git-ignore provides a command line interface for adding entries to a project's .gitignore file.
cd anywhere within the working tree of the repository whose .gitignore you'd like to modify and run
git-ignore "pattern1" [pattern2] [pattern3] ... [-m "Optional message"]
Each specified pattern is appended to .gitignore. For example,
git-ignore *.o *.a *.so -m "Ignore build files"
adds the following to .gitignore:
# Ignore build files
*.o
*.a
*.so
If you move git-ignore to some directory in your $PATH (perhaps /usr/bin), you can use git ignore in place of git-ignore:
git ignore *.o *.a *.so -m "Ignore build files"