jdarpinian/C and shell polyglot.cmd

The C polyglot builds and runs itself on all platforms. But without a shebang line, the kernel doesn't see it as executable. So while you can execute it from a bash prompt you will not be able to invoke it with e.g. execve. The shebang version fixes this, but is no longer valid C as a result, only bash and cmd.

In the shell/cmd polyglot, I do not like the terminal escape sequences very much as the output may not be a terminal. I needed to look them up to actually understand what the script does.

I came up with a way to fix the script even when the output is not a terminal, but I needed to sacrifice the shebang -- in cmd, the shebang generates an error and there is no clean way around that. I also evaded several Bash-specific shell features; still, I did not manage to make it a POSIX shell script due to the need to create a function whose name is at-sign.

:; @() { true; } # Polyglot of shell and cmd
@ goto startbatch
# Start your shell script here

echo running in shell

# End your shell script here
exit
:startbatch
:: Start your cmd script here

@echo off
echo running in cmd

:: End your cmd script here
exit /B

I am thinking of using this boilerplate to implement cross-platform pipelines in Azure DevOps Server using the script task. I like it more than having separate tasks with a condition on Agent.OS as all the tasks are visible in the pipeline run, regardless of the condition. The documentation talks about using the script task with polyglot scripts, but it uses the simplest echo as an example. When I need anything even slightly more complicated, e.g. print a hash character as part of a logging command, I am out of luck with simple tweaks of the syntax (Bash treats # as start of comment, cmd outputs quotes literally, and there is no shared escaping mechanism between the two).

This polyshell is interesting too... Though even more convoluted.
https://github.com/llamasoft/polyshell

@TomasMalecek there's actually no reason to bother making a dummy function for @ or anything, just redirect errors, and no more errors!

#!/usr/bin/env -S 2>/dev/null=2>NUL sh
@goto batch 2>NUL;rm -f NUL

echo running in sh script
echo "you live in ${HOME}"
errors-still-work-fine
exit

:batch
@echo off
echo running in cmd script
echo you live in %USERPROFILE%
errors-still-work-fine
exit /B

the script itself should be posix, but it requires /bin/env that supports -S for the shebang to work (gnu one works, which is default in most places, but it's not posix, and busybox and toybox's env do not work)
or you can just run it directly using dash/ash/bash or whatever other bourne-style shell you like, ignoring the shebang entirely ;P

I've tested my version with both windows XP, and Wine's cmd
the 2>/dev/null=2>NUL in the shebang is making sure that both wine and real cmd will not error on the shebang, it doesn't affect anything else, and it's also why you need -S
if you don't care about shebang erroring on windows you can remove that and use a normal #!/bin/sh or #!/bin/bash