Can I have a conditional shebang?

I have a script that can be run with different interpreters:

#!/usr/bin/env default-interpreter
[my script]

But I would like to use an alternative-interpreter if it's available, like this:

#!/usr/bin/env alternative-interpreter
[my script]

Is there a way to create a shebang that looks for alternative-interpreter and falls back to default-interpreter in case the first one isn't available?

Solution 1:

Not directly, no. Best to write a wrapper Bourne shell script and shebang that:

#!/path/to/my/wrapper

and the wrapper starts with:

#!/bin/sh
for shell in first second third; do
    if /usr/bin/env "${shell}" "$@"; do exit $?; done
done
# We didn't find any of them.
exit 1

This lets env(1) use the ${PATH} search list to locate the programs, in the order given in the for-loop.