The long line of eval at the beginning of a Perl script

Question

I am relatively new to Perl and working on Perl files written by someone else, and I meet the following statement at the beginning of the scripts:

eval '(exit $?0)' && eval 'exec perl -w -S $0 ${1+" $@ "}' && eval 'exec perl -w -S $0 $argv:q' if 0; 

What do these two lines do? What is code verification? and what makes the if 0; ?

Answer 1

This is a variant of hacking exec. In those days when interpreters could be reliably specified with #! , this was used to create the exec perl shell. if 0 on the second line is never read by the shell, which reads only the first line, and execs perl, which reads if 0 and does not re-execute.

This is an interesting option, but I think it’s not quite right. It seems to be configured to work with the bourne shell or with the csh variants, using an initial evaluation to define a shell that parses it, and then using the appropriate syntax to pass perl arguments. The middle sentence is sh syntax, and the last sentence is suitable for csh. If the second && was || , and the initial eval '(exit $?0)' really failed in csh, then it would fulfill these goals, but as it is written, I do not think this works for csh. Is there a preceding command that set $? to some shell-based value? But even if that were the case, and $? set to a nonzero value, then nothing will be done if && not replaced by || . Something funny is happening.