grep

Basic grep

grep $expression file

case insensitive

grep -i $expression file

search for entire word

grep -w $expression file

recursive search (search directory)

grep -r $expression directory/

show lines above and below match (context)

grep -C 2 $expression file

count matches

grep -c $expression file

list files with matches

grep -l $expression *

Perl compatible regex

grep -P "\d{2}" file