How to use the grep command on Debian 10

Grep stands for Global Regular Expression Print. This is a useful command and is widely used by Linux system engineers, while searching for a string or patterns with regular files and in the system.

In this article, TipsMake.com will show you how to use grep command (many examples are included).

Note : The article has tested all commands and examples on machines running Debian 10.

Prerequisites

You need to have a computer running Debian 10 with root access.

Install grep on Debian 10

By default, grep is installed on most systems including Debian 10. If it is not already installed, open the terminal and issue the following command as root.

 apt-get install grep 

When you are asked to confirm, press Y and then type from the keyboard. Wait for the installation process to finish.

Check the version of grep by running the following command in the terminal.

 grep --version 

The command will return the version along with other details as shown below.

Use the grep command

Search for a specific file or directory in the system

When you want to search or locate a specific file in your system, the command's syntax is as follows.

 ls -l <đường dẫn của thư mục muốn tìm kiếm> | grep -i 'tên file hoặc thư mục' 

Need to put the word you want to find in quotation marks if it contains spaces. Assuming you are looking for 'network daemon' , the command will look like the following.

 ls -l /etc/network/ | grep -i 'interfaces daemon' 

Search for a complete word with grep

You may notice, grep has returned all sorts of results with the word 'network' , for example networks, networked, networking or abcnetworking, etc. If you want to limit your search to include specific words If so, you must use the -w option as follows.

 ls -l /etc/ | grep -i -w network 

Search for a specific piece of text in a file

Suppose you have a large file and you want to search for a specific piece of text. The syntax of the command will be as follows.

 grep – i 'văn bản bạn muốn tìm kiếm' 'tên file và đường dẫn' 

Do a search in multiple files

If you want to search for a document from a large number of files and subfolders in a large directory, you can do this using the -r option .

 grep -i -r "fox" 

Here is a sample output showing the word 'fox' included in both test.txt and tree.txt :

You can also provide a directory path and it will search all the files in that directory and its subfolders.

Suppose you want to find 'interfaces' in / etc / and its subfolders. The command should be as follows.

 grep -i -r interfaces /etc/ 

Here is the sample output:

Search for two different words with a single grep command

You can search for two different words with an egrep command (a variant of grep ) as follows. Let's say you want to search for fox and lazy words in multiple files with the -r option . You must run the following command on the terminal.

 egrep -w -r "fox|lazy" 

Here is the sample output:

Number the lines that fit the text

Another useful option is -n numbering of lines of text. The following is an example illustrating how to use the -n option .

 grep -i -n "fox" test.txt 

The following is a sample output for the numbering of lines matching the word 'fox' .

Reverse search

This is in contrast to what we did above. If you want to return a text that does not include the word you specify, you can use the -v option .

The following is an example that illustrates the use of the -v option .

 grep -v -i "fox" test.txt 

Here is the sample output:

All of the above options ( -n , etc.) can also be applied with the -v option .

Count matches

If you want to count the number of results that match a specific text, you can use the -c option .

Count the word 'fox' in test.txt inside the current directory. Run the following command in the terminal:

 grep -i -c fox test.txt 

Below is a sample output after executing the above command, showing that the word 'fox' has appeared 3 times in the test.txt file .

Displays file names that match specific text

If you want to find files containing your specific words, you can use the -l option with -r as follows.

Assuming all the files are located in the current directory and the specific word you are looking for is 'fox'.

 grep -i -r -l fox 

Here is a sample output showing the word 'fox' is present in test.txt , as well as in the subdirectory and asif.txt file:

Displays file names that match specific text

Show only matching text

By default, grep displays entire lines that match the text or words you want. If you want grep to display words that match, you can use the -o option as follows.

 grep -i -o fox test.txt 

Here is a sample output.

Display lines starting with specific word (s)

If you want to retrieve all lines starting with a specific word, you can use the ^ operator as follows.

Suppose you want to return all lines beginning with 'unix' and the file is log.txt located in the current directory.

Run the following command in the terminal:

 grep -i "^unix" log.txt 

Here is the sample output:

Display lines that end with specific word (s)

If you want to return all lines from a file ending with a specific word (s), you can use the $ operator as follows.

Assuming the word is 'linux' and the file you want to search for is rev.txt located in the current directory.

Run the following command:

 grep -i "linux$" rev.txt 

Here is the sample output: