how does qx select a "/bin/sh equivalent"?

bronto has asked for the wisdom of the Perl Monks concerning the following question:

Dearest monks and nuns.

Recently I have been bitten by an unexpected behaviour of a commercial software. In short, if I started a daemon with qx in a perl script, it failed to work properly.

I am still investigating. Anyway, it seems that the daemon fails to work properly if it doesn't run under a "classic" Bourne shell, while qx runs it inside a Bash shell.

The following script:

#!/usr/bin/perl

use strict ;
use warnings ;

my $output = qx'echo $SHELL' ;

print $output ;
[download]

returns

/bin/bash

The man page for qx says:

      qx/STRING/
      ‘STRING‘
              A string which is (possibly) interpolated and then exe-
              cuted as a system command with "/bin/sh" or its equiva-
              lent.[...]
[download]

This leads me to the question: how does qx select the aforementioned "/bin/sh equivalent"?

Ciao!
--bronto

In theory, there is no difference between theory and practice. In practice, there is.

Comment on how does qx select a "/bin/sh equivalent"? Select or Download Code

Replies are listed 'Best First'.
Re: how does qx select a "/bin/sh equivalent"? by almut (Canon) on Jun 10, 2008 at 15:37 UTC
Looking at `$SHELL` isn't necessarily a reliable way to determine what shell variant is being used. As `strace` shows, Perl is doing an `execve("/bin/sh", ["sh", "-c", "echo $SHELL"] ...` (at least on Linux), and that's essentially all it can do... Other than that, it's the shell's job to behave like a classic bourne shell when being called as `/bin/sh` (which `bash` actually claims to do...).	[reply] [d/l] [select]
Re: how does qx select a "/bin/sh equivalent"? by ikegami (Patriarch) on Jun 10, 2008 at 16:20 UTC
To see what Perl uses, `$ perl -le'use Config; print $Config{sh}' /bin/sh` [download] But note that on many systems, `/bin/sh` is bash. `$ /bin/sh -c 'echo $SHELL' /bin/bash` [download] If you want to use an alternate shell, `sub backticks_mysh { open(my $pipe, '-\|', '/bin/mysh', '-c', $_[0]) or return; local $/ = wantarray ? $/ : undef; <$pipe> }` [download]	[reply] [d/l] [select]
Re^2: how does qx select a "/bin/sh equivalent"? by gnujsa (Acolyte) on Jul 30, 2012 at 00:40 UTC
Ok, it was some time ago, but I do not think that the $SHELL variable is reliable. look at my `$ /bin/sh -c 'echo $SHELL' /bin/bash` [download] although: $ file /bin/sh /bin/sh: symbolic link to `dash' [download] and, `$ file /bin/dash /bin/dash: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), d +ynamically linked (uses shared libs), for GNU/Linux 2.6.26, BuildID[s +ha1]=0x890a514bd261794a6d39da58ea2372fdf48e98d6, stripped` [download] I don't really know why. Maybe DASH keeps the environment of the calling shell, but I do not trust $SHELL.	[reply] [d/l] [select]
Re^3: how does qx select a "/bin/sh equivalent"? by ikegami (Patriarch) on Sep 17, 2012 at 05:25 UTC
I used `$SHELL` to demonstrate that `/bin/sh` is not always the bourne shell. I don't see how your reply relates.	[reply] [d/l] [select]
Re: how does qx select a "/bin/sh equivalent"? by pc88mxer (Vicar) on Jun 10, 2008 at 15:51 UTC
This leads me to the question: how does qx select the aforementioned "/bin/sh equivalent"? The shell that is used is determined at the time that the perl executable was compiled. On Unix-like systems it will probably be `/bin/sh`, but on other platforms (OS2, Win32, VMS) it could be very different. Also, the shell is not guaranteed to be "equivalent" to `/bin/sh`. On non-Unix systems perhaps "counterpart" is a better way to describe it.	[reply] [d/l] [select]

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