Grep – A Unix/Linux Command with Examples

Grep is a powerful tool for searching a text, Grep means “Global regular expression print”. Basically, Grep searches a text file for the specified regular expression and output any line containing a match to standard output. So Grep command can be used to search some kind of text, word, pattern or sentence in a text file or a number of text files.

• Suggested Read: 12 Awesome Linux Find Command Examples

In this tutorial, we will show you how to use Grep command with some practical examples.

Prerequisites

• A system running the Linux operating system.
• Access to a terminal/command line.

To Search a Specific String in a File

• To search for a string tecadmin in the file file1.txt, run the following command:

  grep tecadmin file1.txt
  

  This command will print all lines that contain a word tecadmin:

  tecadmin is a popular linux blog
  i love tecadmin
  tecadminlinux
  

  

• To search for an exact string tecadmin in the file file1.txt, run the following command:

  grep -w tecadmin file1.txt
  

  This command will print all lines that contain a whole word tecadmin:

  tecadmin is a popular linux blog
  i love tecadmin
  

  

• To search for a string tecadmin case insensitively in the file file1.txt, run the following command:

  grep -i tecadmin file1.txt
  

  This command will print all lines that contain a word tecadmin case insensitively:

  tecadmin is a popular linux blog
  i love tecadmin
  tecadminlinux
  Tecadmin is my favourite blog
  This is Tecadmin website
  

  

To Search a Specific String in Multiple File

• To search for a string linux in file file1.txt and file2.txt, run the following command:

  grep -i linux file1.txt file2.txt
  

  This command will print all lines that contain a word linux in file1.txt and file2.txt:

  file1.txt:tecadmin is a popular linux blog
  file1.txt:tecadminlinux
  file2.txt:Linux is an open-source operating system.
  file2.txt:linux is made by linus torvalds.
  file2.txt:linux is most popular operating system.
  

  

• To search for a string linux in all files in the current directory and sub-directories, run the following command:

  grep -r linux *
  

  This command will print all lines that contain a word linux in all files in the current directory and sub-directories:

  file1.txt:tecadmin is a popular linux blog
  file1.txt:tecadminlinux
  file2.txt:linux is made by linus torvalds.
  file2.txt:linux is most popular operating system.
  file3.txt:linux vs windows
  file3.txt:Ubuntu is a linux operating system
  

  

Inverse Grep Search

You can use grep command with -v option to print all lines that do not match a specific pattern of characters.

For example, print all lines that don’t contain the string linux in file1.txt and file2.txt, run the following command:

grep -v linux file1.txt file2.txt

This command will exclude all the lines that contain the string linux:

file1.txt:i love tecadmin
file1.txt:Tecadmin is my favourite blog
file1.txt:This is Tecadmin website
file2.txt:Linux is an open-source operating system.

To List Filenames That Matches Specific Pattern

You can display only filenames that contain a specific string using -l option.

For example, list all filenames in the current directory that matches the string tecadmin, run the following command:

grep -l tecadmin *

You should see the following output:

file1.txt

Display the Number of Matches

You can use grep with -c option to display all files with the number of lines that matches the given string.

For example, to display all files with the number of lines that matches a string linux in the current directory, run the following command:

grep -c linux *

You should see the following output:

file1.txt:2
file2.txt:2
file3.txt:2

Display Line Number with Matching Pattern

You can use grep with -n option to print line numbers with matching patterns.

For example, to display line number that matches the pattern linux in the current directory, run the following command:

grep -n linux * 

You should see the following output:

file1.txt:1:tecadmin is a popular linux blog
file1.txt:3:tecadminlinux
file2.txt:2:linux is made by linus torvalds.
file2.txt:3:linux is most popular operating system.
file3.txt:1:linux vs windows
file3.txt:2:Ubuntu is a linux operating system

You can also display one line before and after the matching string using the option c and n with grep command.

For example, display one line before and after the matching string linux in file4.txt, run the following command:

grep -n -C 1 linux file4.txt

You should see the following output:

1-Hi, i am tecadmin user
2:i am linux operating system
3-i am windows operating system

Display Only Matching Pattern

By default, grep command prints the entire line which matches a pattern.

You can print only matching patterns using the -o option.

For example, search for file1.txt that matches the string/pattern linux with the following command:

grep -o linux file1.txt 

You should see the following output:

linux
linux

Conclusion

In the above tutorial, you learned how to use grep command to search for a specific string in files. I hope you have now enough knowledge of grep command and how it is used in various conditions.