How to use grep to search for strings in files on the shell

How to use grep to search for strings in files on the shell

The grep command, which stands for global regular expression print, is one of the most versatile commands in a Linux terminal environment. It is an immensely powerful program that allows the user to sort input according to complex rules, which makes it a rather popular link in numerous command chains. The grep command is primarily used to search text or any file for lines that contain a match to the specified words/strings. By default, grep displays the matched lines, and it can be used to search for lines of text that match a regular expression(s), and it outputs only the matched lines.if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-box-3-0’)};

The basic grep command syntax is as follows:if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-medrectangle-3-0’)};

grep 'word' filename
grep 'word' file1 file2 file3
grep 'string1 string2'  filename
cat otherfile | grep 'something'
command | grep 'something'
command option1 | grep 'data'
grep --color 'data' fileName

In the first example, I will search for the user “tom” in the Linux passwd file. To search the /etc/passwd file for the user “tom”, you need to enter the following command:

grep tom /etc/passwd

Given below is the sample Output:


You have the option to instruct grep to ignore word case, i.e., match abc, Abc, ABC, and all possible combinations with the -i option as shown below:

grep -i "tom" /etc/passwd

The -i option of grep command

If you have a bunch of text files in a directory hierarchy, e.g, the Apache configuration files in /etc/apache2/ and you want to find the file where a specific text is defined, then use the -r option of the grep command to do a recursive search. This will perform a recursive search operation trough files for the string “” (as shown below) in the directory /etc/apache2/ and all its sub-directories:

grep -r "" /etc/apache2/

Alternatively, the following command may be used:

grep -R "" /etc/apache2/

Given below are the Sample outputs for a similar search on an Nginx server:

grep -r "" /etc/nginx/
/etc/nginx/sites-available/ if ($http_host != "") {

Here, you would see the result for on a distinct line preceded by the name of the file (for instance /etc/nginx/sites-available/ in which it was found. The inclusion of the file names in the output data may be easily suppressed by using the -h option (as explained below): grep -h -R “” /etc/nginx/. Given below is the sample Output:if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-medrectangle-4-0’)};

grep -r "" /etc/nginx/
if ($http_host != "") {

When you are searching for abc, grep will match all sorts of things, viz., kbcabc, abc123, aarfbc35, and lots more combinations without obeying word boundaries. You can compel the grep command to select only those lines that contain matches to form whole words (those that match only abc word), as shown below:

grep -w "abc" file.txt


Grep for whole words only

To search for two different words, you must use the egrep command as shown below:

egrep -w 'word1|word2' /path/to/file

The grep command has the ability to report the number of times a particular pattern has been matched for each file using the -c (count) option (as shown below):if(typeof __ez_fad_position != ‘undefined’){__ez_fad_position(‘div-gpt-ad-howtoforge_com-box-4-0’)};

grep -c 'word' /path/to/file

In addition, users may use the ‘-n’ option preceding each output line with the number of the line in the text file from which it was obtained (as shown below):

grep -n 'root' /etc/passwd

Given below are the Sample outputs:


Users may make use of the -v option to print inverts the match, which means it would match only those lines that do not contain the given word. For instance, print all lines that do not contain the word par by using the following command:

grep -v par /path/to/file

You must use the -l option to list file names whose contents mention a particular word, for instance, the word ‘primary’, using the following command:

grep -l 'primary' *.c

Lastly, you have the option to compel grep to display output in specific colors by using the following command:

grep --color root /etc/passwd

Given below are Sample Outputs:

List only names of matching files with grep

There could be situations wherein you might want to search multiple patterns in a given file (or set of files). In such scenarios, you should use the ‘-e’ command-line option that grep provides.

For example, suppose you want to search for words “how”, “to”, and “forge” in all the text files present in your current working directory, then here’s how you can do this:

grep -e how -e to -e forge *.txt

Here’s the command in action:

Multiple search patterns in Grep

The ‘-e’ command-line option also helps in scenarios wherein the pattern begins with a hyphen (-). For example, if you want to search for, say, “-how”, then the following command won’t be helpful:

grep -how *.txt

It’s when you use the -e command-line option, the command understands what exactly you are trying to search in this case:

grep -e -how *.txt

Here are both commands in action:

Grep -e switch

In case you want to limit the grep output to a particular number of lines, you can do that using the ‘-m’ command-line option. For example, suppose you want to search for the word “how” in testfile1.txt which contains the following lines:

Limit Grep output

But the requirement is for grep to stop searching after 3 lines containing the searched pattern have been found. So, to do this, you can run the following command:

grep "how" -m3 testfile1.txt

Here’s the command in action:

Grep -m switch

Moving on, here is what the command’s man page says:

If the input is standard input from a regular file, and NUM matching lines are output, grep ensuresthat the standard input is positioned to just after the last matching line before exiting, regardless of the presence of trailing context lines. This enables a calling process to resume a search.

