less is a pager or a program similar to more, but which allows backward movement in the file as well as forward movement. Less has a massive number of options. Every time I work with less command, I find something new and amazing. less is capable of displaying not just a text file but it can display an ISO image, a PDF file or even a MS word doc file contains from shell prompt. Sometime it saves lots of my time:

Watch a log file
Watch maillog in real time, useful to troubleshoot mail server problem
less +F /var/log/maillog
less +F /var/log/message
You can also use tail command:

tail -f /var/log/maillog

Start to display at a specific part in the file
# httpd -tOutput:

Syntax error on line 53 of /etc/httpd/httpd.conf
....
..
....

Ok let us display an error at line number 53 (no need to scroll down all 52 lines)# less +53 /etc/httpd/httpd.confSearch and highlight particular text:# less "+/*VirtualHost" /etc/httpd/httpd.confUse less preprocessor to display following extensions files on screen
lessfile and lesspipe are programs that can be used to modify the way the contents of a file are displayed in less. What this means is that less can automatically open up tar files, uncompress gzipped files, and even display something reasonable for graphics files. Just put the following command in your login script (if you are using bash put in ~/.bash_profile):
$ vi ~/.bash_profileAppend following line:eval "$(lesspipe)"Save file and load the changes:

. ~/.bash_profile

a) Display a PDF File:$ less file.pdfb) Display a MS-Word File:$ less file.docc) Display listing of tar file (and other compressed files):$ less myfile.tar
$ less myfile.tar.gzd) Display an ISO image file listing without mounting them (remember to mount an ISO file you need to have a root account permission):$ less KNOPPIX_V4.0.2CD-2005-09-23-EN.isoe) Display .deb or rpm file information and file listing (useful to find out where files going to install):$ less foo.deb
$ less foo.rpmless command supports following extensions:

• *.arj
• *.tar.bz2
• *.bz
• *.bz2
• *.deb, *.udeb
• *.doc
• *.gif, *.jpeg, *.jpg, *.pcd, *.png, *.tga, *.tiff, *.tif
• *.iso, *.raw, *.bin
• *.lha, *.lzh
• *.pdf
• *.rar, *.r[0-9][0-9]
• *.rpm
• *.tar.gz, *.tgz, *.tar.z, *.tar.dz
• *.gz, *.z, *.dz
• *.tar
• *.jar, *.war, *.xpi, *.zip
• *.zoo

Finally, lesskey command lets you change the meaning of any key or sequence of keys for less command. lesskey man page has detailed information on this topic. Read man page of less command for more info.