grep.md

title	date	icon	background	tags	categories	intro
Grep	2020-11-25 10:28:43 -0800	icon-grep	bg-emerald-400	egrep search pattern text	Linux Command	This cheat sheet is intended to be a quick reminder for the main concepts involved in using the command line program grep and assumes you already understand its usage.

Getting started {.cols-5}

Usage {.col-span-2}

Search standard output (i.e. a stream of text)

$ grep [options] search_string

Search for an exact string in file:

$ grep [options] search_string path/to/file

Print lines in myfile.txt containing the string "mellon"

$ grep 'mellon' myfile.txt

Wildcards are accepted in filename.

Option examples {.col-span-3}

Option	Example	Operation
-i	grep -i ^DA demo.txt	Forgets about case sensitivity
-w	grep -w "of" demo.txt	Search only for the full word
-A	grep -A 3 'Exception' error.log	Display 3 lines after matching string
-B	grep -B 4 'Exception' error.log	Display 4 lines before matching string
-C	grep -C 5 'Exception' error.log	Display 5 lines around matching string
-r	grep -r 'quickref.me' /var/log/nginx/	Recursive search (within subdirs)
-v	grep -v 'warning' /var/log/syslog	Return all lines which don't match the pattern
-e	grep -e '^al' filename	Use regex (lines starting with 'al')
-E	grep -E 'ja(s|cks)on' filename	Extended regex (lines containing jason or jackson)
-c	grep -c 'error' /var/log/syslog	Count the number of matches
-l	grep -l 'robot' /var/log/*	Print the name of the file(s) of matches
-o	grep -o search_string filename	Only show the matching part of the string
-n	grep -n "go" demo.txt	Show the line numbers of the matches

Grep regular expressions {.cols-3}

Refer

• Regex syntax (quickref.me)
• Regex examples (quickref.me)

Please refer to the full version of the regex cheat sheet for more complex requirements.

Wildcards

-	-
.	Any character.
?	Optional and can only occur once.
*	Optional and can occur more than once.
+	Required and can occur more than once.

Quantifiers

-	-
{n}	Previous item appears exactly n times.
{n,}	Previous item appears n times or more.
{,m}	Previous item appears n times maximum.
{n,m}	Previous item appears between n and m times.

POSIX

-	-
[:alpha:]	Any lower and upper case letter.
[:digit:]	Any number.
[:alnum:]	Any lower and upper case letter or digit.
[:space:]	Any whites­pace.

Character

-	-
[A-Z­a-z]	Any lower and upper case letter.
[0-9]	Any number.
[0-9­A-Z­a-z]	Any lower and upper case letter or digit.

Position

^	Beginning of line.
$	End of line.
^$	Empty line.
\<	Start of word.
\>	End of word.