问题:

What exacly do the following?

#! /usr/bin/perl -w
    eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
        if 0; #$running_under_some_shell

the if 0 is never true, so the eval part will never executed,
and the eval is strange too - what is the value of $0 in this context (inside single quotes?)

Ps: taken from the result of the find2perl command

回答1:

Best guess - as in this comment #$running_under_some_shell, it's to detect if the script is being run by some shell other than perl, e.g. bash.

the if 0 is never true, so the eval part will never executed,

Not by perl, no. By other shells such as bash it won't spot the line continuation and will just execute the eval statement. This then re-runs the script under perl. (Oddly with different options than the hashbang line.)

and the eval is strange too - what is the value of $0 in this context (inside single quotes?)

Again, this will be expanded by bash not perl: here it means the path to find2perl to pass into the perl interpreter.

回答2:

I found some discussion here: http://www.perlmonks.org/?node_id=825147

The extended hashbang is there so you can run your Perl script with almost any /bin/sh under the sun, even a shell/kernel that does not honor the hashbang and it will still launch perl in the end.