Introduction to grep command

Image: CC BY 4.0 by cybrkyd

grep is a Linux command line tool used for text-searching in files. It is a versatile tool, capable of searching through single and multiple files as well as recursively through directories.

grep does not utilise a search index; instead, it performs searches in real-time and does so very quickly. It matches a pattern of characters in a specific file or directory of files and then displays the results.

grep command with examples

The basic grep command requires only two arguments, (i) the pattern or string to look for and (ii) the name of the file.

grep [string] [file]

For example, to search for the word cyberspace in the file words:

$ grep cyberspace words

cyberspace
cyberspace's
cyberspaces

The results will display the entire line where the string is found. If there are no results, the command prints nothing.

How to find words with spaces or special characters with grep

Search patterns can be enclosed in either single or double inverted commas. This allows a search of more than one word or words with spaces or special characters.

$ grep "cyberspace's" words

cyberspace's

The same rule applies to file names. Enclose the file name in either single or double inverted commas if it contains spaces:

$ grep "cyberspace's" 'words of the world.txt'

cyberspace's

How to ignore case when searching with grep

grep is case sensitive so search for England instead of england.

To ignore case, specify the -i flag:

$ grep -i england words

England
England's

How to find the exact word with grep

grep prints every match found by default.

$ grep Peter words

Peter
Peter's
Peters
Petersen
Petersen's
Peterson
Peterson's
Peters's

To find the exact word, specify the -w flag:

$ grep -w Peter words

Peter
Peter's

How to find the line number with grep

grep can display the line number in the results. This is useful when searching through a large file. To display line numbers in results, specify the -n flag.

$ grep -n -w Peter words

14768:Peter
14769:Peter's
14770:Peters
14771:Petersen
14772:Petersen's
14773:Peterson
14774:Peterson's
14775:Peters's

Using regular expressions (regex) with grep when searching

The search query can be fine-tuned with regular expressions for more accurate results. To search for exactly Peter and not wanting to see Peter’s, a regex can be added. In this example, $ specifies end-of-line:

$ grep Peter$ words

Peter

Search multiple files using grep command

One grep command can search the contents of multiple files. To query more than one file, separate the file names with a space.

$ grep -w cyberspace$ words "words of the world.txt"

words:cyberspace
words of the world.txt:cyberspace

How to search one directory using grep command

All files in one directory can be searched by using an asterisk (*).

$ grep cyberspace *

american-english:cyberspace
british-english:cyberspace
words:cyberspace

How to search a directory recursively using grep command

To search a directory and all of its sub-directories, the -r flag can be used.

$ grep -r cyberspace

my-sub-dir/file-x.txt:cyberspace
american-english:cyberspace
british-english:cyberspace
words:cyberspace