Overview

Teaching: 15 min
Exercises: 0 min

Questions

• How can I perform the same actions on many different files?

Objectives

• Use grep to select lines from text files that match simple patterns.

So he has been receiving substantial money from Bishop Industries and has sent many emails to a Martin Bishop. It’d be great if we knew more about Martin Bishop.

Let’s find out who Martin Bishop is. We’re going to look for information in Dr Gill’s contact book.

$ cat contacts.txt

Oh dear, it’s rather large. However it does have information against everyone’s name. The first column is their name, the second is their company, the third is the position at the company and the fourth is their location.

Searching within a file

To search within a file, we use the grep command. It stands for the less catchy: “global regular expression print”. It is however one of the most invaluable tools in the Unix shell.

grep takes two arguments as standard: the search term and the file to search. It will find and print out every line in the file that contains the pattern.

Let’s search the address book for contacts based in Bolivia. Notice that quotation marks are used around the word “bolivia” in the command.

$ grep "bolivia" contacts.txt

Hmm, it didn’t find anything. grep is case-sensitive by default. To make it case-insensitive, use the -i argument.

$ grep -i "bolivia" contacts.txt

Excellent.

The Power of Grep

Grep has so many arguments. Let’s just mention -v. It inverts the search query, so will print any line that doesn’t contain the search query. For instance, the command below will print out every line that doesn’t contain the letter “n”.

$ grep -v n contacts.txt

Piping Grep

Grep is another command that can have data piped into it. Let’s try using grep twice. Can we find a contact of Dr Gill’s that lives in Belgium and works for Reducto Services.

$ grep "Belgium" contacts.txt | grep "Reducto Services"

Exercise

Find out who Martin Bishop is using grep and the contacts.txt file.

Solution

$ grep "Martin Bishop" contacts.txt

Bonus Points

• Find the contacts entry that lives in Singapore, has an “m” in their entry but not a “u”.

Key Points

• grep selects lines in files that match patterns.

• --help is a flag supported by many bash commands, and programs that can be run from within Bash, to display more information on how to use these commands or programs.