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08.18.09
Perl On The Command Line
 By
Jay Fougere
One of the beautiful uses of Perl is to do quick and dirty string manipulations on the command line. In a Unix environment this is especially nice as the output of one command can be piped into Perl for manipulation.
For instance, let's say that I want to view only a couple of specific fields in a log file in realtime, I can tail the log file and pipe it through a small Perl program written on the command line. As an example, let's say I want to view permanent mail delivery errors due to an invalid recipient address in a standard Postfix maillog. I could simply run:
And I would recieve some long and ugly lines that tell me more information than what I really need. Now, let's say I just want to see the addresses that are failing. I could run the same command and then pipe it through perl to grab the fields I am interested in. The '-e' flag for perl means "execute".
And I will recieve a list of the rejected email addresses.
Another favorite of mine is to strip comments from configuration files.
As you are probably aware, $_ is the current line going through the program. The three regular expressions are fairly simple. The unless statement says that if $_ begins with '#' or if $_ begins with one or more spaces followed immediately by a '#' or if $_ matches only one or more spaces, ignore the line. Otherwise we print it to standard out. You could also redirect this output to a new file name for later use
About
the Author:
Jay Fougere is the IT manager for the iEntry network. He also writes occasional articles. If you have any IT questions, please direct them to Jay@ientry.com.
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