grep

Matches patterns in input text.
Supports simple patterns and regular expressions.

 - Search for an exact string:
   grep search_string path/to/file

 - Search in case-insensitive mode:
   grep -i search_string path/to/file

 - Search recursively (ignoring non-text files) in current directory for an exact string:
   grep -RI search_string .

 - Use extended regular expressions (supporting ?, +, {}, () and |):
   grep -E ^regex$ path/to/file

 - Print 3 lines of [C]ontext around, [B]efore, or [A]fter each match:
   grep -C|B|A 3 search_string path/to/file

 - Print file name with the corresponding line number for each match:
   grep -Hn search_string path/to/file

 - Use the standard input instead of a file:
   cat path/to/file | grep search_string

 - Invert match for excluding specific strings:
   grep -v search_string