Question

Why is this exists check returning different results for pwd versus the other examples? What’s happening here?

[me@unixbox1:~/perltests]> cat testopensimple.pl
#!/usr/bin/perl

use strict;
use warnings;

# |<command> for writing
# <command>| for reading

testopen("| pwd");
testopen("pwd |");
testopen("| hostname");
testopen("| cd");
testopen("| sh");
testopen("| sleep 2");

sub testopen {
        my $command = shift;

        print "Exists: " . (-e $command ? "true" : "false") . "\n";
        print "testopen($command)\n";
        eval {
                open(my $fh, $command) or die "$! $@";
                my $data = join('', <$fh>);
                close($fh) or die "$! $@";
                print $data . "\n";
        };
        if ($@) {
                print $@ . "\n";
        }
}

[me@unixbox1:~/perltests]> perl testopensimple.pl
Exists: true
testopen(| pwd)
/home/me/perltests

Exists: true
testopen(pwd |)
/home/me/perltests

Exists: false
testopen(| hostname)
unixbox1

Exists: false
testopen(| cd)

Exists: false
testopen(| sh)

Exists: false
testopen(| sleep 2)

Update:

I’m not convinced that it’s a shell built-in vs external command issue. Try it with a command like netstat which is external. pwd is the only command I’ve found thus far that returns true on the exists check with pipes.

Update 2:

Through the several iterations of testing I was doing, files named ‘| pwd’ and ‘pwd |’ ended up being created. That explains what I’m seeing. Thanks.

Answer 1

-e '| pwd' will only return true if you have a file named | pwd in the current directory.

-e 'pwd |' will only return true if you have a file named pwd | in the current directory.

$ perl -E'say -e "| pwd" ? 1 : 0'
0

$ touch '| pwd'

$ perl -E'say -e "| pwd" ? 1 : 0'
1

$ perl -E'say -e "pwd |" ? 1 : 0'
0

$ touch 'pwd |'

$ perl -E'say -e "pwd |" ? 1 : 0'
1