d\n' | ruby -pe 'gsub(/:/, "-")'
1-2-3-4
a-b-c-dYou might wonder how $_ is modified without the use of ! methods. The reason is that these methods are part of Kernel (see ruby-doc: Kernel for details) and are available only when the -n and -p options are used.
sub(/regexp/, repl)is a shortcut for$_.sub(/regexp/, repl)and$_will be updated if the substitution succeedsgsub(/regexp/, repl)is a shortcut for$_.gsub(/regexp/, repl)and$_gets updated if the substitution succeeds
Consider the sample input file shown below with fields separated by a single space character.
$ cat table.txt
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14Here are some examples that are based on specific fields rather than the entire line. The -a option will cause the input line to be split based on whitespaces and the array contents can be accessed using the $F global variable. Leading and trailing whitespaces will be suppressed, so there's no possibility of empty fields. More details will be discussed in the Default field separation section.
# print the second field of each input line
# same as: awk '{print $2}' table.txt
$ ruby -ane 'puts $F[1]' table.txt
bread
cake
banana
# print lines only if the last field is a negative number
# same as: awk '$NF<0' table.txt
$ ruby -ane 'print if $F[-1].to_f < 0' table.txt
blue cake mug shirt -7
# change 'b' to 'B' only for the first field
# same as: awk '{gsub(/b/, "B", $1)} 1' table.txt
$ ruby -ane '$F[0].gsub!(/b/, "B"); puts $F * " "' table.txt
Brown bread mat hair 42
Blue cake mug shirt -7
yellow banana window shoes 3.14You can use a BEGIN{} block when you need to execute something before the input is read and an END{} block to execute something after all of the input has been processed.
# same as: awk 'BEGIN{print "---"} 1; END{print "%%%"}'
# note the use of ; after the BEGIN block
$ seq 4 | ruby -pe 'BEGIN{puts "---"}; END{puts "%%%"}'
---
1
2
3
4
%%%When it comes to automation and scripting, you'd often need to construct commands that can accept input from users, use data from files and the output of a shell command and so on. As mentioned before, this book assumes bash as the shell being used. To access environment variables of the shell, you can use the special hash variable ENV with the name of the environment variable as a string key.
# existing environment variable
# output shown here is for my machine, would differ for you
$ ruby -e 'puts ENV["HOME"]'
/home/learnbyexample
$ ruby -e 'puts ENV["SHELL"]'
/bin/bash
# defined along with the command
# note that the variable definition is placed before the command
$ word='hello' ruby -e 'puts ENV["word"]'
hello
# the characters are preserved as is
$ ip='hi\nbye' ruby -e 'puts ENV["ip"]'
hi\nbyeHere's another example when a regexp is passed as an environment variable content.
$ cat word_anchors.txt
sub par
spar
apparent effort
two spare computers
cart part tart mart
# assume 'r' is a shell variable containing user provided regexp
$ r='\Bpar\B'
$ rgx="$r" ruby -ne 'print if /#{ENV["rgx"]}/' word_anchors.txt
apparent effort
two spare computersYou can also make use of the -s option to assign a global variable.
$ r='\Bpar\B'
$ ruby -sne 'print if /#{$rgx}/' -- -rgx="$r" word_anchors.txt
apparent effort
two spare computers
You can call external commands using the system Kernel method. See ruby-doc: system for documentation.
$ ruby -e 'system("echo Hello World")'
Hello World
$ ruby -e 'system("wc -w <word_anchors.txt")'
12
$ ruby -e 'system("seq -s, 10 > out.txt")'
$ cat out.txt
1,2,3,4,5,6,7,8,9,10Return value of system or the global variable $? can be used to act upon the exit status of the command issued.
$ ruby -e 'es=system("ls word_anchors.txt"); puts es'
word_anchors.txt
true
$ ruby -e 'system("ls word_anchors.txt"); puts $?'
word_anchors.txt
pid 6087 exit 0
$ ruby -e 'system("ls xyz.txt"); puts $?'
ls: cannot access 'xyz.txt': No such file or directory
pid 6164 exit 2To save the result of an external command, use backticks or %x.
$ ruby -e 'words = `wc -w <word_anchors.txt`; puts words'
12
$ ruby -e 'nums = %x/seq 3/; print nums'
1
2
3
This chapter introduced some of the common options for Ruby CLI usage, along with some of the typical text processing examples. While specific purpose CLI tools like grep, sed and awk are usually faster, Ruby has a much more extensive standard library and ecosystem. And you do not have to learn a lot if you are already comfortable with Ruby but not familiar with those CLI tools. The next section has a few exercises for you to practice the CLI options and text processing use cases.
1) For the input file ip.txt, display all lines containing is.
$ cat ip.txt
Hello World
How are you
This game is good
Today is sunny
12345
You are funny
##### add your solution here
This game is good
Today is sunny2) For the input file ip.txt, display the first field of lines not containing y. Consider space as the field separator for this file.
##### add your solution here
Hello
This
123453) For the input file ip.txt, display all lines containing no more than 2 fields.
##### add your solution here
Hello World
123454) For the input file ip.txt, display all lines containing is in the second field.
##### add your solution here
Today is sunny5) For each line of the input file ip.txt, replace the first occurrence of o with 0.
##### add your solution here
Hell0 World
H0w are you
This game is g0od
T0day is sunny
12345
Y0u are funny6) For the input file table.txt, calculate and display the product of numbers in the last field of each line. Consider space as the field separator for this file.
$ cat table.txt
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14
##### add your solution here
-923.16000000000017) Append . to all the input lines for the given stdin data.
$ printf 'last\nappend\nstop\ntail\n' | ##### add your solution here
last.
append.
stop.
tail.8) Use contents of the s variable to display matching lines from the input file ip.txt. Assume that s doesn't have any regexp metacharacters. Construct the solution such that there's at least one word character immediately preceding the contents of the s variable.
$ s='is'
##### add your solution here
This game is good9) Use system to display the contents of the filename present in the second field of the given input line. Consider space as the field separator.
$ s='report.log ip.txt sorted.txt'
$ echo "$s" | ##### add your solution here
Hello World
How are you
This game is good
Today is sunny
12345
You are funny
$ s='power.txt table.txt'
$ echo "$s" | ##### add your solution here
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14Now that you are familiar with basic Ruby CLI usage, this chapter will dive deeper into line processing examples. You'll learn various ways for matching lines based on regular expressions, fixed string matching, line numbers, etc. You'll also see how to group multiple statements and learn about the control flow keywords next and exit.
As mentioned before, in a conditional context:
/regexp/is a shortcut for$_ =~ /regexp/!/regexp/is a shortcut for$_ !~ /regexp/
Here are some examples:
$ cat table.txt
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14
$ ruby -ne 'print if /-|ow\b/' table.txt
blue cake mug shirt -7
yellow banana window shoes 3.14
$ ruby -ne 'print if !/[ksy]/' table.txt
brown bread mat hair 42
But, this is not applicable for all types of expressions. For example:
# /at$/ will be 'true' as it is treated as just a Regexp object here $ printf 'gate\napple\nwhat\n' | ruby -ne '/at$/ && print' gate apple what # same as: ruby -ne 'print if /at$/' $ printf 'gate\napple\nwhat\n' | ruby -ne '$_ =~ /at$/ && print' what
If required, you can also use different delimiters with %r. See ruby-doc: %r Regexp Literals for details.
$ cat paths.txt
/home/joe/report.log
/home/ram/power.log
/home/rambo/errors.log
# leaning toothpick syndrome
$ ruby -ne 'print if /\/home\/ram\//' paths.txt
/home/ram/power.log
$ ruby -ne 'print if %r{/home/ram/}' paths.txt
/home/ram/power.log
$ ruby -ne 'print if !%r#/home/ram/#' paths.txt
/home/joe/report.log
/home/rambo/errors.logYou can use regexp related global variables to extract only the matching portions. Consider this input file.
$ cat ip.txt
it is a warm and cozy day
listen to what I say
go play in the park
come back before the sky turns dark
There are so many delights to cherish
Apple, Banana and Cherry
Bread, Butter and Jelly
Try them all before you perishHere are some examples of extracting only the matched portions.
# note that this will print only the first match for each input line
$ ruby -ne 'puts $& if /\b[a-z]\w*[ty]\b/' ip.txt
it
what
play
sky
many
# extract the capture group portions
$ ruby -ne 'puts "#{$1}::#{$2}" if /(\b[bdp]\w+).*(\b[a-f]\w+)/i' ip.txt
back::dark
delights::cherish
Banana::Cherry
Bread::and
matchmethod and regexp global variables.
As seen in the previous section, using $_ =~ /regexp/ also sets global variables. If you just need a true or false result, using the match? method is better suited for performance reasons. The difference would be more visible for large input files.
# same result as: ruby -ne 'print if /[AB]|the\b/'
$ ruby -ne 'print if $_.match?(/[AB]|the\b/)' ip.txt
go play in the park
come back before the sky turns dark
Apple, Banana and Cherry
Bread, Butter and JellyThe transliteration method tr helps you perform transformations character-wise. See ruby-doc: tr for documentation.
# rot13
$ echo 'Uryyb Jbeyq' | ruby -pe '$_.tr!("a-zA-Z", "n-za-mN-ZA-M")'
Hello World
# ^ at the start of the first argument complements the specified characters
$ echo 'apple:123:banana' | ruby -pe '$_.tr!("^0-9\n", "-")'
------123-------
# an empty second argument deletes the specified characters
$ echo 'apple:123:banana' | ruby -pe '$_.tr!("^0-9\n", "")'
123
# if the second list is shorter than the number of characters in the first list,
# the last character in the second list will be used to fill the gaps
$ s='orange apple appleseed cab'
$ echo "$s" | ruby -pe 'gsub(/\b(?!apple\b)\w++/) {$&.tr("a-z", "1-9")}'
991975 apple 199959554 312You can use the tr_s method to squeeze repeated characters.
$ echo 'APPLESEED gobbledygook' | ruby -pe '$_.tr_s!("a-zA-Z", "a-zA-Z")'
APLESED gobledygok
# transliteration as well as squeeze
$ echo 'APPLESEED gobbledygook' | ruby -pe '$_.tr_s!("A-Z", "a-z")'
aplesed gobbledygookThese examples combine line filtering and substitution in different ways. As noted before, the sub and gsub Kernel methods update $_ if the substitution succeeds and always return the value of $_.
# change commas to hyphens if the input line does NOT contain '2'
# prints all input lines even if the substitution fails
$ printf '1,2,3,4\na,b,c,d\n' | ruby -pe 'gsub(/,/, "-") if !/2/'
1,2,3,4
a-b-c-d
# perform substitution only for the filtered lines
# prints filtered input lines, even if the substitution fails
$ ruby -ne 'print gsub(/ark/, "[\\0]") if /the/' ip.txt
go play in the p[ark]
come back before the sky turns d[ark]
Try them all before you perish
# print only if the substitution succeeds
# $_.gsub! is required for this scenario
$ ruby -ne 'print if $_.gsub!(/\bw\w*t\b/, "{\\0}")' ip.txt
listen to {what} I sayIt is good to remember that Ruby is a programming language. You can make use of control structures and combine multiple conditions using logical operators, methods like all?, any?, etc. You don't have to create a single complex regexp.
$ ruby -ne 'print if /ark/ && !/sky/' ip.txt
go play in the park
$ ruby -ane 'print if /\bthe\b/ || $F.size == 5' ip.txt
listen to what I say
go play in the park
come back before the sky turns darkWhen the next statement is executed, rest of the code will be skipped and the next input line will be fetched for processing. It doesn't affect the BEGIN and END blocks as they are outside the file content loop.
$ ruby -ne '(puts "%% #{$_}"; next) if /\bpar/;
puts /s/ ? "X" : "Y"' word_anchors.txt
%% sub par
X
Y
X
%% cart part tart mart
()is used in the above example to group multiple statements to be executed for a singleifcondition. You'll see more such examples in the coming chapters.
The exit method will cause the Ruby script to terminate immediately. This is useful to avoid processing unnecessary input content after a termination condition is reached.
# quits after an input line containing 'say' is found
$ ruby -ne 'print; exit if /say/' ip.txt
it is a warm and cozy day
listen to what I say
# the matching line won't be printed in this case
$ ruby -pe 'exit if /say/' ip.txt
it is a warm and cozy dayUse tac to get all lines starting from the last occurrence of the search string in the entire file.
$ tac ip.txt | ruby -ne 'print; exit if /an/' | tac
Bread, Butter and Jelly
Try them all before you perishYou can optionally provide a status code as an argument to the exit method.
$ printf 'sea\neat\ndrop\n' | ruby -ne 'print; exit(2) if /at/'
sea
eat
$ echo $?
