wsl.exe sets first argument as SHELL breaking posix compatibility and gdb

[throwable-one]

Windows Version

Microsoft Windows [Version 10.0.19045.3570]

WSL Version

1.2.5.0

Are you using WSL 1 or WSL 2?

• WSL 2
• WSL 1

Kernel Version

Linux version 5.15.90.1-microsoft-standard-WSL2 (oe-user@oe-host) (x86_64-msft-linux-gcc (GCC) 9.3.0, GNU ld (GNU Binutils) 2.34.0.20200220) #1 SMP Fri Jan 27 02:56:13 UTC 2023

Distro Version

Ubuntu 20.04

Other Software

python3
gdb

Repro Steps

wsl.exe --distribution Ubuntu-20.04 --exec "python3" -c "import os; print(os.environ['SHELL'])"
python3

The output is python, but according to POSIX SHELL variable should point to the valid language interpreter.
https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html

SHELL
This variable shall represent a pathname of the user's preferred command language interpreter. If this interpreter does not conform to the Shell Command Language in XCU Shell Command Language, utilities may behave differently from those described in POSIX.1-2017.

That is a problem.
gdb uses SHELL var to execute commands:
https://ftp.gnu.org/old-gnu/Manuals/gdb/html_node/gdb_19.html#SEC20

Warning: On Unix systems, GDB runs your program using the shell indicated by your SHELL environment variable if it exists (or /bin/sh if not)

As result. gdb doesn't work:

c:\>wsl.exe --distribution Ubuntu-20.04 --exec "/bin/gdb"
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word".
(gdb) file /bin/bash
Reading symbols from /bin/bash...
(No debugging symbols found in /bin/bash)
(gdb) run
Starting program: /usr/bin/bash
GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
Copyright (C) 2020 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Type "show copying" and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
Type "show configuration" for configuration details.
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>.
Find the GDB manual and other documentation resources online at:
    <http://www.gnu.org/software/gdb/documentation/>.

For help, type "help".
Type "apropos word" to search for commands related to "word".
/mnt/c/exec /usr/bin/bash : No such file or directory.
(gdb)

Expected Behavior

SHELL shouldn't be set at all or point to valid shell

Actual Behavior

SHELL points to the first argument

Diagnostic Logs

No response

[zcobol]

@throwable-one why not use -- instead of --exec.

From wsl.exe help:

Arguments for running Linux binaries:

    If no command line is provided, wsl.exe launches the default shell.

    --exec, -e <CommandLine>
        Execute the specified command without using the default Linux shell.

    --shell-type <standard|login|none>
        Execute the specified command with the provided shell type.

    --
        Pass the remaining command line as-is.

You'll notice that the environment is not the same, depending on invocation method:

PS C:\Users\zcobol> wsl.exe --exec python3 -c "import os; print(os.environ['SHELL'])"
python3

PS C:\Users\zcobol> wsl.exe -- python3 -c "import os; print(os.environ['SHELL'])"
/bin/bash

Use print(os.environ) to see the all differences.

[benhillis]

Fixed with https://github.com/microsoft/WSL/releases/tag/2.0.8.

[asampal]

It seems wsl is doing the right thing when using --exec, as @zcobol pointed out. Why was a fix needed?

[throwable-one]

@zcobol But WSL would use shell then, not my bare command.

@asampal There is nothing in WSL documentation that says I can't use arbitrary command with --exec, hence --exec python is valid usage. But POSIX claims explicitly that SHELL variable should point to a shell.

@benhillis thank you for this fix