So for example, if you have a bash script that has a loop, and you want to fetch one match per loop iteration, then using ‘grep -m1’ will do the needful.

If you want, you can also make the grep command obtain patterns from a file. The tool’s -f command-line option lets you do this. 

For example, suppose you want to search all the .txt files in the current directory for words “how” and “to”, but want to supply these input strings through a file named, say, “input,” then here’s how you can do this:

grep -f input *.txt

Here’s the command in action:

Grep optain patterns for file

Up until now, we have seen that by default grep matches and displays complete lines that contain search patterns. But if the requirement is to make grep only display those lines that completely match the searched pattern, then this can be done using the ‘-x’ command-line option.

For example, suppose testfile1.txt file contains the following lines:

Display exact matches only

And the pattern you want to search is “how are you?”. So to make sure that grep only displays lines that completely match this pattern, use it in the following way:

grep -x "how are you?" *.txt

Here’s the command in action:

Grep -x switch in action

There might be situations wherein you don’t need the grep command to produce anything in the output. Instead, you just want to know whether or not a match was found based on the command’s exit status. This can be achieved using the -q command-line option.

While the -q option mutes the output, the tool’s exit status can be confirmed by the ‘echo $?’ command. In the case of grep, the command exits with ‘0’ status when it’s successful (meaning, a match was found), while it exits with status ‘1’ when no match was found.

The following screenshot shows both the successful and unsuccessful scenarios:

Grep do not display output

By default, the grep command displays the name of files containing the search pattern (as well as matched lines). This is quite logical, as that’s what expected of this tool. However, there might be cases wherein the requirement could be to get names of those files that do not contain the searched pattern.

This is also possible with grep – the -L options lets you do this. So, for example, to find all those text files in the current directory that does not contain the word “how”, you can run the following command:

grep -L "how" *.txt

Here’s the command in action:

Grep inverse match

If you want, you can also force grep to mute any error messages it displays in the output. This can be done using the -s command line option. For example, consider the following scenario in which grep produces error/warning related to the directory it encounters:

Suppress errors in grep

So in this kind of scenario, the -s command-line option helps. See below.

The grep -s switch

So you can see that the error/warning got muted.

As clear from the example used in the previous point, the grep command doesn’t do a recursive search by default. To make sure your grep search is recursive, use the -d command-line option and pass the value ‘recurse’ to it.

grep -d recurse "how" *

Note1: The directory related error/warning message we discussed in the previous point can also be muted using the -d option – all you have to do is to pass the value ‘skip’ to it.

Note2: Use ‘–exclude-dir=[DIR]’ option to exclude directories matching the pattern DIR from recursive searches.

As we have already discussed, the -l command-line option of grep is used when you only want the tool to display filenames in the output. For example:

Grep null file termination

Now, what you should know here is that each name in the above output is separated/terminated by a newline character. Here’s how you can verify that:

Redirect the output to a file, and then print the file contents:

Grep -l switch

So the output of the cat command confirms the presence of a newline character between the file names.

But as you might already know, the newline character can be part of a file name as well. So when dealing with cases where-in filenames contain newline and they are separated/terminated by newline as well, it becomes difficult to work on the grep output (especially when accessing the output through a script).

It would be good if the separating/terminating character is not a newline. Well, you’ll be glad to know that grep provides a command-line option -Z that makes sure filenames are followed by a NULL character and not a newline.

So, in our case, the command becomes:

grep -lZ "how" *.txt

Here’s how we confirmed the presence of NULL character:

Check for NULL character

Following is a related command-line option that you should know:

 -z, --null-data
Treat the input as a set of lines, each terminated by a zero byte (the ASCII NUL character) insteadof a newline. Like the -Z or --null option, this option can be used with commands like sort -z to process arbitrary file names.

In our second GREP command tutorial, you can find even more examples of how to use this Linux command.

  • How to perform pattern search in files using Grep
About the Author

Leave a Reply