2Any code in the END block will still be executed before exiting. This doesn't apply if exit was called from the BEGIN block.
$ ruby -pe 'exit if /cake/' table.txt
brown bread mat hair 42
$ ruby -pe 'exit if /cake/; END{puts "bye"}' table.txt
brown bread mat hair 42
bye
$ ruby -pe 'BEGIN{puts "hi"; exit; puts "hello"}; END{puts "bye"}' table.txt
hi
exitwith multiple input files, as Ruby will stop even if there are other files remaining to be processed.
Line numbers can also be specified as a matching criteria by using the $. global variable.
# print only the third line
$ ruby -ne 'print if $. == 3' ip.txt
go play in the park
# print the second and sixth lines
$ ruby -ne 'print if $. == 2 || $. == 6' ip.txt
listen to what I say
There are so many delights to cherish
# transliterate only the second line
$ printf 'gates\nnot\nused\n' | ruby -pe '$_.tr!("a-z", "*") if $. == 2'
gates
***
used
# print from a particular line number to the end of the input
$ seq 14 25 | ruby -ne 'print if $. >= 10'
23
24
25The global variable $< contains the file handle for the current file input being processed. Use the eof method to check for the end of the file condition. See ruby-doc: eof for documentation. You can also use ARGF instead of $< here, see the ARGV and ARGF section for details.
# same as: tail -n1 ip.txt
$ ruby -ne 'print if $<.eof' ip.txt
Try them all before you perish
$ ruby -ne 'puts "#{$.}:#{$_}" if $<.eof' ip.txt
9:Try them all before you perish
# multiple file example
# same as: tail -q -n1 ip.txt table.txt
$ ruby -ne 'print if $<.eof' ip.txt table.txt
Try them all before you perish
yellow banana window shoes 3.14For large input files, use the exit method to avoid processing unnecessary input lines.
$ seq 3542 4623452 | ruby -ne '(print; exit) if $. == 2452'
5993
$ seq 3542 4623452 | ruby -ne 'print if $. == 250; (print; exit) if $. == 2452'
3791
5993
# here is a sample time comparison
$ time seq 3542 4623452 | ruby -ne '(print; exit) if $. == 2452' > f1
real 0m0.055s
$ time seq 3542 4623452 | ruby -ne 'print if $. == 2452' > f2
real 0m1.130s
$ rm f1 f2You can use the Flip-Flop operator to select between a pair of matching conditions like line numbers and regexp. See ruby-doc: Flip-Flop for documentation.
# the range is automatically compared against $. in this context
$ seq 14 25 | ruby -ne 'print if 3..5'
16
17
18
# 'print if 3...5' gives the same result as above,
# you can use the include? method to exclude the end range
$ seq 14 25 | ruby -ne 'print if (3...5).include?($.)'
16
17
# the range is automatically compared against $_ in this context
# note that all the matching ranges are printed
$ ruby -ne 'print if /to/../pl/' ip.txt
listen to what I say
go play in the park
There are so many delights to cherish
Apple, Banana and Cherry
Line numbers and regexp filtering can be mixed.
$ ruby -ne 'print if 6../utter/' ip.txt
There are so many delights to cherish
Apple, Banana and Cherry
Bread, Butter and Jelly
# same logic as: ruby -pe 'exit if /\bba/'
# inefficient, but this will work for multiple file inputs
$ ruby -ne 'print if !(/\bba/..$<.eof)' ip.txt table.txt
it is a warm and cozy day
listen to what I say
go play in the park
brown bread mat hair 42
blue cake mug shirt -7Both conditions can match the same line too! Also, if the second condition doesn't match, lines starting from the first condition to the last line of the input will be matched.
# 'and' matches the 7th line
$ ruby -ne 'print if 7../and/' ip.txt
Apple, Banana and Cherry
# 'and' will be tested against 8th line onwards
$ ruby -ne 'print if 7.../and/' ip.txt
Apple, Banana and Cherry
Bread, Butter and Jelly
# there's a line containing 'Banana' but the matching pair isn't found
# so, all lines till the end of the input is printed
$ ruby -ne 'print if /Banana/../XYZ/' ip.txt
Apple, Banana and Cherry
Bread, Butter and Jelly
Try them all before you perishTo match strings literally, use the include? method for line filtering. Use string argument instead of regexp for fixed string matching with substitution methods.
$ printf 'int a[5]\nfig\n1+4=5\n' | ruby -ne 'print if /a[5]/'
$ printf 'int a[5]\nfig\n1+4=5\n' | ruby -ne 'print if $_.include?("a[5]")'
int a[5]
$ printf 'int a[5]\nfig\n1+4=5\n' | ruby -pe 'sub(/a[5]/, "b")'
int a[5]
fig
1+4=5
$ printf 'int a[5]\nfig\n1+4=5\n' | ruby -pe 'sub("a[5]", "b")'
int b
fig
1+4=5The above examples use double quotes for the string argument, which allows escape sequences like \t, \n, etc and interpolation with #{}. This isn't the case with single quoted string values. Using single quotes within the script from the command line requires messing with shell metacharacters. So, use %q instead or pass the fixed string to be matched as an environment variable.
# double quotes allow escape sequences and interpolation
$ ruby -e 'a=5; puts "value of a:\t#{a}"'
value of a: 5
# use %q as an alternate to specify single quoted strings
$ echo 'int #{a}' | ruby -ne 'print if $_.include?(%q/#{a}/)'
int #{a}
$ echo 'int #{a}' | ruby -pe 'sub(%q/#{a}/, "b")'
int b
# or pass the string as an environment variable
$ echo 'int #{a}' | s='#{a}' ruby -ne 'print if $_.include?(ENV["s"])'
int #{a}
# \\ is special within single quotes, so ENV is the better choice here
$ echo 'int #{a\\}' | s='#{a\\}' ruby -pe 'sub(ENV["s"], "b")'
int bTo provide a fixed string in the replacement section, environment variables comes in handy again. Need to use block form, since \ is special in the replacement section.
# \\ will be treated as \ and \0 will backreference the entire matched portion
$ echo 'int a' | s='x\\y\0z' ruby -pe 'sub(/a/, ENV["s"])'
int x\yaz
# use block form to avoid such issues
$ echo 'int a' | s='x\\y\0z' ruby -pe 'sub(/a/) {ENV["s"]}'
int x\\y\0zUse the start_with? and end_with? methods to restrict the matching to the start or end of the input line. The line content in the $_ variable contains the \n line ending character as well. You can either use the chomp method explicitly or use the -l command line option (which will be discussed in detail in the Record separators chapter). For now, it is enough to know that -l will remove the line separator and add it back when print is used.
$ cat eqns.txt
a=b,a-b=c,c*d
a+b,pi=3.14,5e12
i*(t+9-g)/8,4-a+b
# start of the line
$ s='a+b' ruby -ne 'print if $_.start_with?(ENV["s"])' eqns.txt
a+b,pi=3.14,5e12
# end of the line
# -l option is needed here to remove \n from $_
$ s='a+b' ruby -lne 'print if $_.end_with?(ENV["s"])' eqns.txt
i*(t+9-g)/8,4-a+bUse the index method if you need more control over the location of the matching strings. You can use either the return value (which gives you the index of the matching string) or use the optional second argument to specify an offset to start searching. See ruby-doc: index for details.
# same as: $_.include?("a+b")
$ ruby -ne 'print if $_.index("a+b")' eqns.txt
a+b,pi=3.14,5e12
i*(t+9-g)/8,4-a+b
# same as: $_.start_with?("a+b")
$ ruby -ne 'print if $_.index("a+b")==0' eqns.txt
a+b,pi=3.14,5e12
# since 'index' returns 'nil' if there's no match,
# you need some more processing for < or <= comparison
$ ruby -ne '$i = $_.index("="); print if $i && $i < 6' eqns.txt
a=b,a-b=c,c*d
# for > or >= comparison, use the optional second argument
$ s='a+b' ruby -ne 'print if $_.index(ENV["s"], 1)' eqns.txt
i*(t+9-g)/8,4-a+bIf you need to match the entire input line or a particular field, you can use the comparison operators.
$ printf 'a.b\na+b\n' | ruby -lne 'print if /^a.b$/'
a.b
a+b
$ printf 'a.b\na+b\n' | ruby -lne 'print if $_ == %q/a.b/'
a.b
$ printf '1 a.b\n2 a+b\n' | ruby -lane 'print if $F[1] != %q/a.b/'
2 a+bYou can use the -i option to write back the changes to the input file instead of displaying the output on terminal. When an extension is provided as an argument to -i, the original contents of the input file gets preserved as per the extension given. For example, if the input file is ip.txt and -i.orig is used, the backup file will be named as ip.txt.orig.
$ cat colors.txt
deep blue
light orange
blue delight
# no output on the terminal as -i option is used
# space is NOT allowed between -i and the extension
$ ruby -i.bkp -pe 'sub(/blue/, "-green-")' colors.txt
# changes are written back to 'colors.txt'
$ cat colors.txt
deep -green-
light orange
-green- delight
# original file is preserved in 'colors.txt.bkp'
$ cat colors.txt.bkp
deep blue
light orange
blue delightMultiple input files are treated individually and the changes are written back to respective files.
$ cat t1.txt
have a nice day
bad morning
what a pleasant evening
$ cat t2.txt
worse than ever
too bad
$ ruby -i.bkp -pe 'sub(/bad/, "good")' t1.txt t2.txt
$ ls t?.*
t1.txt t1.txt.bkp t2.txt t2.txt.bkp
$ cat t1.txt
have a nice day
good morning
what a pleasant evening
$ cat t2.txt
worse than ever
too goodSometimes backups are not desirable. In such cases, you can use the -i option without an argument. Be careful though, as changes made cannot be undone. It is recommended to test the command with sample inputs before applying the -i option on the actual file. You could also use the option with backup, compare the differences with a diff program and then delete the backup.
$ cat fruits.txt
banana
papaya
mango
$ ruby -i -pe 'gsub(/(..)\1/) {$&.upcase}' fruits.txt
$ cat fruits.txt
bANANa
PAPAya
mangoThis chapter showed various examples of processing only the lines of interest instead of the entire input file. Filtering can be specified using a regexp, fixed string, line number or a combination of them. You also saw how to combine multiple statements inside () for compact CLI usage. The next and exit methods are useful to control the flow of code. The -i option is handy for in-place editing.
1) For the given input, display except the third line.
$ seq 34 37 | ##### add your solution here
34
35
372) Display only the fourth, fifth, sixth and seventh lines for the given input.
$ seq 65 78 | ##### add your solution here
68
69
70
713) For the input file ip.txt, replace all occurrences of are with are not and is with is not only from line number 4 till the end of file. Also, only the lines that were changed should be displayed in the output.
$ cat ip.txt
Hello World
How are you
This game is good
Today is sunny
12345
You are funny
##### add your solution here
Today is not sunny
You are not funny4) For the given stdin, display only the first three lines. Avoid processing lines that are not relevant.
$ seq 14 25 | ##### add your solution here
14
15
165) For the input file ip.txt, display all lines from the start of the file till the first occurrence of game.
##### add your solution here
Hello World
How are you
This game is good6) For the input file ip.txt, display all lines that contain is but not good.
##### add your solution here
Today is sunny7) For the input file ip.txt, extract the word before the whole word is as well as the word after it. If such a match is found, display the two words around is in reversed order. For example, hi;1 is--234 bye should be converted to 234:1. Assume that the whole word is will not be present more than once in a single line.
##### add your solution here
good:game
sunny:Today8) For the input file hex.txt, replace all occurrences of 0xA0 with 0x50 and 0xFF with 0x7F.
$ cat hex.txt
start: 0xA0, func1: 0xA0
end: 0xFF, func2: 0xB0
restart: 0xA010, func3: 0x7F
##### add your solution here
start: 0x50, func1: 0x50
end: 0x7F, func2: 0xB0
restart: 0x5010, func3: 0x7F9) For the input file text.txt, replace all occurrences of in with an and write back the changes to text.txt itself. The original contents should get saved to text.txt.orig.
$ cat text.txt
can ran want plant
tin fin fit mine line
##### add your solution here
$ cat text.txt
can ran want plant
tan fan fit mane lane
$ cat text.txt.orig
can ran want plant
tin fin fit mine line10) For the input file text.txt, replace all occurrences of an with in and write back the changes to text.txt itself. Do not create backups for this exercise. Note that you should have solved the previous exercise before starting this one.
$ cat text.txt
can ran want plant
tan fan fit mane lane
##### add your solution here
$ cat text.txt
cin rin wint plint
tin fin fit mine line
$ diff text.txt text.txt.orig
1c1
< cin rin wint plint
---
> can ran want plant11) Find the starting index of first occurrence of is or the or was or to for each input line of the file idx.txt. Assume that every input line will match at least one of these terms.
