Back-ticks and bash

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

This is a puzzle of a cross-domain fashion in multiple environments.
Problem: an Perl installation script that installs a Perl program that auto-starts at boot, and can be started or stopped from the command line. In any release of RHEL, CentOS, Debian, Ubuntu, or Kali.

I have found a bash script that seems to reliably identify 'system5-init', 'upstart', and 'systemd' as foundations for process control. Running the script in back-ticks has interpretation problems. Using 'system' will only return the exit value, not the STDOUT (which is possibly usable, but rather opaque).

#create the bash script to determine whic Init System
  my $tmpScript = "/tmp/whichSysInit";
  my $whichInit = <<"END";
      #!/bin/bash
      if [[ `/sbin/init --version 2>/dev/null` =~ upstart ]]; then 
          echo using upstart;
      elif [[ `systemctl` =~ -\.mount ]]; then 
          echo using systemd;
      elif [[ -f /etc/init.d/cron && ! -h /etc/init.d/cron ]]; then 
          echo using sysv-init;
      else echo cannot tell; 
      fi

END

  open  OPT, "> $tmpScript";
  print OPT $whichInit;
  close OPT;
  chmod 0555, $tmpScript;

  #run that script to get answer
  my $rslt= `$tmpScript`;
  #unlink $tmpScript; # cleanup

  print " Initialization is $rslt\n";

  #react to the result
  #if ($rslt =~/using\s(?<si>\S+)/){
  #  my $sysInit= $+{si};
[download]

When I run the file left in /tmp/ from the command line, it echos the appropriate string.
When run the file in back-ticks the following errors are produced:

/tmp/whichSysInit: 2: /tmp/whichSysInit: [[: not found
/tmp/whichSysInit: 1: /tmp/whichSysInit: systemctl: not found
/tmp/whichSysInit: 4: /tmp/whichSysInit: [[: not found
/tmp/whichSysInit: 6: /tmp/whichSysInit: [[: not found
Can not determine System Init mechanism <>.
 Exiting
[download]

I searched for the program '[[', but none was found.
Linux ubuntu 3.11.0-26-generic #45~precise1-Ubuntu SMP Tue Jul 15 04:02:35 UTC 2014 x86_64 x86_64 x86_64 GNU/Linux

The shell in the environmant is '/bin/bash'
Maybe the '#!' on line 1 isn't run? And bash isn't what is interpreting the script?
Thoughts?

It is always better to have seen your target for yourself, rather than depend upon someone else's description.

Comment on Back-ticks and bash Select or Download Code

Replies are listed 'Best First'.
Re: Back-ticks and bash by haukex (Archbishop) on Oct 08, 2016 at 16:08 UTC
Hi Wiggins, Maybe the '#!' on line 1 isn't run? That seems to be the case because the line doesn't begin with `#!`, it begins with the whitespace of the indentation. AFAIK Perl uses `/bin/sh` to execute backticks, and so it seems your shell script is run with `/bin/sh` instead of bash, and I guess sh doesn't have the `[[` builtin. Remove the indentation and your code works for me. However, why use a bash script at all, when you could just do all the stuff your bash script is doing in Perl? If you want to capture external commands' STDOUT and STDERR, I'd suggest something like IPC::Run3 or Capture::Tiny. Hope this helps, -- Hauke D	[reply] [d/l] [select]
Re^2: Back-ticks and bash by Wiggins (Hermit) on Oct 09, 2016 at 13:32 UTC
Yes, this is what it takes. The leading space breaks the rule. Thanks, it was right there in front of me! It is always better to have seen your target for yourself, rather than depend upon someone else's description.	[reply]
Re: Back-ticks and bash by shmem (Chancellor) on Oct 08, 2016 at 20:04 UTC
And bash isn't what is interpreting the script? Yes. Init scripts are always interpreted by `/bin/sh`, even if the default $SHELL is /bin/bash. `qwurx [shmem] ~> /bin/bash -c 'if [[ 1 -eq 1 ]]; then echo yup; fi' yup qwurx [shmem] ~> /bin/sh -c 'if [[ 1 -eq 1 ]]; then echo yup; fi' /bin/sh: 1: [[: not found` [download] update: not sure what might befall within the systemd usurpation... sigh perl -le'print map{pack c,($-++?1:13)+ord}split//,ESEL'	[reply] [d/l] [select]
Re: Back-ticks and bash by hippo (Archbishop) on Oct 08, 2016 at 16:10 UTC
Hmmm. Is it not the case that the leading whitespace prevents the shebang from being recognised? At that point it will be run by whatever the O/S chooses, which is probably not bash. I'm not entirely sure that I see the point of making the tests in a bash script anyway. Why not perform the tests directly inside your perl script? Surely that would be less hassle.	[reply]
Re: Back-ticks and bash by kcott (Archbishop) on Oct 09, 2016 at 06:14 UTC
G'day Wiggins, "I searched for the program '[[', but none was found." It's part of the shell syntax (see the bash manpage). It's used in conjunction with '`]]`'. Both are reserved words. Searching in the manpage finds, among others: `RESERVED WORDS ... The following words are recognized as reserved ... [[ + ]]` [download] and, later `SHELL GRAMMAR ... [[ expression ]] ... the evaluation of the conditional expression expr +ession. ... ...` [download] — Ken	[reply] [d/l] [select]
Re^2: Back-ticks and bash by Wiggins (Hermit) on Oct 09, 2016 at 13:29 UTC
What I remenbered was /usr/bin/[ which is an external program used for doing tests I believe, and that is still there. I always find bash syntax confusing and obscure. It is always better to have seen your target for yourself, rather than depend upon someone else's description.	[reply]
Re^3: Back-ticks and bash by choroba (Cardinal) on Oct 09, 2016 at 20:10 UTC
`[` in bash is a synomym for `test` but it's not still a builtin. For backward compatibility, it behaves similarly to `/usr/bin/[` , which means variables among its arguments are still expanded by the shell, which is not true for `[[` . In other words, you have to write things like `if [ "$x" = abc ]` [download] i.e. you need to quote the variables, while with `[[` you don't have to, as the shell processes the parameters before expansion: `if [[ $x = abc ]]` [download] Also, `[[` introduces pattern matching `[[ $x = ab* ]]` and `=~` you might now from Perl. ($q=q:Sq=~/;[c](.)(.)/;chr(-\|\|-\|5+lengthSq)`"S\|oS2"`map{chr \|+ord }map{substrSq`S_+\|`\|}3E\|-\|`7**2-3:)=~y+S\|`+$1,++print+eval$q,q,a, [download]	[reply] [d/l] [select]