How can I run an external command and capture its output in Perl?

Question

I\'m new to Perl and want to know of a way to run an external command (call it prg) in the following scenarios:

1. Run prg, get its

Answer 1

In most cases you can use the qx// operator (or backticks). It interpolates strings and executes them with the shell, so you can use redirections.

• To capture a command's STDOUT (STDERR is unaffected):

  $output = `cmd`;
  

• To capture a command's STDERR and STDOUT together:

  $output = `cmd 2>&1`;
  

• To capture a command's STDERR but discard its STDOUT (ordering is important here):

  $output = `cmd 2>&1 1>/dev/null`;
  

• To exchange a command's STDOUT and STDERR in order to capture the STDERR but leave its STDOUT to come out the old STDERR:

  $output = `cmd 3>&1 1>&2 2>&3 3>&-`;
  

• To read both a command's STDOUT and its STDERR separately, it's easiest to redirect them separately to files, and then read from those files when the program is done:

  system("program args 1>program.stdout 2>program.stderr");