Environment variables in the config files not supported

[kuyagic]

Environment variables in the config files is not properly expanded on windows
I set the following

[backup]
exclude-if-present = [".nobackup", "CACHEDIR.TAG"]
iglob-file = ["C:/ProgramData/rustic/ignore.iglob.list"]

this is working.
but both %PROGRAMDATA%/rustic/ignore.iglob.list and %PROGRAMDATA%\\rustic\\ignore.iglob.list not working.

Variables are not expanded. This affects both Windows and *Unix

[aawsome]

I marked this as enhancement wish as currently there is no substitution of variables in the config file. See also #996

[aawsome]

This feature is a bit more involved as there are more questions arising:

• Do we only want variable substitution in filenames or elsewhere in the config?
• Do we want to allow substition only within strings?
• Do we want to first parse the TOML, then substitute what is appropriate or first substitute and then parse (which would allow to define TOML structures within env variables)?

[simonsan]

There is this library https://crates.io/crates/expand_str, which is an alternative to https://docs.rs/crate/shellexpand/3.1.0 which is useful for tilde expansion.

[aawsome]

I would suggest to start with expanding env variables where shellexpand is also a good candidate. I would actually keep it really simple and just expand everything before we parse the toml.
There is the rustic show-config command which is able to show which config rustic is using, so enough possibility for users to debug any strange behavior.