$ cat idx.txt
match after the last newline character
and then you want to test
this is good bye then
you were there to see?
##### add your solution here
12
4
2
912) Display all lines containing [4]* for the given stdin data.
$ printf '2.3/[4]*6\n2[4]5\n5.3-[4]*9\n' | ##### add your solution here
2.3/[4]*6
5.3-[4]*913) For the given input string, change all lowercase alphabets to x only for words starting with m.
$ s='ma2T3a a2p kite e2e3m meet'
$ echo "$s" | ##### add your solution here
xx2T3x a2p kite e2e3m xxxx14) For the input file ip.txt, delete all characters other than lowercase vowels and the newline character. Perform this transformation only between a line containing you up to line number 4 (inclusive).
##### add your solution here
Hello World
oaeou
iaeioo
oaiu
12345
You are funny15) For the input file sample.txt, display from the start of the file till the first occurrence of are, excluding the matching line.
$ cat sample.txt
Hello World
Good day
How are you
Just do-it
Believe it
Today is sunny
Not a bit funny
No doubt you like it too
Much ado about nothing
He he he
##### add your solution here
Hello World
Good day16) For the input file sample.txt, display from the last occurrence of do till the end of the file.
##### add your solution here
Much ado about nothing
He he he17) For the input file sample.txt, display from the 9th line till a line containing you.
##### add your solution here
Today is sunny
Not a bit funny
No doubt you like it too18) Display only the odd numbered lines from ip.txt.
##### add your solution here
Hello World
This game is good
1234519) For the table.txt file, print only the line number for lines containing air or win.
$ cat table.txt
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14
##### add your solution here
1
320) For the input file table.txt, calculate the sum of numbers in the last column, excluding the second line.
##### add your solution here
45.1421) Print the second and fourth line for every block of five lines.
$ seq 15 | ##### add your solution here
2
4
7
9
12
1422) For the input file ip.txt, display all lines containing e or u but not both.
##### add your solution here
Hello World
This game is good
Today is sunnyThis chapter will dive deep into field processing. You'll learn how to set input and output field separators, how to use regexps for defining fields and how to work with fixed length fields.
The -a option splits the input based on one or more sequence of whitespace characters. In addition, whitespaces at the start or end of input gets trimmed and won't be part of the field contents. Using -a is equivalent to $F = $_.split. From ruby-doc: split:
If
$;isnil(its default value), the split occurs just as iffield_sepwere given as a space character. Whenfield_sepis' 'andlimitisnil, the split occurs at each sequence of whitespace.
$ echo ' a b c ' | ruby -ane 'puts $F.size'
3
# note that the leading whitespaces aren't part of the field content
$ echo ' a b c ' | ruby -ane 'puts "(#{$F[0]})"'
(a)
# trailing whitespaces are removed as well
$ echo ' a b c ' | ruby -ane 'puts "(#{$F[-1]})"'
(c)
# here's another example with more whitespace characters thrown in
$ printf ' one \t\f\v two\t\r\tthree \t\r ' | ruby -ane 'puts $F.size'
3
$ printf ' one \t\f\v two\t\r\tthree \t\r ' | ruby -ane 'puts $F[1] + "."'
two.You can use the -F command line option to specify a custom field separator. The value passed to this option will be treated as a regexp. Note that the -a option is also necessary for -F to work.
# use ':' as the input field separator
$ echo 'goal:amazing:whistle:kwality' | ruby -F: -ane 'puts $F[0], $F[-1], $F[1]'
goal
kwality
amazing
# use quotes to avoid clashes with shell special characters
$ echo 'one;two;three;four' | ruby -F';' -ane 'puts $F[2]'
three
$ echo 'load;err_msg--\ant,r2..not' | ruby -F'\W+' -ane 'puts $F[2]'
ant
$ echo 'hi.bye.hello' | ruby -F'\.' -ane 'puts $F[1]'
bye
# count the number of vowels for each input line
$ printf 'COOL\nnice car\n' | ruby -F'(?i)[aeiou]' -ane 'puts $F.size - 1'
2
3No need to use field separation to access individual characters. See ruby-doc: Encoding for details on handling different string encodings.
$ echo 'apple' | ruby -ne 'puts $_[0]'
a
$ ruby -e 'puts Encoding.default_external'
UTF-8
$ LC_ALL=C ruby -e 'puts Encoding.default_external'
US-ASCII
$ echo 'fox:αλεπού' | ruby -ne 'puts $_[4..5]'
αλ
# use the -E option to explicitly specify external/internal encodings
$ echo 'fox:αλεπού' | ruby -E UTF-8:UTF-8 -ne 'puts $_[4..5]'
αλIf the custom field separator doesn't affect the newline character, then the last element can contain the newline character.
# last element will not have the newline character with the -a option
# as leading/trailing whitespaces are trimmed with default split
$ echo 'cat dog' | ruby -ane 'puts "[#{$F[-1]}]"'
[dog]
# last element will have the newline character since the field separator is ':'
$ echo 'cat:dog' | ruby -F: -ane 'puts "[#{$F[-1]}]"'
[dog
]
# unless the input itself doesn't have newline characters
$ printf 'cat:dog' | ruby -F: -ane 'puts "[#{$F[-1]}]"'
[dog]The newline character can also show up as the entire content of the last field.
# both the leading and trailing whitespaces are trimmed
$ echo ' a b c ' | ruby -ane 'puts $F.size'
3
# leading empty element won't be removed here
# and the last element will have only the newline character as the value
$ echo ':a:b:c:' | ruby -F: -ane 'puts $F.size; puts "[#{$F[-1]}]"'
5
[
]As mentioned before, the -l option is helpful if you wish to remove the newline character (more details will be discussed in the Record separators chapter). A side effect of removing the newline character before applying split is that the trailing empty fields will also get removed (you can explicitly call the split method with -1 as limit to prevent this).
# -l will remove the newline character
$ echo 'cat:dog' | ruby -F: -lane 'puts "[#{$F[-1]}]"'
[dog]
# -l will also cause 'print' to append the newline character
$ echo 'cat:dog' | ruby -F: -lane 'print "[#{$F[-1]}]"'
[dog]
# since the newline character is chomped, last element is empty
# which is then removed due to the default 'split' behavior
$ echo ':a:b:c:' | ruby -F: -lane 'puts $F.size'
4
# explicit call to split with -1 as the limit will preserve the empty element
$ echo ':a:b:c:' | ruby -lane 'puts $_.split(/:/, -1).size'
5There are a few ways to affect the separator to be used while displaying multiple values. The value of the $, global variable is used as the separator when multiple arguments are passed to the print method. This is usually used in combination with the -l option so that a newline character is appended automatically as well. The join method also uses $, as the default value.
$ ruby -lane 'BEGIN{$, = " "}; print $F[0], $F[2]' table.txt
brown mat
blue mug
yellow windowThe other options include manually building the output string within double quotes. Or, use the join method. Note that the -l option is used in the examples below as a good practice even when not needed.
$ ruby -lane 'puts "#{$F[0]} #{$F[2]}"' table.txt
brown mat
blue mug
yellow window
$ echo 'Sample123string42with777numbers' | ruby -F'\d+' -lane 'puts $F.join(",")'
Sample,string,with,numbers
$ s='goal:amazing:whistle:kwality'
$ echo "$s" | ruby -F: -lane 'puts $F.values_at(-1, 1, 0).join("-")'
kwality-amazing-goal
# you can also use the '*' operator
$ echo "$s" | ruby -F: -lane '$F.append(42); puts $F * "::"'
goal::amazing::whistle::kwality::42The -F option uses the split method to generate the fields. In contrast, the scan method allows you to define what should the fields be made up of. The scan method does not have the concept of removing empty trailing fields nor does it have the limit argument.
$ s='Sample123string42with777numbers'
# define fields to be one or more consecutive digits
$ echo "$s" | ruby -ne 'puts $_.scan(/\d+/)[1]'
42
$ s='coat Bin food tar12 best Apple fig_42'
# whole words made up of lowercase alphabets and digits only
$ echo "$s" | ruby -ne 'puts $_.scan(/\b[a-z0-9]+\b/) * ","'
coat,food,tar12,best
$ s='items: "apple" and "mango"'
# get the second double quoted item
$ echo "$s" | ruby -ne 'puts $_.scan(/"[^"]+"/)[1]'
"mango"
# no need to use 'scan' to extract the first matching portion
$ echo "$s" | ruby -ne 'puts $_[/"[^"]+"/]'
"apple"A simple split fails for CSV input where fields can contain embedded delimiter characters. For example, a field content "fox,42" when , is the delimiter.
$ s='eagle,"fox,42",bee,frog'
# simply using , as the separator isn't sufficient
$ echo "$s" | ruby -F, -lane 'puts $F[1]'
"foxWhile the ruby-doc: CSV library should be preferred for robust CSV parsing, regexp is enough for simple formats.
$ echo "$s" | ruby -lne 'puts $_.scan(/"[^"]*"|[^,]+/)[1]'
"fox,42"The unpack method is more than just a different way of using string slicing. It supports various formats and pre-processing, see ruby-doc: Packed Data for details.
In the example below, a indicates arbitrary binary string. The optional number that follows indicates length of the field.
$ cat items.txt
apple fig banana
50 10 200
# here field widths have been assigned such that
# extra spaces are placed at the end of each field
$ ruby -ne 'puts $_.unpack("a8a4a6") * ","' items.txt
apple ,fig ,banana
50 ,10 ,200
$ ruby -ne 'puts $_.unpack("a8a4a6")[1]' items.txt
fig
10 You can specify characters to be ignored with x followed by an optional length.
# first field is 5 characters
# then 3 characters are ignored and 3 characters for the second field
# then 1 character is ignored and 6 characters for the third field
$ ruby -ne 'puts $_.unpack("a5x3a3xa6") * ","' items.txt
apple,fig,banana
50 ,10 ,200Using * will cause remaining characters of that particular format to be consumed. Here Z is used to process strings that are separated by the ASCII NUL character.
$ printf 'banana\x0050\x00' | ruby -ne 'puts $_.unpack("Z*Z*") * ":"'
banana:50
# first field is 5 characters, then 3 characters are ignored
# all the remaining characters are assigned to the second field
$ ruby -ne 'puts $_.unpack("a5x3a*") * ","' items.txt
apple,fig banana
50 ,10 200Unpacking isn't always needed, simple string slicing might suffice.
$ echo 'b 123 good' | ruby -ne 'puts $_[2,3]'
123
$ echo 'b 123 good' | ruby -ne 'puts $_[6,4]'
good
# replacing arbitrary slice
$ echo 'b 123 good' | ruby -lpe '$_[2,3] = "gleam"'
b gleam goodHaving seen command line options and features commonly used for field processing, this section will highlight some of the built-in array and Enumerable methods. There are just too many to meaningfully cover them all in detail, so consider this to be just a brief overview of features.
First up, regexp based field selection. grep(cond) and grep_v(cond) are specialized filter methods that perform cond === object test check. See stackoverflow: What does the === operator do in Ruby? for more details.
$ s='goal:amazing:42:whistle:kwality:3.14'
# fields containing 'in' or 'it' or 'is'
$ echo "$s" | ruby -F: -lane 'puts $F.grep(/i[nts]/) * ":"'
amazing:whistle:kwality
# fields NOT containing a digit character
$ echo "$s" | ruby -F: -lane 'puts $F.grep_v(/\d/) * ":"'
goal:amazing:whistle:kwality
# no more than one field can contain 'r'
$ ruby -lane 'print if $F.grep(/r/).size <= 1' table.txt
blue cake mug shirt -7
yellow banana window shoes 3.14The map method transforms each element according to the logic passed to it.
$ s='goal:amazing:42:whistle:kwality:3.14'
$ echo "$s" | ruby -F: -lane 'puts $F.map(&:upcase) * ":"'
GOAL:AMAZING:42:WHISTLE:KWALITY:3.14
$ echo '23 756 -983 5' | ruby -ane 'puts $F.map {_1.to_i ** 2} * " "'
529 571536 966289 25
$ echo 'AaBbCc' | ruby -lne 'puts $_.chars.map(&:ord) * " "'
65 97 66 98 67 99
$ echo '3.14,17,6' | ruby -F, -ane 'puts $F.map(&:to_f).sum'
26.14The filter method (which has other aliases and opposites too) is handy to construct all kinds of selection conditions. You can combine with map by using the filter_map method.
$ s='hour hand band mat heated pineapple'
$ echo "$s" | ruby -ane 'puts $F.filter {_1[0]!="h" && _1.size<6}'
band
mat
$ echo "$s" | ruby -ane 'puts $F.filter_map {|w|
w.gsub(/[ae]/, "X") if w[0]=="h"}'
hour
hXnd
hXXtXdThe reduce method can be used to perform an action against all the elements of an array and get a singular value as the result.
