$ egrep -i by:(.*)$ test.txt By: Tobias Highfill What is grep? In - - PowerPoint PPT Presentation

$ egrep -i by:(.*)$ test.txt

By: Tobias Highfill

What is grep?

In UNIX systems grep is a command line tool to search files using regular expressions. What are regular expressions? Regular expressions are strings used for pattern matching. What are strings? How did you get into this class?

Regex crash course

• Most characters stand for themselves
• Metacharacters represent other characters

and can only be represented by escaping

• Sections of text can be captured using

parenthesis

• Characters like “*” “+” and “?” allow

segments to repeat

Example regex

by:(.*)$

• Normal characters
• Capturing group
• Repeat character (0 or more times)
• Metacharacters

○ “.” matches any character ○ “$” matches the end of the line

Different grep flavors

• grep

○ vanilla, normal

• egrep

○ extended grep with additional character support

• fgrep

○ “fast grep,” doesn’t use regex so it’s way faster

How to use grep

grep commands have three sections:

• 1. Options*
• 2. Pattern
• 3. Files*

*optional grep [OPTIONS]... PATTERN [FILES]...

Wait files are optional?

• If no files are provided grep will read the

standard input and analyze that.

• This makes grep amazing for piping.
• Don’t want to read the entire man page for a

program? Grep for keywords! man gcc | fgrep -in target

Common grep options

• -c, --count
• -e regexp, --regexp=regexp
• -f file, --file=file
• -i, --ignore-case
• -h, --no-filename
• -l, --files-with-matches
• -n, --line-number
• -s, --no-messages
• -v, --invert-match
• -x, --line-regexp

When am I ever gonna use this?

• Searching large files for pertinent information
• Searching your file system for a particular

pattern

• Great for piping!
• Finding the source code for your regex golf

solver

Source: xkcd.com/1313

Demonstration

I will now demonstrate grep using cygwin