This is the last remnant of the old percent expansion.
It has the downsides of it, in that it is annoying to combine with
anything:
```fish
echo %self/foo
```
prints "%self/foo", not fish's pid.
We have introduced $fish_pid in 3.0, which is much easier to use -
just like a variable, because it is one.
If you need backwards-compatibility for < 3.0, you can use the
following shim:
```fish
set -q fish_pid
or set -g fish_pid %self
```
So we introduce a feature-flag called "remove-percent-self" to turn it
off.
"%self" will simply not be special, e.g. `echo %self` will print
"%self".
Issue #10194 reports Cobra completions do
set -l args (commandline -opc)
eval $args[1] __complete $args[2..] (commandline -ct | string escape)
The intent behind "eval" is to expand variables and tildes in "$args".
Fair enough. Several of our own completions do the same, see the next commit.
The problem with "commandline -o" + "eval" is that the former already
removes quotes that are relevant for "eval". This becomes a problem if $args
contains quoted () or {}, for example this command will wrongly execute a
command substituion:
git --work-tree='(launch-missiles)' <TAB>
It is possible to escape the string the tokens before running eval, but
then there will be no expansion of variables etc. The problem is that
"commandline -o" only unescapes tokens so they end up in a weird state
somewhere in-between what the user typed and the expanded version.
Remove the need for "eval" by introducing "commandline -x" which expands
things like variables and braces. This enables custom completion scripts to
be aware of shell variables without eval, see the added test for completions
to "make -C $var/some/dir ".
This means that essentially all third party scripts should migrate from
"commandline -o" to "commandline -x". For example
set -l tokens
if commandline -x >/dev/null 2>&1
set tokens (commandline -xpc)
else
set tokens (commandline -opc)
end
Since this is mainly used for completions, the expansion skips command
substitutions. They are passed through as-is (instead of cancelling or
expanding to nothing) to make custom completion scripts work reasonably well
in the common case. Of course there are cases where we would want to expand
command substitutions here, so I'm not sure.