# sum of input numbers, with initial value of 100
$ echo '3.14,17,6' | ruby -F, -lane 'puts $F.map(&:to_f).reduce(100, :+)'
126.14
# product of input numbers
$ echo '3.14,17,6' | ruby -F, -lane 'puts $F.map(&:to_f).reduce(:*)'
320.28000000000003
# with initial value of 2
$ echo '3.14,17,6' | ruby -F, -lane 'puts $F.reduce(2) {|op,n| op*n.to_f}'
640.5600000000001Here are some examples with the sort, sort_by and uniq methods for arrays and strings.
$ s='floor bat to dubious four'
$ echo "$s" | ruby -ane 'puts $F.sort * ":"'
bat:dubious:floor:four:to
$ echo "$s" | ruby -ane 'puts $F.sort_by(&:size) * ":"'
to:bat:four:floor:dubious
# numeric sort example
$ echo '23 756 -983 5' | ruby -lane 'puts $F.sort_by(&:to_i) * ":"'
-983:5:23:756
$ echo 'dragon' | ruby -lne 'puts $_.chars.sort.reverse * ""'
rongda
$ s='try a bad to good i teal by nice how'
# longer words first, ascending alphabetic order as tie-breaker
$ echo "$s" | ruby -ane 'puts $F.sort_by {|w| [-w.size, w]} * ":"'
good:nice:teal:bad:how:try:by:to:a:i
$ s='3,b,a,3,c,d,1,d,c,2,2,2,3,1,b'
# note that the input order of elements is preserved
$ echo "$s" | ruby -F, -lane 'puts $F.uniq * ","'
3,b,a,c,d,1,2Here's an example for sorting in descending order based on header column names.
$ cat marks.txt
Dept Name Marks
ECE Raj 53
ECE Joel 72
EEE Moi 68
CSE Surya 81
EEE Tia 59
ECE Om 92
CSE Amy 67
$ ruby -ane 'idx = $F.each_index.sort {$F[_2] <=> $F[_1]} if $.==1;
puts $F.values_at(*idx) * "\t"' marks.txt
Name Marks Dept
Raj 53 ECE
Joel 72 ECE
Moi 68 EEE
Surya 81 CSE
Tia 59 EEE
Om 92 ECE
Amy 67 CSEThe shuffle method randomizes the order of elements.
$ s='floor bat to dubious four'
$ echo "$s" | ruby -ane 'puts $F.shuffle * ":"'
bat:floor:dubious:to:four
$ echo 'foobar' | ruby -lne 'print $_.chars.shuffle * ""'
bofraoUse the sample method to get one or more elements of an array in random order.
$ s='hour hand band mat heated pineapple'
$ echo "$s" | ruby -ane 'puts $F.sample'
band
$ echo "$s" | ruby -ane 'puts $F.sample(2)'
pineapple
handThis chapter discussed various ways in which you can split (or define) the input into fields and manipulate them. Many more examples will be discussed in later chapters.
1) For the input file brackets.txt, extract only the contents between () or )( from each input line. Assume that () characters will be present only once every line.
$ cat brackets.txt
foo blah blah(ice) 123 xyz$
(almond-pista) choco
yo )yoyo( yo
##### add your solution here
ice
almond-pista
yoyo2) For the input file scores.csv, extract Name and Physics fields in the format shown below.
$ cat scores.csv
Name,Maths,Physics,Chemistry
Blue,67,46,99
Lin,78,83,80
Er,56,79,92
Cy,97,98,95
Ort,68,72,66
Ith,100,100,100
##### add your solution here
Name:Physics
Blue:46
Lin:83
Er:79
Cy:98
Ort:72
Ith:1003) For the input file scores.csv, display names of those who've scored above 70 in Maths.
##### add your solution here
Lin
Cy
Ith4) Display the number of word characters for the given inputs. Word definition here is same as used in regular expressions. Can you construct a solution with gsub and one without the substitution functions?
# solve using gsub
$ echo 'hi there' | ##### add your solution here
7
# solve without using the substitution functions
$ echo 'u-no;co%."(do_12:as' | ##### add your solution here
125) For the input file quoted.txt, extract the sequence of characters surrounded by double quotes and display them in the format shown below.
$ cat quoted.txt
1 "grape" and "mango" and "guava"
("c 1""d""a-2""b")
##### add your solution here
"grape","guava","mango"
"a-2","b","c 1","d"6) Display only the third and fifth characters from each input line.
$ printf 'restore\ncat one\ncricket' | ##### add your solution here
so
to
ik7) Transform the given input file fw.txt to get the output as shown below. If the second field is empty (i.e. contains only space characters), replace it with NA.
$ cat fw.txt
1.3 rs 90 0.134563
3.8 6
5.2 ye 8.2387
4.2 kt 32 45.1
##### add your solution here
1.3,rs,0.134563
3.8,NA,6
5.2,ye,8.2387
4.2,kt,45.18) For the input file scores.csv, display the header as well as any row which contains b or t (irrespective of case) in the first field.
##### add your solution here
Name,Maths,Physics,Chemistry
Blue,67,46,99
Ort,68,72,66
Ith,100,100,1009) Extract all whole words containing 42 but not at the edge of a word. Assume a word cannot contain 42 more than once.
$ s='hi42bye nice1423 bad42 cool_42a 42fake'
$ echo "$s" | ##### add your solution here
hi42bye
nice1423
cool_42a10) For the input file scores.csv, add another column named GP which is calculated out of 100 by giving 50% weightage to Maths and 25% each for Physics and Chemistry.
##### add your solution here
Name,Maths,Physics,Chemistry,GP
Blue,67,46,99,69.75
Lin,78,83,80,79.75
Er,56,79,92,70.75
Cy,97,98,95,96.75
Ort,68,72,66,68.5
Ith,100,100,100,100.011) For the input file mixed_fs.txt, retain only the first two fields from each input line. The input and output field separators should be space for first two lines and , for the rest of the lines.
$ cat mixed_fs.txt
rose lily jasmine tulip
pink blue white yellow
car,mat,ball,basket
light green,brown,black,purple
apple,banana,cherry
##### add your solution here
rose lily
pink blue
car,mat
light green,brown
apple,banana12) For the given space separated numbers, filter only numbers in the range 20 to 1000 (inclusive).
$ s='20 -983 5 756 634223 1000'
$ echo "$s" | ##### add your solution here
20 756 100013) For the given space separated words, randomize the order of characters for each word.
$ s='this is a sample sentence'
# sample randomized output shown here, could be different for you
$ echo "$s" | ##### add your solution here
shti si a salemp sneentce14) For the given input file words.txt, filter all lines containing characters in ascending and descending order.
$ cat words.txt
bot
art
are
boat
toe
flee
reed
# ascending order
##### add your solution here
bot
art
# descending order
##### add your solution here
toe
reed15) For the given space separated words, extract the three longest words.
$ s='I bought two bananas and three mangoes'
$ echo "$s" | ##### add your solution here
mangoes
bananas
bought16) Convert the contents of split.txt as shown below.
$ cat split.txt
apple,1:2:5,mango
wry,4,look
pencil,3:8,paper
##### add your solution here
apple,1,mango
apple,2,mango
apple,5,mango
wry,4,look
pencil,3,paper
pencil,8,paper17) For the input file varying_fields.txt, construct a solution to get the output shown below.
$ cat varying_fields.txt
hi,bye,there,was,here,to
1,2,3,4,5
##### add your solution here
hi:bye:to
1:2:518) The fields.txt file has fields separated by the : character. Delete : and the last field if there is a digit character anywhere before the last field. Solution shouldn't use the substitution functions.
$ cat fields.txt
42:cat
twelve:a2b
we:be:he:0:a:b:bother
apple:banana-42:cherry:
dragon:unicorn:centaur
##### add your solution here
42
twelve:a2b
we:be:he:0:a:b
apple:banana-42:cherry
dragon:unicorn:centaur19) The sample string shown below uses cat as the field separator (irrespective of case). Use space as the output field separator and add 42 as the last field.
$ s='applecatfigCaT12345cAtbanana'
$ echo "$s" | ##### add your solution here
apple fig 12345 banana 4220) For the input file sample.txt, filter lines containing 5 or more lowercase vowels.
##### add your solution here
How are you
Believe it
No doubt you like it too
Much ado about nothingSo far, you've seen examples where Ruby automatically splits input line by line based on the newline character. Just like you can control how those lines are further split into fields using the -a, -F options and other features, Ruby provides a way to control what constitutes a line in the first place. The term record is used to describe the contents that gets placed in the $_ global variable.
By default, the newline character is used as the input record separator. You can change the $/ global variable to specify a different input record separator. Unlike field separators, you can only use a string value, regexp isn't allowed.
# changing the input record separator to a comma character
# note the content of the second record where newline is just another character
# by default, the record separator stays with the record contents
$ printf 'this,is\na,sample,ok' | ruby -ne 'BEGIN{$/ = ","}; puts "#{$.})#{$_}"'
1)this,
2)is
a,
3)sample,
4)ok
# use the -l option to chomp the record separator
$ printf 'this,is\na,sample,ok' | ruby -lne 'BEGIN{$/ = ","}; puts "#{$.})#{$_}"'
1)this
2)is
a
3)sample
4)okHere's an example where the record separator has multiple characters:
$ cat report.log
blah blah Error: second record starts
something went wrong
some more details Error: third record
details about what went wrong
# uses 'Error:' as the input record separator
# prints all the records containing 'something'
$ ruby -lne 'BEGIN{$/ = "Error:"}; print if /something/' report.log
second record starts
something went wrong
some more details The -0 command line option can be used to specify a single character record separator, represented with zero to three octal digits.
$ s='this:is:a:sample:string'
# the : character is represented by 072 in octal
# -l is used here to chomp the separator
$ echo "$s" | ruby -0072 -lne 'puts "#{$.}) #{$_}"'
1) this
2) is
3) a
4) sample
5) string
# print all records containing 'a'
$ echo "$s" | ruby -0072 -lne 'puts $_ if /a/'
a
sampleThe character that gets appended to the print method when the -l option is active is based on the value of the input record separator at that point. Here are some examples to clarify this point.
$ s='this:is:a:sample:string'
# here, the record separator is still the default \n when -l is used
# so \n gets appended when 'print' is used
# note that chomp isn't affected by such differences in order
# same as: echo "$s" | ruby -lne 'BEGIN{$/=":"}; print if /a/'
$ echo "$s" | ruby -l -0072 -ne 'print if /a/'
a
sample
# here -l is defined after -0, so : gets appended for 'print'
$ echo "$s" | ruby -0072 -lne 'print if /a/'
a:sample:By default, the -a option will split the input record based on whitespaces and remove the leading/trailing whitespaces. Now that you've seen how the input record separator can be something other than newline, here's an example to show the full effect of the default record splitting.
# ':' character is the input record separator here
$ s=' a\t\tb:1000\n\n\t \n\n123 7777:x y \n \n z :apple banana cherry'
$ printf '%b' "$s" | ruby -0072 -lane 'puts $F * ","'
a,b
1000,123,7777
x,y,z
apple,banana,cherry
chompwill remove\r\nline endings as well from the input record. But, you'll get only\nin the output if you are relying on the-loption.$ printf 'apple\r\nfig\r\n' | cat -v apple^M fig^M $ printf 'apple\r\nfig\r\n' | ruby -lne 'print' | cat -v apple fig
If the -0 option is used without an argument, the ASCII NUL character will be considered as the input record separator.
$ printf 'apple\0banana\0' | cat -v
apple^@banana^@
# could also be golfed to: ruby -l0pe ''
$ printf 'apple\0banana\0' | ruby -l -0 -ne 'print'
apple
bananaAny octal value of 400 and above will cause the entire input to be slurped as a single string. Idiomatically, 777 is used. This is same as setting $/ = nil. Slurping the entire file makes it easier to solve some problems, but be careful to not use it for large files, as that might cause memory issues.
$ cat paths.txt
/home/joe/report.log
/home/ram/power.log
/home/rambo/errors.log
$ ruby -0777 -pe 'sub(%r{(?<!\A)/.+/}m, "/")' paths.txt
/home/errors.log
# replicate entire input as many times as needed
$ seq 2 | ruby -0777 -ne 'print $_ * 2'
1
2
1
2As a special case, using -00 or setting $/ to an empty string will invoke paragraph mode. Two or more consecutive newline characters will act as the record separator. Consider the below sample file:
$ cat para.txt
Hello World
Hi there
How are you
Just do-it
Believe it
banana
papaya
mango
Much ado about nothing
He he he
Adios amigoHere are some examples of processing the input file paragraph wise.
