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From: mayer@sono.uucp (Ronald &)
Newsgroups: comp.lang.perl
Subject: `...` and open(F,'...|') VS system(...) with nonexistant programs
Message-ID: <MAYER.91May24152103@porky.sono.uucp>
Date: 24 May 91 22:21:03 GMT
Sender: mayer@sono.uucp (Ronald Mayer)
Organization: Acuson; Mountain View, California
Lines: 10


When attempting to execute unix program from perl, I'm unable to both
    1) Determine if the program was actually executed
and
    2) Get the output from the program into a perl string

As shown below, the only ways I know of getting the output of a unix
program into a perl string, `...` and open(F,'...|'), don't seem to be
able to distinguish between a program with no output which returns
false and a program which doesn't exist.  This is in contrast to
perl's 'system(...)', which returns a different value for nonexistant
programs.

#! /usr/local/bin/perl
### Unix programs: 'true' returns 0; 'false' returns 1; 'undef' doesnt exist.
`true`, print "$?\n";       # 0
`false`, print "$?\n";      # 256
`undef`, print "$?\n";      # 256 (how do I distinguish this from `false`?)

open(F,'true|') || die; <F>; close(F); print "$?\n"; # 0
open(F,'false|')|| die; <F>; close(F); print "$?\n"; # 256
open(F,'undef|')|| die; <F>; close(F); print "$?\n"; # 256, same question...

print system('true'),"\n";  # 0
print system('false'),"\n"; # 256
print system('undef'),"\n"; # 65280 easily distinguishable from system('false')


Does anyone know why? (I kinda assume that '``' and 'open' are using
'sh' or 'csh', which return the same value when executing 'false' as
they do when they execute a nonexistant program.  Is this correct?)

Does anyone know of a workaround?  (Other than parsing `which filename`?)

        Ron Mayer