Introduction to Linux shell commands > Global Regex Print | Python Programming (70053 Autumn Term 2021/2022) | Department of Computing

Introduction to Linux shell commands

Chapter 3: Advanced Commands

Global Regex Print

face Harry Coppock

grep is a very powerful tool that allows you to search for patterns in files. The grep utility searches any given input files, selecting lines that match one or more patterns.
By default, a pattern matches an input line if the regular expression in the pattern matches the input line.

note: This is not a tutorial on regex. For those interested I would point you towards this tutorial video and also to this regex cheat sheet

Nosy Josiah interrupts: We will cover regex in more detail later on in our course! kthxbye! *poof*

grep has the following structure.

grep [options] PATTERNS FILES

where options allows you to specify how to search, see man grep for all the options - there are many! PATTERNS is the pattern you want to search for and FILES is where you want to search for

I have detailed some useful options below:

Exercise:

cd to /vol/bitbucket/hc-teaching/grep_demo. Here there are 3 .txt files to help demonstrate grep. Run the following commands:

First use cat or less to view the file names.txt
grep "John Williams" names.txt the vanilla grep command, simply returns the lines in the file names.txt which match the string "John Williams"
grep -w "John Williams" names.txt only matches whole words.
grep -i "john williams" names.txt grep is by default case sensitive, pass the option i to make it case insensitive
grep -n "john williams" names.txt the option -n is useful as it returns the line numbers which matches were made.
grep -C 2 "john williams" names.txt this returns the match and +- 2 lines of context around the match.
grep -win -C 2 "john williams" names.txt remember you can always combine arguments!
grep -win "john williams" ./ searches all files in the directory, this however, returns an error as there is a directory in your working directory. To combat this you can do:
- grep -win "john williams" ./*.txt utilises * which matches any content which ends with .txt therefore will not attempt to read the directory
- grep -inr "john williams" . uses the recursive argument r to recursively search down directories.
grep "...-...-..." names.txt here . specifies any character, so we are trying to match the patter 3x any character - 3x any character - 3x any charater.
- grep -P "\d{3}-\d{3}-\d{3}" names.txt if you want to use PERL compatible regex you need to specify the option -P