# all paragraphs containing 'do'
# note that the record separator is preserved as there's no chomp
$ ruby -00 -ne 'print if /do/' para.txt
Just do-it
Believe it
Much ado about nothing
He he he
Adios amigo
# all paragraphs containing exactly two lines
# note that there's an empty line at the end of the output
$ ruby -F'\n' -00 -ane 'print if $F.size == 2' para.txt
Hi there
How are you
Just do-it
Believe it
If the paragraphs are separated by more than two consecutive newlines, the extra newlines will not be part of the record content.
$ s='a\n\n\n\n\n\n\n\n12\n34\n\nhi\nhello\n'
# note that the -l option isn't being used here
$ printf '%b' "$s" | ruby -00 -ne 'print if $. <= 2'
a
12
34
Any leading newlines (only newlines, not other whitespace characters) in the input data file will be trimmed and not lead to empty records. This is similar to how -a treats whitespaces for default field separation.
$ s='\n\n\na\nb\n\n12\n34\n\nhi\nhello\n\n\n\n'
# note that -l is used to chomp the record separator here
$ printf '%b' "$s" | ruby -00 -lne 'puts "#{$_}\n---" if $. <= 2'
a
b
---
12
34
---
# max. of two trailing newlines will be preserved if -l isn't used
$ printf '%b' "$s" | ruby -00 -lne 'puts "#{$_}\n---" if $<.eof'
hi
hello
---
$ printf '%b' "$s" | ruby -00 -ne 'END{puts $.}'
3
$ printf '%b' "$s" | ruby -00 -ne 'BEGIN{$/="\n\n"}; END{puts $.}'
5The empty line at the end is a common problem when dealing with custom record separators. You could either process the output further to remove it or add extra logic to handle the issue. Here's one possible workaround:
# no empty line at the end
$ ruby -l -00 -ne '(print $s, $_; $s="\n") if /are/' para.txt
Hi there
How are you
# single empty line between the paragraphs
$ ruby -l -00 -ne '(print $s, $_; $s="\n") if /are|an/' para.txt
Hi there
How are you
banana
papaya
mangoYou can use $\ to specify the string that gets appended to the print method. This is will override changes due to the -l option, if any.
# recall that the input record separator isn't removed by default
$ seq 2 | ruby -ne 'print'
1
2
# this will add four more characters after the already present newline
# same as: ruby -pe 'BEGIN{$\ = "---\n"}'
$ seq 2 | ruby -ne 'BEGIN{$\ = "---\n"}; print'
1
---
2
---
# change the NUL character to a dot and newline characters
# -l here helps to chomp the NUL character
# -l also sets NUL to be added to print, but gets overridden in the BEGIN block
$ printf 'apple\0banana\0' | ruby -0 -lpe 'BEGIN{$\ = ".\n"}'
apple.
banana.Many a times, you'd need to change the output record separator depending upon the contents of the input record or some other condition. The cond ? expr1 : expr2 ternary operator is often used in such scenarios. The below example assumes that the input is evenly divisible, you'll have to add more logic if that is not the case.
# same as: ruby -pe 'sub(/\n/, "-") if $. % 3 != 0'
$ seq 6 | ruby -lpe '$\ = $. % 3 != 0 ? "-" : "\n"'
1-2-3
4-5-6This chapter showed you how to change the way the input content is split into records and how to set the string to be appended when print is used. The paragraph mode is useful for processing multiline records separated by one or more empty lines. You also learned how to set ASCII NUL as the record separator and how to slurp the entire input as a single string.
1) The input file jumbled.txt consists of words separated by various delimiters. Display the last four words that contain an or at or in or it, one per line.
$ cat jumbled.txt
overcoats;furrowing-typeface%pewter##hobby
wavering:concession/woof\retailer
joint[]seer{intuition}titanic
##### add your solution here
wavering
joint
intuition
titanic2) Emulate paste -sd, with Ruby.
# this command joins all input lines with the ',' character
$ paste -sd, ip.txt
Hello World,How are you,This game is good,Today is sunny,12345,You are funny
# make sure there's no ',' at the end of the line
# and that there's a newline character at the end of the line
##### add your solution here
Hello World,How are you,This game is good,Today is sunny,12345,You are funny
# if there's only one line in input, again make sure there's no trailing ','
# and that there's a newline character at the end of the line
$ printf 'fig' | paste -sd,
fig
$ printf 'fig' | ##### add your solution here
fig3) For the input file sample.txt, extract all paragraphs having words starting with do.
$ cat sample.txt
Hello World
Good day
How are you
Just do-it
Believe it
Today is sunny
Not a bit funny
No doubt you like it too
Much ado about nothing
He he he
# note that there's no extra empty line at the end of the output
##### add your solution here
Just do-it
Believe it
Today is sunny
Not a bit funny
No doubt you like it too4) For the input file sample.txt, change each paragraph to a single line by joining lines using . and a space character as the separator. Also, add a final . to each paragraph.
# note that there's no extra empty line at the end of the output
##### add your solution here
Hello World.
Good day. How are you.
Just do-it. Believe it.
Today is sunny. Not a bit funny. No doubt you like it too.
Much ado about nothing. He he he.5) For the given input, use ;; as the record separators and : as the field separators. Filter records whose second field is greater than 50.
$ s='mango:100;;apple:25;;grapes:75'
# note that the output has ;; at the end, not a newline character
$ printf "$s" | ##### add your solution here
mango:100;;grapes:75;;6) The input file f1.txt has varying amount of empty lines between the records. Change them to be always two empty lines. Also, remove empty lines at the start and end of the file.
##### add your solution here
hello
world
apple
banana
cherry
tea coffee
chocolate7) The sample string shown below uses cat as the record separator. Display only the even numbered records separated by a single empty line.
$ s='applecatfigcat12345catbananacatguava:cat:mangocat3'
$ echo "$s" | ##### add your solution here
fig
banana
:mangoYou have already seen blocks like BEGIN, END and methods like next and exit. This chapter will discuss features that are useful to make decisions around individual files when there are multiple files passed as input.
The ARGV array contains the list of files passed to the Ruby script. Once an input file is processed, it is removed from this list. You can dynamically manipulate this array if you wish to change the flow of input being processed. The $* global variable can also be used instead of ARGV.
# note that only the -e option is used here
# can also use 'puts $*' instead of 'puts ARGV'
$ ruby -e 'puts ARGV' f[1-3].txt greeting.txt
f1.txt
f2.txt
f3.txt
greeting.txt
# ARGV continuously ejects the filename being processed
# f1.txt and f2.txt have 1 line each, table.txt has 3 lines
$ ruby -ne 'puts "#{ARGV.size}: " + ARGV * ","' f[12].txt table.txt
2: f2.txt,table.txt
1: table.txt
0:
0:
0: ARGF (or the $< global variable) represents the filehandle of the current file (that was passed as an argument to the Ruby script) being processed. If ARGV is empty, then ARGF will process stdin data if available. If you explicitly call the close method on ARGF, it will reset the $. variable. See ruby-doc: ARGF for documentation.
# logic to do something at the start of each input file
# can also use $<.eof instead of ARGF.eof
$ ruby -ne 'puts "--- #{ARGF.filename} ---" if $. == 1;
print;
ARGF.close if ARGF.eof' greeting.txt table.txt
--- greeting.txt ---
Hi there
Have a nice day
Good bye
--- table.txt ---
brown bread mat hair 42
blue cake mug shirt -7
yellow banana window shoes 3.14
# do something at the end of a file
# same as: tail -q -n1 greeting.txt table.txt
$ ruby -ne 'print if ARGF.eof' greeting.txt table.txt
Good bye
yellow banana window shoes 3.14Here are some more examples.
# same as: awk 'FNR==2{print; nextfile}' greeting.txt table.txt
$ ruby -ne '(print; ARGF.close) if $.==2' greeting.txt table.txt
Have a nice day
blue cake mug shirt -7
# same as: head -q -n1 and awk 'FNR>1{nextfile} 1'
# can also use: ruby -pe 'ARGF.close'
$ ruby -pe 'ARGF.close if $.>=1' greeting.txt table.txt
Hi there
brown bread mat hair 42You can use methods like read, readline, readlines, gets, etc to explicitly get data from a specific filehandle. ARGF is the default source for some of these methods like readlines, gets, etc as they are part of Kernel (see ruby-doc: Kernel for details).
# note that only the -e option is used
# same as: ruby -e 'puts ARGF.gets' greeting.txt
$ ruby -e 'puts gets' greeting.txt
Hi there
$ ruby -e 'puts gets, "---", ARGF.read' greeting.txt
Hi there
---
Have a nice day
Good bye
$ ruby -e 'puts readlines' greeting.txt
Hi there
Have a nice day
Good bye
$ ruby -e 'puts ARGF.readchar' greeting.txt
HThe STDIN filehandle is useful to distinguish between files passed as arguments and the stdin data. See the Comparing records section for more examples.
# with no file arguments, readline uses the stdin data
$ printf 'apple\nmango\n' | ruby -e 'puts readline'
apple
# with file arguments, readline doesn't work on stdin data
$ printf 'apple\nmango\n' | ruby -e 'puts readline' greeting.txt
Hi there
# use STDIN to work on stdin data irrespective of file arguments
$ printf 'apple\nmango\n' | ruby -e 'puts STDIN.readline' greeting.txt
appleYou have seen examples where the exit method was used to avoid processing unnecessary records for the current and any other files yet to be processed. Sometimes, you need to skip only the contents of the current file and move on to the next file for processing. The close method seen previously comes in handy for such cases.
# avoids unnecessary processing compared to ruby -ne 'print if !(/\bba/..$<.eof)'
# same as: awk '/\<ba/{nextfile} 1' ip.txt table.txt
$ ruby -ne '/\bba/ ? ARGF.close : print' ip.txt table.txt
it is a warm and cozy day
listen to what I say
go play in the park
brown bread mat hair 42
blue cake mug shirt -7
# print filename if it contains 'I' anywhere in the file
# same as: grep -l 'I' f[1-3].txt greeting.txt
# same as: ruby -0777 -ne 'puts ARGF.filename if /I/'
# but slurping is dependent on size of input files and available memory
$ ruby -ne '(puts ARGF.filename; ARGF.close) if /I/' f[1-3].txt greeting.txt
f1.txt
f2.txt
# print filename if it contains a word ending with 'e'
# and 'bat' or 'mat' (irrespective of case) anywhere in the file
# same as: ruby -0777 -ne 'puts ARGF.filename if /(?=.*?e\b)(?i).*[bm]at/m'
$ ruby -ne '$m1=true if /e\b/; $m2=true if /[bm]at/i;
(puts ARGF.filename; $m1=$m2=false; ARGF.close; next) if $m1 && $m2;
$m1=$m2=false if ARGF.eof' f[1-3].txt greeting.txt
f3.txtThis chapter introduced features for processing multiple file inputs and constructing file level decisions. These will also show up in more examples in the coming chapters.
1) Print the last field of first two lines for the input files table.txt and ip.txt. Assume space as the field separators for these two files. To make the output more informative, print filenames and a separator as shown in the output below. Assume that the input files will have at least two lines.
# table.txt ip.txt are passed as file inputs
##### add your solution here
>table.txt<
42
-7
----------
>ip.txt<
World
you
----------2) For the input files sample.txt, secrets.txt, ip.txt and table.txt, display only the names of files that contain at or fun in the third field. Assume space as the field separator.
##### add your solution here
secrets.txt
ip.txt
table.txt3) Print the first two lines from the input files ip.txt, sample.txt and table.txt. Also, add a separator between the results as shown below (note that the separator isn't present at the end of the output). Assume that the input files will have at least two lines.
##### add your solution here
Hello World
How are you
---
Hello World
---
brown bread mat hair 42
blue cake mug shirt -74) Print only the second field of the third line, if any, from these input files: ip.txt, sample.txt and copyright.txt. Consider space as the field separator.
##### add your solution here
game
day
blaOften, you need to consider multiple lines at a time to make a decision, such as the paragraph mode examples seen earlier. Sometimes, you need to match a particular record and then get records surrounding the matched record. Solution to these type of problems often use state machines. See softwareengineering: FSM examples if you are not familiar with state machines.
You might need to define a condition that should satisfy something for one record and something else for the very next record. There are many ways to tackle this problem. One possible solution is to use a variable to save the previous record and then create the required conditional expression using those variables and $_ which has the current record content.
# match and print two consecutive records
# first record should contain 'he' and second record should contain 'you'
# note that this will not work if you use normal variable instead of $p
$ ruby -ne 'puts $p, $_ if /you/ && $p=~/he/; $p = $_' para.txt
Hi there
How are you
# same filtering as above, but print only the first record
$ ruby -ne 'puts $p if /you/ && $p=~/he/; $p = $_' para.txt
Hi there
# same filtering as above, but print only the second record
$ ruby -ne 'print if /you/ && $p=~/he/; $p = $_' para.txt
How are youSometimes you want not just the matching records, but the records relative to the matches as well. For example, it could be to see the comments at the start of a function block that was matched while searching a program file. Or, it could be to see extended information from a log file while searching for a particular error message.
