Re: 'exec -a' and $0 substitution

[alex xmb ratchev]

root@localhost:~# unset -v z ; cat srcpath ; bash srcpath ; ./srcpath ;
"${z=$PWD/srcpath}" ; bash "$z"
[[ ${z=$BASH_SOURCE} == /* ]] && t=$z || t=$PWD/$z
printf %s\\n "$t"
/root/srcpath
/root/./srcpath
/root/srcpath
/root/srcpath

On Thu, Jan 26, 2023, 12:51 AM Sergei Trofimovich <slyich@gmail.com> wrote:

> Hello bash maintainers! nixpkgs package collection likes to wrap binaies
> and shell scripts to pass extra environment variables via wrappers.
>
> One example is dejagnu's runtest:
>
>     $ cat `which runtest` | unnix
>     #! /<<NIX>>/bash-5.2-p15/bin/bash -e
>     PATH=...
>     export PATH
>     exec -a "$0" "/<<NIX>>/dejagnu-1.6.3/bin/.runtest-wrapped"  "$@"
>
> Script's idea is to call .runtest-wrapped as if it had a 'runtest' name.
> It's especially important for `runtest` because it internally checks for
> own $0:
>
>     mypath=${0-.}
>     ...
>     if [ "$target" != runtest ] ; then
>         target="--target ${target}"
>     else
>         target=""
>     fi
>
> To the bug: looks like 'exec -a' does not work for bash scripts, but
> does work for other executables. An example:
>
>   Wrap C executable:
>
>     $ cat b.bash
>     #!/usr/bin/env bash
>
>     exec -a "$0" $PWD/a "$@"
>
>     $ cat a.c
>     #include <stdio.h>
>
>     int main(int argc, char * argv[])
>     {
>         for (int i = 0; i < argc; ++i) {
>             printf ("A[%i]='%s'\n", i, argv[i]);
>         }
>     }
>     $ gcc a.c -o a
>
>   Result:
>
>     $ bash -x ./b.bash 1 2 3
>     + exec -a ./b.bash /home/slyfox/bugs/a 1 2 3
>     A[0]='./b.bash'
>     A[1]='1'
>     A[2]='2'
>     A[3]='3'
>
>   Works as expected: './a' sees './b.bash' as an argv[0].
>
>   Wrap 'bash' script:
>
>     $ cat a.bash
>     #!/usr/bin/env bash
>
>     exec -a "$0" $PWD/foo "$@"
>
>     $ cat foo
>     #!/usr/bin/env bash
>
>     echo "$0" "$@"
>
>   Result:
>
>     $ bash -x ./a.bash 1 2 3
>     + exec -a ./a.bash /home/slyfox/bugs/foo 1 2 3
>     /home/slyfox/bugs/foo 1 2 3
>
>   For some reason '-a ./a.bash' did not take an effect.
>
> Such a discrepancy makes these wrappers to break occasionally like in
> https://sourceware.org/PR30052.
>
> I fear it's a side-effect of the way 'bash' gets executed via shebang by
> kernel. But maybe not? Somehow direct script execution still manages to
> preserve script's name. Is it an intended behaviour that could not be
> easily changed? Or it's a bug?
>
> Thank you!
>
> --
>
>   Sergei
>
>