The grep command is a very useful tool, that when you master will help you tremendously in your day to day.

If you’re wondering, grep stands for global regular expression print

You can use grep to search in files, or combine it with pipes to filter the output of another command.

For example here’s how we can find the occurences of the document.getElementById line in the file:

grep -n document.getElementById

Using the -n option it will show the line numbers:

grep -n document.getElementById

One very useful thing is to tell grep to print 2 lines before, and 2 lines after the matched line, to give us more context. That’s done using the -C option, which accepts a number of lines:

grep -nC 2 document.getElementById

Search is case sensitive by default. Use the -i flag to make it insensitive.

As mentioned, you can use grep to filter the output of another command. We can replicate the same functionality as above using:

less | grep -n document.getElementById

The search string can be a regular expression, and this makes grep very powerful.

Another thing you might find very useful is to invert the result, excluding the lines that match a particular string, using the -v option:

The grep command works on Linux, macOS, WSL, and anywhere you have a UNIX environment