Consider this sample input file:
$ cat context.txt
blue
toy
flower
sand stone
light blue
flower
sky
water
language
english
hindi
spanish
tamil
programming language
python
kotlin
rubyCase 1: Here's an example that emulates grep --no-group-separator -A<n> functionality. The $n && $n>=0 && $n-=1 trick used in the example below works like this:
- If
$nhasn't been initialized yet, the expression becomesfalseand thus preventsnilbeing compared to a number - If
$n=11>=0 && 0--> evaluates totrueand$nbecomes00>=0 && -1--> evaluates totrueand$nbecomes-1-1>=0 &&--> evaluates tofalseand$nno longer changes
# same as: grep --no-group-separator -A1 'blue'
# print the matching line as well as the one that follows it
$ ruby -ne '$n=1 if /blue/; print if $n && $n>=0 && $n-=1' context.txt
blue
toy
light blue
flower
# for overlapping cases, $n gets re-assigned before $n becomes negative
$ ruby -ne '$n=1 if /toy|flower/; print if $n && $n>=0 && $n-=1' context.txt
toy
flower
sand stone
flower
skyOnce you've understood the above examples, the rest of the examples in this section should be easier to comprehend. They are all variations of the logic used above and re-arranged to solve the use case being discussed.
Case 2: Print n records after the matching record. This is similar to the previous case, except that the matching record isn't printed.
# print 2 lines after the matching line
# note that comparison here is $n>0 and not $n>=0
$ ruby -ne 'print if $n && $n>0 && $n-=1; $n=2 if /language/' context.txt
english
hindi
python
kotlinCase 3: Here's how to print nth record after the matching record.
# print only the 3rd line found after the matching line
# overlapping cases won't work as $n gets re-assigned before going to 0
$ ruby -ne 'print if $n.to_i>0 && ($n-=1)==0; $n=3 if /language/' context.txt
spanish
rubyCase 4: Printing the matched record and n records before it.
# this won't work if there are less than n records before a match
$ ruby -e 'ip=readlines; n=2; ip.each_with_index { |s, i|
puts ip[i-n..i] if s.match?(/stone/) }' context.txt
toy
flower
sand stone
# this will work even if there are less than n records before a match
$ ruby -e 'ip=readlines; n=2; ip.each_with_index { |s, i|
c=i-n; c=0 if c<0;
puts ip[c..i] if s.match?(/toy/) }' context.txt
blue
toyTo prevent confusion with overlapping cases, you can add a separation line between the results.
$ ruby -e 'ip=readlines; n=2; ip.each_with_index { |s, i|
c=i-n; c=0 if c<0;
(print $s; puts ip[c..i]; $s="---\n") if s.match?(/toy|flower/) }
' context.txt
blue
toy
---
blue
toy
flower
---
sand stone
light blue
flowerCase 5: Print nth record before the matching record.
$ ruby -e 'ip=readlines; n=2; ip.each_with_index { |s, i| c=i-n;
puts ip[c] if c>=0 && s.match?(/language/) }' context.txt
sky
spanish
# if the count is small enough, you can save them in variables
# this one prints the 2nd line before the matching line
# $.>2 is needed as first 2 records shouldn't be considered for a match
$ ruby -ne 'print $p2 if $.>2 && /toy|flower/; $p2=$p1; $p1=$_' context.txt
blue
sand stoneYou can also use the logic from Case 3 by applying tac twice. This avoids the need to use an array variable.
$ tac context.txt | ruby -ne 'print if $n.to_i>0 && ($n-=1)==0;
$n=2 if /language/' | tac
sky
spanishThis section will cover cases where the input file will always contain the same number of starting and ending patterns, arranged in an alternating fashion. For example, there cannot be two starting patterns appearing without an ending pattern between them and vice versa. Lines of text inside and between such groups are optional.
The sample file shown below will be used to illustrate examples in this section. For simplicity, assume that the starting pattern is marked by start and the ending pattern by end. They have also been given group numbers to make it easier to analyze the output.
$ cat uniform.txt
mango
icecream
--start 1--
1234
6789
**end 1**
how are you
have a nice day
--start 2--
a
b
c
**end 2**
par,far,mar,tarCase 1: Processing all the groups of records based on the distinct markers, including the records matched by markers themselves. For simplicity, the below command will just print all such records.
$ ruby -ne '$f=true if /start/; print if $f; $f=false if /end/' uniform.txt
--start 1--
1234
6789
**end 1**
--start 2--
a
b
c
**end 2**
ruby -ne 'print if /start/../end/'but compared to the Flip-Flop operator, the state machine format is more suitable to adapt for the various cases to follow.
Case 2: Processing all the groups of records but excluding the records matched by markers themselves.
$ ruby -ne '$f=false if /end/; print "* ", $_ if $f;
$f=true if /start/' uniform.txt
* 1234
* 6789
* a
* b
* cCase 3-4: Processing all the groups of records but excluding one of the markers.
$ ruby -ne '$f=true if /start/; $f=false if /end/; print if $f' uniform.txt
--start 1--
1234
6789
--start 2--
a
b
c
$ ruby -ne 'print if $f; $f=true if /start/; $f=false if /end/' uniform.txt
1234
6789
**end 1**
a
b
c
**end 2**The next four cases are obtained by just using if !$f instead of if $f from the cases shown above.
Case 5: Processing all input records except the groups of records bound by the markers.
# same as: ruby -ne 'print if !(/start/../end/)'
$ ruby -ne '$f=true if /start/; print if !$f; $f=false if /end/' uniform.txt
mango
icecream
how are you
have a nice day
par,far,mar,tarCase 6: Processing all input records except the groups of records between the markers.
$ ruby -ne '$f=false if /end/; print if !$f; $f=true if /start/' uniform.txt
mango
icecream
--start 1--
**end 1**
how are you
have a nice day
--start 2--
**end 2**
par,far,mar,tarCase 7-8: Similar to case 6, but include only one of the markers.
$ ruby -ne 'print if !$f; $f=true if /start/; $f=false if /end/' uniform.txt
mango
icecream
--start 1--
how are you
have a nice day
--start 2--
par,far,mar,tar
$ ruby -ne '$f=true if /start/; $f=false if /end/; print if !$f' uniform.txt
mango
icecream
**end 1**
how are you
have a nice day
**end 2**
par,far,mar,tarInstead of working with all the groups (or blocks) bound by the markers, this section will discuss how to choose blocks based on an additional criteria.
Here's how you can process only the first matching block.
$ ruby -ne '$f=true if /start/; print if $f; exit if /end/' uniform.txt
--start 1--
1234
6789
**end 1**
# use other tricks discussed in the previous section as needed
$ ruby -ne 'exit if /end/; print if $f; $f=true if /start/' uniform.txt
1234
6789Getting the last block alone involves a lot more work, unless you happen to know how many blocks are present in the input file.
# reverse input linewise, change the order of comparison, reverse again
# difficult to adjust if the record separator is something other than newline
$ tac uniform.txt | ruby -ne '$f=true if /end/; print if $f; exit if /start/' | tac
--start 2--
a
b
c
**end 2**
# or, save the blocks in a buffer and print the last one alone
# << operator concatenates the given string to the variable in-place
$ ruby -ne '($f=true; buf=$_; next) if /start/;
buf << $_ if $f;
$f=false if /end/;
END{print buf}' uniform.txt
--start 2--
a
b
c
**end 2**Only the nth block.
$ seq 30 | ruby -ne 'BEGIN{n=2; c=0}; c+=1 if /4/; (print; exit if /6/) if c==n'
14
15
16All blocks greater than the nth block.
$ seq 30 | ruby -ne 'BEGIN{n=1; c=0}; ($f=true; c+=1) if /4/;
print if $f && c>n; $f=false if /6/'
14
15
16
24
25
26Excluding the nth block.
$ seq 30 | ruby -ne 'BEGIN{n=2; c=0}; ($f=true; c+=1) if /4/;
print if $f && c!=n; $f=false if /6/'
4
5
6
24
25
26All blocks, only if the records between the markers match an additional condition.
# additional condition here is a record with entire content as '15'
$ seq 30 | ruby -ne '($f=true; buf=$_; next) if /4/; buf << $_ if $f;
($f=false; print buf if buf.match?(/^15$/)) if /6/;'
14
15
16Sometimes, you can have markers in random order and mixed in different ways. In such cases, to work with blocks without any other marker present in between them, the buffer approach comes in handy again.
$ cat broken.txt
qqqqqqqqqqqqqqqq
error 1
hi
error 2
1234
6789
state 1
bye
state 2
error 3
xyz
error 4
abcd
state 3
zzzzzzzzzzzzzzzz
$ ruby -ne '($f=true; buf=$_; next) if /error/;
buf << $_ if $f;
(print buf if $f; $f=false) if /state/' broken.txt
error 2
1234
6789
state 1
error 4
abcd
state 3This chapter covered various examples of working with multiple records. State machines play an important role in deriving solutions for such cases. Knowing various corner cases is also crucial, otherwise a solution that works for one input may fail for others. Next chapter will discuss use cases where you need to process a file input based on the contents of another file.
1) For the input file sample.txt, print lines containing do only if the previous line is empty and the line before that contains you.
##### add your solution here
Just do-it
Much ado about nothing2) For the input file sample.txt, match lines containing do or not case insensitively. Each of these terms occur multiple times in the file. The goal is to print only the second occurrences of these terms (independent of each other).
# for reference, here are all the matches
$ grep -i 'do' sample.txt
Just do-it
No doubt you like it too
Much ado about nothing
$ grep -i 'not' sample.txt
Not a bit funny
Much ado about nothing
##### add your solution here
No doubt you like it too
Much ado about nothing3) For the input file sample.txt, print the matching lines containing are or bit as well as n lines around the matching lines. The value for n is passed to the Ruby command as an environment value.
$ n=1 ##### add your solution here
Good day
How are you
Today is sunny
Not a bit funny
No doubt you like it too
# note that the first and last line are empty for this case
$ n=2 ##### add your solution here
Good day
How are you
Just do-it
Today is sunny
Not a bit funny
No doubt you like it too
4) For the input file broken.txt, print all lines between the markers top and bottom. The first Ruby command shown below doesn't work because it is matching till the end of file as the second marker isn't found. Assume that the input file cannot have two top markers without a bottom marker appearing in between and vice-versa.
$ cat broken.txt
top
3.14
bottom
---
top
1234567890
bottom
top
Hi there
Have a nice day
Good bye
# wrong output
$ ruby -ne '$f=false if /bottom/; print if $f; $f=true if /top/' broken.txt
3.14
1234567890
Hi there
Have a nice day
Good bye
# expected output
##### add your solution here
3.14
12345678905) For the input file concat.txt, extract contents from a line starting with %%% until but not including the next such line. The block to be extracted is indicated by the variable n passed as an environment value.
$ cat concat.txt
%%% addr.txt
How are you
This game is good
Today %%% is sunny
%%% broken.txt
top %%%
1234567890
bottom
%%% sample.txt
Just %%% do-it
Believe it
%%% mixed_fs.txt
pink blue white yellow
car,mat,ball,basket
$ n=2 ##### add your solution here
%%% broken.txt
top %%%
1234567890
bottom
$ n=4 ##### add your solution here
%%% mixed_fs.txt
pink blue white yellow
car,mat,ball,basket6) For the input file ruby.md, replace all occurrences of ruby (irrespective of case) with Ruby. But, do not replace any matches between ```ruby and ``` lines (ruby in these markers shouldn't be replaced either). Save the output in out.md.
##### add your solution here, redirect the output to 'out.md'
$ diff -sq out.md expected.md
Files out.md and expected.md are identical7) Print the last two lines for each of the input files ip.txt, sample.txt and table.txt. Also, add a separator between the results as shown below (note that the separator isn't present at the end of the output). Assume that the input files will have at least two lines.
##### add your solution here
12345
You are funny
---
Much ado about nothing
He he he
---
blue cake mug shirt -7
yellow banana window shoes 3.148) For the input file lines.txt, delete the line that comes after a whole line containing ---. Assume that such lines won't occur consecutively.
$ cat lines.txt
Go There
come on
go there
---
2 apples and 5 mangoes
come on!
---
2 Apples
COME ON
##### add your solution here
Go There
come on
go there
---
come on!
---
COME ON9) For the input file result.csv, use --- to separate entries with the same name in the first column. Assume that the lines with the same first column value will always be next to each other.
