On Jun 21, 4:11 am, [EMAIL PROTECTED] (Gowthamgowtham) wrote:
> Hello All,
>
> I found this in Programming Perl - 3rd edition. Could not understand how
> this works.
>
> #!/bin/sh -- # perl, to stop looping
>
> eval 'exec /usr/bin/perl -S $0 ${1+"$@"}'
>
> if 0;
>
> Questions:
>
> - Who (I mean which program/shell) runs eval 'exec .'?
>
> - What is ${1+"$@"}, looks like this makes up the command line
> arguments to the script?
>
> - What is the purpose of 'if 0;' on a new line?
>
> 'perldoc perlrun' documents that -S switch makes perl search $PATH for the
> script.
perldoc perlrun also says this:
This example works on many platforms that have a shell
compatible with Bourne shell:
#!/usr/bin/perl
eval 'exec /usr/bin/perl -wS $0 ${1+"$@"}'
if $running_under_some_shell;
The system ignores the first line and feeds the program
to /bin/sh, which proceeds to try to execute the Perl
program as a shell script. The shell executes the
second line as a normal shell command, and thus starts
up the Perl interpreter. On some systems $0 doesn't
always contain the full pathname, so the -S tells Perl
to search for the program if necessary. After Perl
locates the program, it parses the lines and ignores
them because the variable $running_under_some_shell is
never true. If the program will be interpreted by csh,
you will need to replace "${1+"$@"}" with $*, even
though that doesn't understand embedded spaces (and
such) in the argument list. To start up sh rather than
csh, some systems may have to replace the #! line with
a line containing just a colon, which will be politely
ignored by Perl.
--
Brad
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