A Regular Expression to Colorize Standard Output on the Command Line
Michael
McDonnell
Winterstorm Solutions, Inc.
2003-09-27
1999
Michael McDonnell and Winterstorm Solutions, Inc.
This document may be redistributed under the terms of the GNU Free Documentation License.
A regular expression is describe that can be used to insert ANSI color codes, thereby colorizing text. A method for using this regular expression on the UNIX command line is given.
Update
On October 7, 2002 Sergey Goldgaber provided a patch for the hilite script that hacks it to work correctly with regular expressions. The patch has been incoporated below. Thanks Sergey!
A Better Alternative
Shortly after I wrote these instructions on how to colorize output, Joakim Andersson wrote Colortail which is a version of the tail program that can colorize its output. There are few situations where you might want to use the technique described in this document but for most situations you probably will find Colortail more useful. The URL for the home page of Colortail
is http://www.student.hk-r.se/~pt98jan/colortail.html.
Introduction
As a Systems Administrator and a CGI developer I often find my self watching
active logfiles as they grow. For instance while debugging a CGI
script I often use the 'tail -f' command to view new error messages generated
by the web server as I test my work.
For a long time I've wanted some way to colorize the output to highlight
certain keywords that I'm interested in the output. This can be really
helpful if your watching for attempts by crackers to break into your system
(something that I've had to do on a number of occasions unDfortunately).
I'm sure this could be handy in many circumstances. For instance:
Examining large amounts of data for particular oddities
Achieving a more aesthetic display for the output of syslogd to a virtual
console or terminal
Anything that requires the 'grep' command
Colorize the output of various shell commands for emphasis
Recently I decided to solve this problem and it turns out to be a very
simple thing to do. Here is what you will need:
An ANSI compatible shell/console/xterm/termcap so you can display color
PERL installed on your system. If you know how to do this with GNU sed please let me know.
If your using color-xterm, the linux console, rxvt, kvt, or have the UNIX
environment variable TERM set to ansi
or xterm
you should be fine.
Give one of the examples below a try and see if it works. If it doesn't
then see if you can get color-ls to work in your environment (or any other
command listitemne tool that uses color). The same technique shown here
could be used to bold or highlight or underline fonts for use on monochrome
terminals too.
A Simple Example
I'm sure that most readers just want to see the example so they can go
and do this in their own context. This command will accept
any text as standard input and return the same text as output but with
any instance of the word 'GET' turned red.
perl -p -e 's/GET/\e[31mGET\e[0m/'
For instance the simplest case would be this:
echo "GET" | perl -p -e 's/GET/\e[31mGET\e[0m/'
This example would product a red word GET
as output.
Here is another example. This one will will colorize the output
of the web server logs. As the logfile grows new listitemnes will be displayed
to the screen. Any instance of the word GET
will be turned yellow
and any instance of the word PUT
will be turned red and cause the bell
to sound.
tail -f /var/log/httpd/access_log | perl -p -e 's/GET/\e[33mGET\e[0m/; s/PUT/\007\e[31mPUT\e[0m/'
For one final example I will show how this trick can be used to colorize
the output of grep. This will find any listitemne in /var/log/maillog that
contains the word fred
and print those listitemnes after turned instances of
the u can use to colorize standard input!
I named the script hilite on my system.
#!/usr/bin/perl
### Usage: hilite <ansi_command> %lt;target_string>
### Purpose: Will read text from standard input and perform specified highlighting
### command before displaying text to standard output.
### License: GNU GPL
$|=1; # don't buffer i/o
$command = "$ARGV0";
$target = "$ARGV1";
$color = "\e[" . $command . "m";
$end = "\e[0m";
while(<STDIN>) {
s/($target)/$color$1$end/;
print $_;
}