You can easily display line numbers using less command. All you have to do is pass either -N or --LINE-NUMBERS option to the less command. This option forces less to show a line number at the beginning of each line in the screen.
Display line numbers in less
The basic syntax is:
less -N filename
less --LINE-NUMBERS filename
In this example, show line numbers for /etc/passwd file using GNU/less, type:
less -N /etc/passwd
less --LINE-NUMBERS /etc/passwd
How do I force less to display line number by default?
The less command can read various options from the environment variable "LESS". For example, to avoid typing less -N filename each time less is invoked, you might type following on tcsh/csh shell:
setenv LESS "-N"
OR if you use sh/bash/ksh, enter:
LESS="-N"; export LESS
Now, run less as follows:
$ less filename
See less(1) Linux command man page for more information.