Linux and Unix grep command tutorial with examples

Tutorial using grep, a UNIX and Linux command to print lines matching a pattern. Examples of finding text in a file, printing line numbers, counting the number of matches, searching recursively and ignoring case sensitivity.

August 3, 2016
Updated October 2, 2023

What is the grep command in UNIX? ¶

The grep command in UNIX is a command line utility for printing lines that match a pattern. It can be used to find text in a file and search a directory structure of files recursively. It also supports showing the context of a match by showing lines before and after the result and has support for regular expressions in pattern matching.

How to find text in a file ¶

To find text in a file pass the string you are looking for to grep followed by the name of the file or files.

grep 'computer' /usr/share/dict/words
computer

The grep tool will print occurrences that it finds to standard output.

How to list line numbers for matches ¶

To list line numbers and file names pass the -n option to grep. This prints matches to standard output along with the line number it was found on.

grep 'computer' -n /usr/share/dict/words
40565

This can be useful if you are looking to edit a file and want to launch vim and go straight to the line.

vim +40565 /usr/share/dict/words

How to print lines before and after a match ¶

To print lines before and after a match the -A and -B options can be used. Both expect a number and will print this number of lines.

grep -B 2 -A 2 'computer' /usr/share/dict/words
computativeness
compute
computer
computist
computus

Using the -A and -B options can be very useful for grepping through log files to see what occurred before and after the item of interest.

A further option is available in -C that will print the context of the match. This is equivalent to using both -A and -B.

grep -C 2 'computer' /usr/share/dict/words
computativeness
compute
computer
computist
computus

The --context option may also be used and defaults to two lines before and after if no number is given.

How to count the number of matches ¶

To count the number of matches use the -c option. This outputs a number count to standard output.

grep -c 'comput*' /usr/share/dict/words
50

How to print the filename for a match ¶

To print the filename for a match use the -H option. This is automatically invoked when grep is given more than one file to search.

grep -H 'computer' /usr/share/dict/words
/usr/share/dict/words:computer

How to search recursively ¶

To search for a pattern recursively use the -R option. This will search through all files in the directory tree that you have permission to read.

grep -R 'passwd' /etc
/etc/pam.d/su:# NIS (man nsswitch) as well as normal /etc/passwd and
/etc/pam.d/chpasswd:# The PAM configuration file for the Shadow 'chpasswd' service

How to search for the inverse of a pattern ¶

To search for the inverse of a pattern use the -v option. This will print inverse matches to standard output.

grep -v 'computer' /usr/share/dict/words
A
a
aa
aal
....

How to ignore case when searching ¶

To ignore case when searching use the -i option. By default grep will respect case.

grep 'COMPUTER' /usr/share/dict/words
# no match
grep -i 'COMPUTER' /usr/share/dict/words
computer

How to use basic regular expressions when searching ¶

To use basic regular expressions all versions of grep support basic character matches. In the following example the pattern matches ‘ia’ characters at the end of the line.

grep 'ia$' /usr/share/dict/words
abasia
Abelia
abepithymia
....

A great book for understanding the power of regular expressions is Mastering Regular Expressions.

How to use extended regular expressions when searching ¶

To use extended regular expressions use the -e option. The following line matches lines that do not contain the words ‘foo’ or ‘bar’.

grep -v -e 'foo' -e 'bar'

Note that in the GNU version of grep there is no difference in available functionality between basic and extended syntaxes.