grep

grep searches for regular expression (regex) patterns in files and prints matching lines.

Useful options

Short options can be chained like so -iv.

flag	effect
-i, --ignore-case	ignore case
-v, --invert-match	show non-matching lines
-E, --extended-regexp	interpret regex patterns as extended regex
-o, --only-matching	print only matched parts of a line
-r, --recursive	reads files under directories recursively