System command in perl

Question

I need to run a system command which would go to a directory and delete sub directories excluding files if present. I wrote the below command to perform this operation:

system("cd /home/faizan/test/cache ; for i in *\; do if [ -d \"$i\" ]\; then echo \$i fi done");

The command above keeps throwing syntax error. I have tried multiple combinations but still not clear how this should go. Please suggest.

Why not create a bash file separately and then call it from your Perl program? — Chankey Pathak, Dec 26 '12 at 10:24
This particular command needs to be executed on around 16 servers through ssh/perl. I need some way to perform this operation using a single perl script on multiple servers — f-z-N, Dec 26 '12 at 10:27
You might be facing [this problem](http://stackoverflow.com/questions/6366530/bash-syntax-error-unexpected-end-of-file) — Chankey Pathak, Dec 26 '12 at 10:33
Tried above but still the same error message. Somehow I feel that it is related to the way the command has been formed but I am not able to pinpoint the mistake. — f-z-N, Dec 26 '12 at 10:39

score 6 · Accepted Answer · answered Dec 26 '12 at 10:46

Well, your command line does contain syntax errors. Try this:

system("cd /home/faizan/test/cache ; for i in *; do if [ -d \"\$i\" ]; then echo \$i; fi; done");

Or better yet, only loop over directories in the first place;

system("for i in /home/faizan/test/cache/*/.; do echo \$i; done");

Or better yet, do it without a loop:

system("echo /home/faizan/test/cache/*/.");

(I suppose you will want to rmdir instead of echo once it is properly debugged.)

Or better yet, do it all in Perl. There is nothing here which requires system().

Thank you..your suggestion worked..I will try to implement this in perl — f-z-N, Dec 26 '12 at 10:52

F. Hauri - Give Up GitHub · Answer 2 · 2012-12-26T11:10:01.593

As question title stand for system command, this will answer directly, but the sample command using bash contain only thing that will be simplier in perl only (take a look at other answer using opendir and -d in perl).

If you want to use system (instead of open $cmdHandle,"bash -c ... |"), the prefered syntax for execution commands like system or exec, is to let perl parsing the command line.

Try this (as you've already done):

perl -e 'system("bash -c \"echo hello world\"")'
hello world

perl -e 'system "bash -c \"echo hello world\"";'
hello world

And now better, same but letting perl ensure command line parsing, try this:

perl -e 'system "bash","-c","echo hello world";'
hello world

There are clearly 3 argument of system command:

bash
-c
the script

or little more:

perl -e 'system "bash","-c","echo hello world;date +\"Now it is %T\";";'
hello world
Now it is 11:43:44

as you can see in last purpose, there is no double double-quotes enclosing bash script part of command line.

**Nota: on command line, using perl -e '...' or perl -e "...", it's a little heavy to play with quotes and double-quotes. In a script, you could mix them:

system 'bash','-c','for ((i=10;i--;));do printf "Number: %2d\n" $i;done';

or even:

system 'bash','-c','for ((i=10;i--;));do'."\n".
                       'printf "Number: %2d\n" $i'."\n".
                       'done';

Using dots . for concatening part of (script part) string, there are always 3 arguments.

score 1 · Answer 3 · answered Dec 26 '12 at 10:50

You're still best off trying this as a bash command first. Formatting that properly makes it much clearer that you're missing statement terminators:

for i in *; do
    if [ -d "$i" ]; then
        echo $i
    fi
done

And condensing that by replacing new lines with semicolons (apart from after do/then):

for i in *; do if [ -d "$i" ]; then echo $i; fi; done

Or as has been mentioned, just do it in Perl (I haven't tested this to the point of actually uncommenting remove_tree - be careful!):

use strict;
use warnings;

use File::Path 'remove_tree';
use feature 'say';

chdir '/tmp';
opendir my $cache, '.';
while (my $item = readdir($cache)) {
    if ($item !~ /^\.\.?$/ && -d $item) {
        say "Deleting '$item'...";
        # remove_tree($item);
    }
}

Thank you for the nice explanation. The problem is that only one server has perl installed and others dont where this has to be executed. I need to ssh to those servers using perl and then execute either a sh script or something similar to above. — f-z-N, Dec 26 '12 at 10:58

score 1 · Answer 4 · answered Dec 26 '12 at 10:57

Using system

my @args = ("cd /home/faizan/test/cache ; for i in *; do if [ -d \"\$i\" ]; then echo \$i; fi; done");
system(@args);

Using Subroutine

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

do_stuff("cd /home/faizan/test/cache ; for i in *; do if [ -d \"\$i\" ]; then echo \$i; fi; done");

System command in perl

4 Answers4