Syntax

The syntax is:
cat -n fileNameHere
OR
cat --number foo.c
OR use the more command/less command as filter when text can not be fitted on the screen:
cat --number foo.c | more
OR
cat --number foo.c | less
The -b / --number-nonblank option number all nonempty output lines, starting with one and the syntax is:
cat -b fileNameHere
OR
cat --number--nonblank filename
Finally, you suppress or remove repeated empty output lines with the -s / --squeeze-blank option:
cat -s -n fileNameHere
cat -s -n /etc/resolv.conf
Sample outputs:

     1	#
     2	# Mac OS X Notice
     3	#
     4	# This file is not used by the host name and address resolution
     5	# or the DNS query routing mechanisms used by most processes on
     6	# this Mac OS X system.
     7	#
     8	# This file is automatically generated.
     9	#
    10	search nixcraft.com
    11	nameserver 8.8.8.8
    12	nameserver 192.168.2.254
    13	nameserver 8.8.4.4

OR (GNU only syntax)
cat --squeeze-blank -n filename

Say hello to nl command

Use the nl command number lines of files under Linux or Unix oses. The syntax is:

nl filename

Examples

Create a text file called hello.c as follows:

/* Purpose: Sample see program to print Hello world on stdout 
 * Author: nixCraft
 * Copyright: None / Copyleft
 */
 
#include<stdio.h>
 
/* our main */
int main(void){
	printf("Hello world\n");
	return 0;
}

Use the cat or nl command to display line numbers:

cat -n hello.c
nl hello.c

Sample outputs:

A note about sed

To just print 3rd line use the sed command:
sed -n 3p /etc/resolv.conf
To just print 3rd and 5th line use the sed command:
sed -n -e 3p -e 5p /etc/resolv.conf
To see specific range, say show lines between 3 to 5, run:
sed -n 3,5p /etc/resolv.conf
For more information see man pages – cat(1),nl(1),sed(1).