Tutorial :How can I call a shell command in my Perl script?



Question:

What would be an example of how I can call a shell command, say 'ls -a' in a Perl script and the way to retrieve the output of the command as well?


Solution:1

How to run a shell script from a Perl program

1. Using system system($command, @arguments);

For example:

system("sh", "script.sh", "--help" );

system("sh script.sh --help");

System will execute the $command with @arguments and return to your script when finished. You may check $! for certain errors passed to the OS by the external application. Read the documentation for system for the nuances of how various invocations are slightly different.

2. Using exec

This is very similar to the use of system, but it will terminate your script upon execution. Again, read the documentation for exec for more.

3. Using backticks or qx//

my $output = `script.sh --option`;

my $output = qx/script.sh --option/;

The backtick operator and it's equivalent qx// excute the command and options inside the operator and return that commands output to STDOUT when it finishes.

There are also ways to run external applications through creative use of open, but this is advanced use; read the documentation for more.


Solution:2

From Perl HowTo, the most common ways to execute external commands from Perl are:

  • my $files = `ls -la` â€" captures the output of the command in $files
  • system "touch ~/foo" â€" if you don't want to capture the command's output
  • exec "vim ~/foo" â€" if you don't want to return to the script after executing the command
  • open(my $file, '|-', "grep foo"); print $file "foo\nbar" â€" if you want to pipe input into the command


Solution:3

Look at the open function in Perl - especially the variants using a '|' (pipe) in the arguments. Done correctly, you'll get a file handle that you can use to read the output of the command. The back tick operators also do this.

You might also want to review whether Perl has access to the C functions that the command itself uses. For example, for ls -a, you could use the opendir function, and then read the file names with the readdir function, and finally close the directory with (surprise) the closedir function. This has a number of benefits - precision probably being more important than speed. Using these functions, you can get the correct data even if the file names contain odd characters like newline.


Solution:4

Examples

  1. `ls -l`;
  2. system("ls -l");
  3. exec("ls -l");


Solution:5

As you become more experienced with using Perl, you'll find that there are fewer and fewer occasions when you need to run shell commands. For example, one way to get a list of files is to use Perl's built-in glob function. If you want the list in sorted order you could combine it with the built-in sort function. If you want details about each file, you can use the stat function. Here's an example:

#!/usr/bin/perl    use strict;  use warnings;    foreach my $file ( sort glob('/home/grant/*') ) {      my($dev,$ino,$mode,$nlink,$uid,$gid,$rdev,$size,$atime,$mtime,$ctime,$blksize,$blocks)          = stat($file);      printf("%-40s %8u bytes\n", $file, $size);  }  


Solution:6

There are a lot of ways you can call a shell command from perl script, such as:

  1. back tick ls which captures the output and gives back to you.
  2. system system('ls');
  3. open

Refer #17 here: perl programming tips


Solution:7

You might want to look into open2 and open3 in case you need bidirectional communication.


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