Perl newbie question about system command

Guttorm · 06-30-2011, 06:23 AM

Hi

I am trying to fix a perl script, and I really suck at perl. But I think this problem will be easy for people who know it.

The problem is, I have an old setup script someone wrote many years ago. It fails if the standard shell is dash and not bash. The only way I've gotten it to work is to point /bin/sh to bash. I looked thru the script and it uses "system" many places, and I think that's the problem.

I searched for it and found this link:

http://stackoverflow.com/questions/5...n-perls-system

My plan is to include this function:

Code:

sub system_bash {
  my @args = ( "bash", "-c", shift );
  system(@args);
}

Then I could simply change all calls to system into system_bash and it should work?

The parameter to the system calls is usually some variable. What if the parameter is a list already? Do I need to test for it somehow, and if it's a list, prepend "bash" and "-c" to the list? How do I do that?

In the script there are lots of places like this:

my $error = system($cmd);
if ($error) {
die/warn "some error message";
}

Shouldn't there be a return in the system_bash function?

theNbomr · 06-30-2011, 09:29 AM

My first instinct would be to stop using 'system()' if it has any dependency on a particular shell, and since anything you can do with Bash you can probably do easier & better with Perl. If it needs to call other stand-alone applications, these can be invoked from Perl in any number of ways.
I haven't tried this, but you may be able to coerce Perl to use Bash by default in system() calls, simply by setting the $SHELL environment variable to "bash".

--- rod.

Guttorm · 06-30-2011, 10:01 AM

The problem is that the script is long and complicated - reading lots of config files and things. The person who wrote it was fairly good at Perl, so I figured he didn't use system if it was uneccessary. But when he wrote it, there was no dash, and as I said, if bash is standard shell, it works fine.

Setting $SHELL in the script doesn't seem to work. I tried this code:

Code:

$SHELL = "bash";
my $cmd = 'for (( i=1 ; i<=10 ; i++ )) ; do echo $i ; done';
system($cmd);

This too works fine if /bin/sh points to bash, but not when it's dash. Or does Perl need something like export? It looks like "/bin/sh" is hard-coded in Perl somewhere.

theNbomr · 06-30-2011, 10:29 AM

Quote:

Originally Posted by Guttorm

This too works fine if /bin/sh points to bash, but not when it's dash. Or does Perl need something like export? It looks like "/bin/sh" is hard-coded in Perl somewhere.

Yes, it looks like it. I just tried invoking perl from a csh commandline, with $SHELL set to 'csh', and it still invoked dash (Debian). Sorry to have sent you on a tangent.

Incidently, setting environment variables in Perl is done by writing to the %ENV hash.

Code:

$ENV{"ABCDEF"}='qwerty';

--- rod.

Sergei Steshenko · 06-30-2011, 11:07 AM

Quote:

Originally Posted by Guttorm

...
Then I could simply change all calls to system into system_bash and it should work?
...

Why do you think so ? I.e. suppose you have a 'foo.sh' script which somewhere inside has

Code:

...
/path/to/bar.sh SOME_ARGS
...

, and 'bar.sh' begins with typical

Code:

#!/bin/sh

, but still expects 'bash' and not 'dash'. So, regardless of what you do in Perl, 'bar.sh' won't work because your '/bin/sh' is 'dash' and not 'bash'.

Sergei Steshenko · 06-30-2011, 11:11 AM

Quote:

Originally Posted by Guttorm

The problem is that the script is long and complicated - reading lots of config files and things. The person who wrote it was fairly good at Perl, so I figured he didn't use system if it was uneccessary. But when he wrote it, there was no dash, and as I said, if bash is standard shell, it works fine.

Setting $SHELL in the script doesn't seem to work. I tried this code:

Code:

$SHELL = "bash";
my $cmd = 'for (( i=1 ; i<=10 ; i++ )) ; do echo $i ; done';
system($cmd);

This too works fine if /bin/sh points to bash, but not when it's dash. Or does Perl need something like export? It looks like "/bin/sh" is hard-coded in Perl somewhere.

First of all, you are doing it wrong - see a post above about '%ENV'; secondly, according to my experiments, Perl ignores $SHELL - it looks like '/bin/sh' is hard-coded into it. I am not even sure this can be changed when Perl is built from sources, but one can find '/bin/sh' in whatever Perl source files and replace it with desired path to shell.

Guttorm · 06-30-2011, 12:17 PM

Good point Sergei. I grepped a bit in the code without finding anything. And the script is meant to setup/upgrade some software, and I searched thru it without finding any shell scripts. There is no mention of "bash" or "bin/sh" anywhere. But there are some use of open, and I think some of them can be like popen, so I will have to look thru those.

But back to that system_bash function. Is there anyway to make it accept a list as parameter instead of just a string? I tried this, but it doesn't work:

Code:

sub system_bash {
  my @args = ( "bash", "-c", shift );
  return system(@args);
}

my @cmd = ("echo", "1", "2", "3");
my $retcode = system_bash(@cmd);

Sergei Steshenko · 06-30-2011, 05:52 PM

Quote:

Originally Posted by Guttorm

... Is there anyway to make it accept a list as parameter instead of just a string? ...

There is - read 'perldoc perlsub' paying special attention to '@_' (without the quotes).

theNbomr · 06-30-2011, 08:43 PM

It is easy enough to just use join() to make a whitespace delimited string.

Code:

sub system_bash {
  my @args = ( "bash", "-c", shift );
  return system( join( " ", @args ) );
}

I'm glad Sergei was able to confirm that modifying $ENV{"SHELL"} doesn't affect how Perl loads a shell; I thought I had gone a bit soft in the head. Still seems an odd and counter-intuitive behavior.

One could manufacture their own system() function clone, but I wouldn't do it the way the OP suggested, since that appears to make the problem recursive. My solution, if I had to use system() at all (and I never prefer to), would be to use a fork() + exec() + wait(). From what I've observed, most/many people use system(), which loads a shell in order to simply load some other program which could have been loaded directly. If you really do need bash instead of dash, you can launch it this way.

Code:

sub mySystem{

    if(  my $childPid = fork() ){
        wait;
    }
    else{
        exec( @_ );
    }
}

--- rod.