ag

TLDR

Find files containing "foo", and print the line matches in context

$ ag [foo]

Find files containing "foo" in a specific directory

$ ag [foo] [path/to/directory]

Find files containing "foo", but only list the filenames

$ ag -l [foo]

Find files containing "FOO" case-insensitively, and print only the match, rather than the whole line

$ ag -i -o [FOO]

Find "foo" in files with a name matching "bar"

$ ag [foo] -G [bar]

Find files whose contents match a regular expression

$ ag '[^ba(r|z)$]'

Find files with a name matching "foo"

$ ag -g [foo]