grep

grep is command-line util for matching lines of text against a pattern of characters or regular expressions in a file.

Getting started

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	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 ‘lsong.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

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

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

`{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.

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

`[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.