Open a terminal.

Install ack-grep by typing sudo apt-get install ack-grep

Change to the directory you want to search under, and type

ack-grep foo

it lists out all the matches in all files under that directory.

—
If you don’t want to install additional software, you can simply use

grep

in the terminal. To recursively search through directories, you can use the

-r

option (see

man grep

for details). The syntax you are looking for is probably:

grep -r "[STRING TO SEARCH FOR]" "[DIRECTORY TO SEARCH]"

So, for instance if I want to search for the string “asdf” in all files in all subdirectories of “/tmp/testdir/” the command looks like this:

grep -r "asdf" "/tmp/testdir/"

The quotation marks are not strictly necessary, but if your string or directory path contains whitespaces, you otherwise would have to mask them using the \ character…