$ cat result.csv
Amy,maths,89
Amy,physics,75
Joe,maths,79
John,chemistry,77
John,physics,91
Moe,maths,81
Ravi,physics,84
Ravi,chemistry,70
Yui,maths,92
##### add your solution here
Amy,maths,89
Amy,physics,75
---
Joe,maths,79
---
John,chemistry,77
John,physics,91
---
Moe,maths,81
---
Ravi,physics,84
Ravi,chemistry,70
---
Yui,maths,9210) The input file multisort.csv has two fields separated by the comma character. Sort this file based on the number of : characters in the second field. Use alphabetic order as the tie-breaker if there are multiple lines with the same number of : characters in the second field.
$ cat multisort.csv
papaya,2
apple,4:5:2
mango,100
dark:chocolate,12:32
cherry,1:2:1:4:2:1
almond,3:14:6:28
banana,23:8
##### add your solution here
mango,100
papaya,2
banana,23:8
dark:chocolate,12:32
apple,4:5:2
almond,3:14:6:28
cherry,1:2:1:4:2:1This chapter focuses on solving problems which depend upon the contents of two or more files. These are usually based on comparing records and fields. Sometimes, record number plays a role and some cases need the entire file content. You'll also see examples with the -r command line option and the gets method.
Consider the following input files which will be compared line wise in this section.
$ cat colors_1.txt
teal
light blue
green
yellow
$ cat colors_2.txt
light blue
black
dark green
yellowThe -r command line option allows you to specify a library required for the script. The Set class is handy for two file processing cases. See ruby-doc: Set for documentation.
# common lines
# same as: grep -Fxf colors_1.txt colors_2.txt
# ARGV.size==1 will be true only for the first file (for two file input)
$ ruby -rset -ne 'BEGIN{s=Set.new}; (s.add($_); next) if ARGV.size==1;
print if s.include?($_)' colors_1.txt colors_2.txt
light blue
yellow
# lines from colors_2.txt not present in colors_1.txt
# same as: grep -vFxf colors_1.txt colors_2.txt
$ ruby -rset -ne 'BEGIN{s=Set.new}; (s.add($_); next) if ARGV.size==1;
print if !s.include?($_)' colors_1.txt colors_2.txt
black
dark green
# reversing the order of input files gives
# lines from colors_1.txt not present in colors_2.txt
$ ruby -rset -ne 'BEGIN{s=Set.new}; (s.add($_); next) if ARGV.size==1;
print if !s.include?($_)' colors_2.txt colors_1.txt
teal
greenAlternatively, you can store the contents of the two input files as arrays and use set operations between them.
# common lines, output order is based on the array to the left of & operator
# note that only the -e option is used and one of the files is passed as stdin
$ ruby -e 'f1=STDIN.readlines; f2=readlines;
puts f1 & f2' <colors_1.txt colors_2.txt
light blue
yellow
# lines from colors_1.txt not present in colors_2.txt
$ ruby -e 'f1=STDIN.readlines; f2=readlines;
puts f1 - f2' <colors_1.txt colors_2.txt
teal
green
# union of the two files, same as f1 | f2 if read as separate arrays
$ ruby -e 'puts readlines.uniq' colors_1.txt colors_2.txt
teal
light blue
green
yellow
black
dark greenIn the previous section, you saw how to compare the contents of whole records between two files. This section will focus on comparing only specific fields. The below sample file will be one of the two file inputs for examples in this section. Consider whitespace as the field separator, so the -a option is enough to get the fields.
$ cat marks.txt
Dept Name Marks
ECE Raj 53
ECE Joel 72
EEE Moi 68
CSE Surya 81
EEE Tia 59
ECE Om 92
CSE Amy 67To start with, here's an example with a single field comparison. The problem statement is to fetch all records from marks.txt if the first field matches any of the departments listed in the dept.txt file.
$ cat dept.txt
CSE
ECE
# note that dept.txt is used to build the set first
$ ruby -rset -ane 'BEGIN{s=Set.new}; (s.add($F[0]); next) if ARGV.size==1;
print if s.include?($F[0])' dept.txt marks.txt
ECE Raj 53
ECE Joel 72
CSE Surya 81
ECE Om 92
CSE Amy 67For multiple field comparison, use subset of an array for comparison.
$ cat dept_name.txt
EEE Moi
CSE Amy
ECE Raj
$ ruby -rset -ane 'BEGIN{s=Set.new}; (s.add($F); next) if ARGV.size==1;
print if s.include?($F[0..1])' dept_name.txt marks.txt
ECE Raj 53
EEE Moi 68
CSE Amy 67In this example, one of the fields is used for numerical comparison.
$ cat dept_mark.txt
ECE 70
EEE 65
CSE 80
# match Dept and minimum marks specified in dept_mark.txt
# since the marks are consistently 2-digits, string comparison is enough
# otherwise, you'd need to convert to numbers before comparison
$ ruby -ane 'BEGIN{d={}}; (d[$F[0]]=$F[1]; next) if ARGV.size==1;
print if d.key?($F[0]) && $F[2] >= d[$F[0]]' dept_mark.txt marks.txt
ECE Joel 72
EEE Moi 68
CSE Surya 81
ECE Om 92Here's an example of adding a new field based on the contents of another field.
$ ruby -lane 'BEGIN{g = %w[D C B A S]};
$F.append($.==1 ? "Grade" : g[$F[-1].to_i/10 - 5]);
print $F * "\t"' marks.txt
Dept Name Marks Grade
ECE Raj 53 D
ECE Joel 72 B
EEE Moi 68 C
CSE Surya 81 A
EEE Tia 59 D
ECE Om 92 S
CSE Amy 67 CThe gets method (or readline) helps you to read a record from a file on demand. This is most useful when you need something based on the record number. The following example shows how you can replace the mth line from a file with the nth line from another file.
# replace the 3rd line of table.txt with the 2nd line of greeting.txt
$ ruby -pe 'BEGIN{m=3; n=2; n.times {$s = STDIN.gets}};
$_ = $s if $. == m' <greeting.txt table.txt
brown bread mat hair 42
blue cake mug shirt -7
Have a nice dayHere's an example where two files are processed simultaneously. This doesn't implement error detection for difference in the number of lines between the two files though.
# print line from greeting.txt if the last column of the corresponding line
# from table.txt is a positive number
# STDIN.gets will override $_ which is why $_ is saved to another variable
$ ruby -ne 'a=$_; n = STDIN.gets.split[-1].to_f;
print a if n > 0' <table.txt greeting.txt
Hi there
Good byeYou can use file slurping for fixed string multiline search and replace requirements. Both sub and gsub methods allow matching fixed string if the first argument is a string instead of a regexp object. Since \ is special in the replacement section, you'll have to use block form to provide the replacement string.
The below example is substituting complete lines. The solution will work for partial lines as well, provided there is no newline character at the end of search.txt and repl.txt files.
$ head -n2 table.txt > search.txt
$ cat repl.txt
2$1$&3\0x\\yz
wise ice go goa
$ ruby -0777 -ne 'ARGV.size==2 ? s=$_ : ARGV.size==1 ? r=$_ :
print(gsub(s) {r})
' search.txt repl.txt table.txt
2$1$&3\0x\\yz
wise ice go goa
yellow banana window shoes 3.14
search.txtandrepl.txtin shell variables for passing them to the Ruby script. Trailing newlines and ASCII NUL characters will cause issues. See stackoverflow: pitfalls of reading file into shell variable for details.
Case 1: replace each matching line with the entire contents of STDIN.
# same as: sed -e '/[ot]/{r dept.txt' -e 'd}' greeting.txt
$ ruby -pe 'BEGIN{r = STDIN.read}; $_ = r if /[ot]/' <dept.txt greeting.txt
CSE
ECE
Have a nice day
CSE
ECECase 2: insert the entire contents of STDIN before each matching line.
# same as: sed '/nice/e cat dept.txt' greeting.txt
$ ruby -pe 'BEGIN{r = STDIN.read}; print r if /nice/' <dept.txt greeting.txt
Hi there
CSE
ECE
Have a nice day
Good byeCase 3: append the entire contents of STDIN after each matching line.
# same as: sed '/nice/r dept.txt' greeting.txt
$ ruby -pe 'BEGIN{r = STDIN.read}; $_ << r if /nice/' <dept.txt greeting.txt
Hi there
Have a nice day
CSE
ECE
Good byeThis chapter discussed use cases where you need to process the contents of two or more files based on entire record/file or field(s). The ARGV.size==1 trick is handy for such cases (where the number is n-1 to match the first file passed among n input files). The gets method is helpful for record number based comparisons.
1) Use the contents of match_words.txt file to display matching lines from jumbled.txt and sample.txt. The matching criteria is that the second word of lines from these files should match the third word of lines from match_words.txt.
$ cat match_words.txt
%whole(Hello)--{doubt}==ado==
just,\joint*,concession<=nice
# 'concession' is one of the third words from 'match_words.txt'
# and second word from 'jumbled.txt'
##### add your solution here
wavering:concession/woof\retailer
No doubt you like it too2) Interleave the contents of secrets.txt with the contents of a file passed as stdin in the format as shown below.
##### add your solution here, use 'table.txt' as stdin
stag area row tick
brown bread mat hair 42
---
deaf chi rate tall glad
blue cake mug shirt -7
---
Bi tac toe - 42
yellow banana window shoes 3.143) The file search_terms.txt contains one search string per line, and these terms have no regexp metacharacters. Construct a solution that reads this file and displays the search terms (matched case insensitively) that were found in every file passed as the arguments after search_terms.txt. Note that these terms should be matched anywhere in the line (so, don't use word boundaries).
$ cat search_terms.txt
hello
row
you
is
at
# ip: search_terms.txt jumbled.txt mixed_fs.txt secrets.txt table.txt oops.txt
##### add your solution here
row
at
# ip: search_terms.txt ip.txt sample.txt oops.txt
##### add your solution here
hello
you
is4) For the input file ip.txt, print all lines that contain are and the line that comes after such a line, if any. Use the gets method to construct the solution.
# note that there shouldn't be an empty line at the end of the output
##### add your solution here
How are you
This game is good
You are funnyBonus: Will grep -A1 'is' ip.txt give identical results for your solution with is as the search term? If not, why?
5) Replace the third to fifth lines of the input file ip.txt with the second to fourth lines from the file para.txt.
##### add your solution here
Hello World
How are you
Start working on that
project you always wanted
to, do not let it end
You are funny6) Insert one line from jumbled.txt before every two lines of copyright.txt.
##### add your solution here
overcoats;furrowing-typeface%pewter##hobby
bla bla 2015 bla
blah 2018 blah
wavering:concession/woof\retailer
bla bla bla
copyright: 20187) Use the entire contents of match.txt to search error.txt and replace matching portions with the contents of jumbled.txt. Partial lines should NOT be matched.
$ cat match.txt
print+this
but not that
$ cat error.txt
print+this
but not that or this
print+this
but not that
if print+this
but not that
print+this
but not that
##### add your solution here
print+this
but not that or this
overcoats;furrowing-typeface%pewter##hobby
wavering:concession/woof\retailer
joint[]seer{intuition}titanic
if print+this
but not that
overcoats;furrowing-typeface%pewter##hobby
wavering:concession/woof\retailer
joint[]seer{intuition}titanic8) Display lines from scores.csv by matching the first field based on a list of names from the names.txt file. Also, change the output field separator to a space character.
$ cat names.txt
Lin
Cy
Ith
##### add your solution here
Lin 78 83 80
Cy 97 98 95
Ith 100 100 1009) The result.csv file has three columns — name, subject and mark. The criteria.txt file has two columns — name and subject. Match lines from result.csv based on the two columns from criteria.txt provided the mark column is greater than 80.
$ cat result.csv
Amy,maths,89
Amy,physics,75
Joe,maths,79
John,chemistry,77
John,physics,91
Moe,maths,81
Ravi,physics,84
Ravi,chemistry,70
Yui,maths,92
$ cat criteria.txt
Amy maths
John chemistry
John physics
Ravi chemistry
Yui maths
##### add your solution here
Amy,maths,89
John,physics,91
Yui,maths,9210) Insert the contents of hex.txt before a line matching cake of table.txt.
##### add your solution here
brown bread mat hair 42
start: 0xA0, func1: 0xA0
end: 0xFF, func2: 0xB0
restart: 0xA010, func3: 0x7F
blue cake mug shirt -7
yellow banana window shoes 3.1411) For the input file ip.txt, replace lines containing are with the contents of hex.txt.
##### add your solution here
Hello World
start: 0xA0, func1: 0xA0
end: 0xFF, func2: 0xB0
restart: 0xA010, func3: 0x7F
This game is good
Today is sunny
12345
start: 0xA0, func1: 0xA0
end: 0xFF, func2: 0xB0
restart: 0xA010, func3: 0x7FOften, you need to eliminate duplicates from an input file. This could be based on the entire line content or based on certain fields. These are typically solved with the sort and uniq commands. Advantages with Ruby include regexp based field separator, record separator other than newline, input doesn't have to be sorted, and in general more flexibility because it is a programming language.
