In fish I want to set an environment variable to a non-utf8 value like \x80 (the character sequence of length one with the value 0x80, i.e., 128). I already tried
set -x a \x80
fish: Invalid token '\x80'
set -x a \x80
^
and
set -x a (string unescape '\x80')
(the latter not creating an error message, but a is set to the empty string, $status is 1)
Linux does not specify a specific encoding for environment variable values, they are just arbitrary null-terminated uint8 strings. So there must be some way to set them in a shell, because the shell cannot not know how other applications are going to decode the values.
In bash and zsh it is easy:
export a=$'\x80'
I want to do this for two reasons:
Many of today's file systems still allow non-unicode file names. Consider one has mounted a read-only file system with some directory named some_dir\x80 and one wants to add this directory to PATH (or any other environment variable).
imagine I want to use an old version of a software that uses latin1 encoding to interpret certain environment variable values and cannot be changed to use unicode.
For example, my family name is "Döbler" (German name), $'D\xf6bler' in latin-1 encoding (as a unix-quote bash literal). This sequence of bytes is not a legal utf-8 string. So I wonder: How can I hand over my family name as latin-1 encoded environment variable value to that software while using fish as shell?
The \x sequence used to check if the given bytes were valid ascii, while the \X (that's an uppercase "X") sequence did not.
This was changed in https://github.com/fish-shell/fish-shell/pull/9247, released in fish 3.6.0 in January 2023. Now \x is the same as \X.
Either upgrade your fish installation (you're at least 20% of all commits ever made to fish behind the current release) or use \X.
After that, use set --show foo to show the value of the variable called "foo".