Suresh Govindachar wrote:
> >> 1) What does the following line do?
> >>
> >> eval 'exec C:\opt\perl\bin\perl.exe -S $0 ${1+"$@"}'
> >> if 0;
> >
> > That's just running perl against the bat file.
>
> But doesn't the if 0 condition mean that nothing happens?
> (The call to perl to run the bat file from the shebang line
> onwards happens with a dos-cmd line).
This is a bunch of convoluted code at the beginning of the bat file to get
perl to read the file regardless of what shell it's runining in. Read
perlrun man page for more info - esp on -S switch :
-S makes Perl use the PATH environment variable to search for the program
(unless the name of the program contains directory separators).
On some platforms, this also makes Perl append suffixes to the filename
while searching for it. For example, on Win32 platforms, the ".bat" and
".cmd" suffixes are appended if a lookup for the original name fails,
and if the name does not already end in one of those suffixes. If your
Perl was compiled with DEBUGGING turned on, using the -Dp switch to
Perl shows how the search progresses.
Typically this is used to emulate #! startup on platforms that don't
support #!. Its also convenient when debugging a script that uses #!,
and is thus normally found by the shell's $PATH search mechanism.
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. Other
systems can't control that, and need a totally devious construct that
will work under any of csh, sh, or Perl, such as the following:
eval '(exit $?0)' && eval 'exec perl -wS $0 ${1+"$@"}'
& eval 'exec /usr/bin/perl -wS $0 $argv:q'
if $running_under_some_shell;
If the filename supplied contains directory separators (i.e., is an
absolute or relative pathname), and if that file is not found,
platforms that append file extensions will do so and try to look for
the file with those extensions added, one by one.
On DOS-like platforms, if the program does not contain directory
separators, it will first be searched for in the current directory
before being searched for on the PATH. On Unix platforms, the program
will be searched for strictly on the PATH.
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