Grep

grep is a command-line utility for searching plain-text data sets for lines that match a regular expression

Grep looks for a pattern inside a file. It ignores most linux special characters and treats them as normal text

Examples
grep 'hello' foo.txt

grep -i 'ea'
 * looks for every line containing 'hello' and displays all the lines


 * looks for every line containing "EA" "Ea" "eA" "ea"

Command syntax
grep options 'regular expression' file(s)

Options

 * -n display line numbers with the output
 * -c display just a count of lines
 * -i case insensitive mode
 * -v display all lines that don't match the pattern
 * Sometimes useful if the lines you want are easier to get by elimination