wc (count lines, words and characters)
wc is the definitive tool for counting lines, words and characters in a text file. Let me show you a practical example:
wc -l /etc/passwd
This will output the number of lines in /etc/passwd. The option -w would display the number of words and -c the number of characters. If you don't supply any options, wc will provide all three values. The above example is useful because the number of lines in /etc/passwd equals the number of users registered in your system.