Showing progress whilst running a system() command in Perl

Question

I have a Perl script which performs some tasks, one of which is to call a system command to "tar -cvf file.tar.....".

This can often take some time so I'd like the command line to echo back a progress indicator, something like a # echoing back to screen whilst the system call is in progress.

I've been doing some digging around and stumbled across fork. Is this the best way to go? Is it possible to fork off the system command, then create a while loop which checks on the staus of the $pid returned by the fork?

I've also seen references to waitpid.... I'm guessing I need to use this also.

fork system("tar ... ")
while ( forked process is still active) {
    print #
    sleep 1
}

Am I barking up the wrong tree?

Many thanks John

Answer 1

I would try something like this

open my $tar, "tar -cvf file.tar..... 2>&/dev/null |"
    or die "can't fork: $!";
my $i = 0;
while (<$tar>) {
    if( i++ % 1000 == 0 ) print;
} 
close $tar or die "tar error: $! $?";

Answer 2

Perl has a nice construction for this, called "pipe opens." You can read more about it by typing perldoc -f open at a shell prompt.

# Note the use of a list for passing the command. This avoids
# having to worry about shell quoting and related errors.
open(my $tar, '-|', 'tar', 'zxvf', 'test.tar.gz', '-C', 'wherever') or die ...;

Here's a snippet showing an example:

  open(my $tar, '-|', 'tar', ...) or die "Could not run tar ... - $!";
  while (<$tar>) {
       print ".";
  }
  print "\n";
  close($tar);

Replace the print "." with something that prints a hash mark every 10 to 100 lines or so to get a nice gaugebar.

Answer 3

An example that doesn't depend on the child process writing any kind of output, and just prints a dot about once a second as long as it's running:

use POSIX qw(:sys_wait_h);
$|++;

defined(my $pid = fork) or die "Couldn't fork: $!";

if (!$pid) { # Child
  exec('long_running_command', @args) 
    or die "Couldn't exec: $!";
} else { # Parent
  while (! waitpid($pid, WNOHANG)) {
    print ".";
    sleep 1;
  }
  print "\n";
}

Although it could probably stand to have more error-checking, and there might actually be something better already on CPAN. Proc::Background seems promising for abstracting this kind of job away but I'm not sure how reliable it is.

Answer 4

For showing progress during a long-running task, you will find Term::ProgressBar useful -- it does the "printing of # across the screen" functionality that you describe.