Using the uniq method on readlines is the easiest and most compact solution if memory isn't an issue.
$ cat purchases.txt
coffee
tea
washing powder
coffee
toothpaste
tea
soap
tea
# note that the -n or -p options aren't needed here
# this will work for multiple input files as well
$ ruby -e 'puts readlines.uniq' purchases.txt
coffee
tea
washing powder
toothpaste
soapIf there are lots of duplicate lines and having the whole input file in an array can cause memory issues, then using a Set might help.
# add? returns nil if element already exists, else adds to the set
$ ruby -rset -ne 'BEGIN{s=Set.new}; print if s.add?($_)' purchases.txt
coffee
tea
washing powder
toothpaste
soap
The set based solution is easy to adapt for removing field based duplicates. Just change $_ to the required fields after setting the appropriate field separator. With uniq, you can use blocks to specify the condition.
$ cat duplicates.txt
brown,toy,bread,42
dark red,ruby,rose,111
blue,ruby,water,333
dark red,sky,rose,555
yellow,toy,flower,333
white,sky,bread,111
light red,purse,rose,333
# based on the last field
# same as: ruby -e 'puts readlines.uniq {_1.split(",")[-1]}'
$ ruby -rset -F, -ane 'BEGIN{s=Set.new}; print if s.add?($F[-1])' duplicates.txt
brown,toy,bread,42
dark red,ruby,rose,111
blue,ruby,water,333
dark red,sky,rose,555Multiple fields example.
# based on the first and third fields
# same as: ruby -e 'puts readlines.uniq {_1.split(",").values_at(0,2)}'
$ ruby -rset -F, -ane 'BEGIN{s=Set.new};
print if s.add?($F.values_at(0,2))' duplicates.txt
brown,toy,bread,42
dark red,ruby,rose,111
blue,ruby,water,333
yellow,toy,flower,333
white,sky,bread,111
light red,purse,rose,333In this section, how many times a duplicate record is found plays a role in determining the output.
First up, printing only a specific numbered duplicate. As seen before, Hash.new(0) will initialize the value of a new key to 0.
# print only the second occurrence of duplicates based on the second field
$ ruby -F, -ane 'BEGIN{h=Hash.new(0)};
print if (h[$F[1]]+=1)==2' duplicates.txt
blue,ruby,water,333
yellow,toy,flower,333
white,sky,bread,111
# print only the third occurrence of duplicates based on the last field
$ ruby -F, -ane 'BEGIN{h=Hash.new(0)};
print if (h[$F[-1]]+=1)==3' duplicates.txt
light red,purse,rose,333Next, printing only the last copy of duplicates. Since the count isn't known, the tac command comes in handy again.
# reverse the input line-wise, retain the first copy and then reverse again
$ tac duplicates.txt | ruby -rset -F, -ane 'BEGIN{s=Set.new};
print if s.add?($F[-1])' | tac
brown,toy,bread,42
dark red,sky,rose,555
white,sky,bread,111
light red,purse,rose,333To get all the records based on a duplicate count, you can pass the input file twice. Then use the two file processing trick to make decisions.
# all duplicates based on the last column
$ ruby -F, -ane 'BEGIN{h=Hash.new(0)}; ARGV.size==1 ? h[$F[-1]]+=1 :
h[$F[-1]]>1 && print' duplicates.txt duplicates.txt
dark red,ruby,rose,111
blue,ruby,water,333
yellow,toy,flower,333
white,sky,bread,111
light red,purse,rose,333
# all duplicates based on the last column, minimum 3 duplicates
$ ruby -F, -ane 'BEGIN{h=Hash.new(0)}; ARGV.size==1 ? h[$F[-1]]+=1 :
h[$F[-1]]>2 && print' duplicates.txt duplicates.txt
blue,ruby,water,333
yellow,toy,flower,333
light red,purse,rose,333
# only unique lines based on the third column
$ ruby -F, -ane 'BEGIN{h=Hash.new(0)}; ARGV.size==1 ? h[$F[2]]+=1 :
h[$F[2]]==1 && print' duplicates.txt duplicates.txt
blue,ruby,water,333
yellow,toy,flower,333This chapter showed how to work with duplicate contents for records and fields. If you don't need regexp based separators and if your input is too big to handle, then specialized command line tools like sort, uniq and huniq will be better suited.
1) Retain only the first copy of a line for the input file lines.txt. Case should be ignored while comparing the lines. For example, hi there and HI TheRE should be considered as duplicates.
$ cat lines.txt
Go There
come on
go there
---
2 apples and 5 mangoes
come on!
---
2 Apples
COME ON
##### add your solution here
Go There
come on
---
2 apples and 5 mangoes
come on!
2 Apples2) Retain only the first copy of a line for the input file twos.txt. Assume space as the field separator with exactly two fields per line. Compare the lines irrespective of the order of the fields. For example, hehe haha and haha hehe should be considered as duplicates.
$ cat twos.txt
hehe haha
door floor
haha hehe
6;8 3-4
true blue
hehe bebe
floor door
3-4 6;8
tru eblue
haha hehe
##### add your solution here
hehe haha
door floor
6;8 3-4
true blue
hehe bebe
tru eblue3) For the input file twos.txt, display only the unique lines. Assume space as the field separator with exactly two fields per line. Compare the lines irrespective of the order of the fields. For example, hehe haha and haha hehe should be considered as duplicates.
##### add your solution here
true blue
hehe bebe
tru eblueExamples in the previous chapters dealt with simple text formats separated by a newline or some other sequence of characters. Such data could be linearly read and processed. However, formats like json, csv, xml and html have a specific structure that require a custom parser before they can be made available for further processing. This chapter will give a brief overview of libraries to parse such structured inputs.
The JSON built-in module helps you to process json data by converting it to a Ruby hash object. See ruby-doc: JSON for documentation.
Here's an example of converting possibly minified json input to a pretty printed output.
$ s='{"greeting":"hi","marks":[78,62,93]}'
# ARGF.read is an alternate for -0777 to slurp the entire input
$ echo "$s" | ruby -rjson -e 'ip = JSON.parse(ARGF.read);
puts JSON.pretty_generate(ip)'
{
"greeting": "hi",
"marks": [
78,
62,
93
]
}You can create a shortcut to make it easier for one-liners.
# check if the shortcut is available
$ type rq
bash: type: rq: not found
# add this to your ~/.bashrc (or the file you use for aliases/functions)
$ rq() { ruby -rjson -e 'ip = JSON.parse(ARGF.read);'"$@" ; }
$ s='{"greeting":"hi","marks":[78,62,93]}'
# get value of the given key
$ echo "$s" | rq 'puts ip["greeting"]'
hi
# you can even pass options after the code snippet
$ echo "$s" | rq 'print ip["marks"]' -l
[78, 62, 93]
# use JSON.pretty_generate(ip) if you need pretty output
$ echo "$s" | rq 'ip["marks"][1] = 100; puts JSON.generate(ip)'
{"greeting":"hi","marks":[78,100,93]}Here's another example.
$ cat sample.json
{
"fruit": "apple",
"blue": ["toy", "flower", "sand stone"],
"light blue": ["flower", "sky", "water"],
"language": {
"natural": ["english", "hindi", "spanish"],
"programming": ["python", "kotlin", "ruby"]
},
"physics": 84
}
# process top-level keys not containing 'e'
$ rq 'ip.each {|k,v| puts "#{k}:#{v}" if !k.match?(/e/)}' sample.json
fruit:apple
physics:84
# process keys within the 'language' key that contain 't'
$ rq 'ip["language"].each {|k,v| puts "#{k}:#{v}" if k.match?(/t/)}' sample.json
natural:["english", "hindi", "spanish"]The CSV built-in class comes in handy for processing csv files. See ruby-doc: CSV for documentation.
Here's a simple example that parses the entire input string in one shot.
$ s='eagle,"fox,42",bee,frog\n1,2,3,4'
$ printf '%b' "$s" | ruby -rcsv -le 'ip=CSV.new(ARGF.read); print ip.read'
[["eagle", "fox,42", "bee", "frog"], ["1", "2", "3", "4"]]
$ printf '%b' "$s" | ruby -rcsv -e 'ip=CSV.new(ARGF.read); puts ip.read[0][1]'
fox,42Here's an example with embedded newline characters. This example directly uses the input filename instead of passing it as command line argument to the Ruby script.
$ cat newline.csv
apple,"1
2
3",good
guava,"32
54",nice
# this will parse the entire input in one shot
$ ruby -rcsv -le 'ip=CSV.read("newline.csv"); print ip'
[["apple", "1\n2\n3", "good"], ["guava", "32\n54", "nice"]]
# this is better suited for large inputs
$ ruby -rcsv -e 'CSV.foreach("newline.csv"){ |row|
puts row[2] if row[0]=~/pp/ }'
goodYou can change the field separator using the col_sep option.
$ ruby -rcsv -e 'CSV.foreach("marks.txt", :col_sep => "\t"){ |r|
puts r * "," if r[0]=="ECE" }'
ECE,Raj,53
ECE,Joel,72
ECE,Om,92The headers option will treat the first row as the header, useful for named field processing.
$ ruby -rcsv -e 'CSV.foreach("marks.txt", :headers => true, :col_sep => "\t"){
|r| puts r["Name"] }'
Raj
Joel
Moi
Surya
Tia
Om
AmyYou can automatically try to convert field value to given data types using the converters option. See ruby-doc: CSV Field Converters for details.
$ ruby -rcsv -le 'CSV.foreach("marks.txt", :converters => :integer,
:col_sep => "\t"){ print _1 }'
["Dept", "Name", "Marks"]
["ECE", "Raj", 53]
["ECE", "Joel", 72]
["EEE", "Moi", 68]
["CSE", "Surya", 81]
["EEE", "Tia", 59]
["ECE", "Om", 92]
["CSE", "Amy", 67]nokogiri is a popular third-party library to parse xml and html formats. It also supports working with malformed data.
To parse an input, you can either pass a string or pass a filehandle. You can also pass a URI filehandle for working with URLs directly.
Both XPath and CSS based selections are available. See also XPath introduction and difference between XPath and CSS Selectors. Here are some examples with xpath methods.
$ cat sample.xml
<doc>
<greeting type="ask">Hi there. How are you?</greeting>
<greeting type="reply">I am good.</greeting>
<color>
<blue>flower</blue>
<blue>sand stone</blue>
<light-blue>sky</light-blue>
<light-blue>water</light-blue>
</color>
</doc>
# all results for the 'blue' tag
# note that the ARGF filehandle is passed directly here
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.xpath("//blue")' sample.xml
<blue>flower</blue>
<blue>sand stone</blue>
# filtering based on the content of 'blue' tags
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.xpath("//blue").grep(/stone/)' sample.xml
<blue>sand stone</blue>
# use 'at_xpath' instead of 'xpath' to get only the first result
# or use 'ip.xpath("//blue")[0]' to get specific elements
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.at_xpath("//blue")' sample.xml
<blue>flower</blue>Here's an example with css methods. Use the text method to get the value of the selected tags.
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.css("light-blue").map(&:text)' sample.xml
sky
water
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.at_css("blue").text' sample.xml
flowerHere's an example of matching attributes.
# you can use //@type if you want all attributes that are named as 'type'
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.xpath("//greeting/@type")' sample.xml
ask
reply
# match a specific attribute value
# same as: ip.css("greeting[type=\"ask\"]")
$ ruby -rnokogiri -e 'ip=Nokogiri.XML(ARGF);
puts ip.xpath("//greeting[@type=\"ask\"]").text' sample.xml
Hi there. How are you?Here's an example with html input.
$ s='https://learnbyexample.github.io/substitution-with-ripgrep/'
$ url="$s" ruby -rnokogiri -ropen-uri -e 'ip=Nokogiri.HTML(URI.open(ENV["url"]));
puts ip.css("@href")[5..7]'
https://learnbyexample.github.io/books
https://learnbyexample.github.io/mini
https://learnbyexample.github.io/tipsThis chapter showed basic examples of processing structured data like json, csv, xml and html using built-in and third-party libraries. As mentioned in the introduction chapter, using Ruby one-liners for such tasks helps you avoid learning the syntax and idioms of a custom command line tool. Another advantage is that you have the entire ecosystem of a programming language at disposal once the structured input has been parsed. If performance becomes a concern, then custom CLI tools like jq, xsv and xmlstarlet will come in handy.
No exercises for this final chapter (author is lazy and doesn't have much experience with these formats). So, I'll leave you with links for further reading and as a source of exercises.