Redirecting standard error in Perl

There are numerous ways to redirect standard error in a Perl script. You can redirect the output from one particular command, you can combine standard error with standard out so that the two are handled together or you can send all standard error to the bit bucket. None of these techniques depends on what the person running the script does. Instead, they can all be set up in your Perl scripts. Let's take a look at how this works.

First, you can redirect standard error from any particular system command in the normal way. In the command shown below, for example, we are throwing away any errors that the nmap command might be generating. This allows us to control the output that the user sees.

@results=`nmap -p $range -P0 -sT $hostname 2> /dev/null`;

Perl also allows you to refer to standard error with the file handle "STDERR". If you want to generate messages that are specifically sent to standard error, you can set them up like this:

print STDERR "This message goes to standard error\n";

If you want to allow your users to redirect the expected output from a script to a file, you might want to send alerts and other messages detailing the script's progress to standard error to avoid having them redirected as well and the user never seeing them.

You can also combine standard error with standard out using a line like this:

open STDERR, '>&STDOUT';

Reminiscent of the 2>&1 syntax of many shells, this line instructs perl to send standard error to the same place it sends standard out.

To send standard error to the bit bucket starting at some particular point in your script, you might use this simple straightforward command:

open STDERR, '>/dev/null';

Any commands following this one in your script can be coded without regard for whether they might generate errors that would normally be sent to standard error. All output will be sent to /dev/null. If you want to reverse this effect later on in your script, you can close standard error much as would close any output file in a perl program:

close STDERR;

Now let's see how all this works by examining a script that uses these various ways of redirecting output and looking at the text the script generates.

Here's the script:

Sign up for ITworld's Daily newsletter
Follow ITworld on Twitter @IT_world

• Email
• Print
• Share
  • Slashdot
  • Digg
  • Stumble
  • Reddit
  • Newsvine