grep command in Unix/Linux

The grep command is one of the most useful tools in Linux and Unix systems. It is used to search for specific words, phrases, or patterns inside text files, and shows the matching lines on your screen.

Syntax of grep Command in Unix/Linux

The basic syntax of the `grep` command is as follows:

grep [options] pattern [files]

• [options]: These are command-line flags that modify the behavior of grep. 
• [pattern]: This is the regular expression you want to search for.
• [file]: This is the name of the file(s) you want to search within. You can specify multiple files for simultaneous searching.

Example of grep Command in Linux

grep Command is useful when you need to quickly find certain keywords or phrases in logs or documents.

Example 1: Search for a word in a file

If you have a file called notes.txt and you want to find all lines containing the word Python, you can use:

grep "python" notes.txt

Here's the output:

Example 2: Search recursively in all files of a directory

If you want to search for the word error in all files under the /var/log directory, you can run:

grep -r "error" /var/log

Note: If you are not logged in as the root user, you may need to use sudo to access protected log files, like this:

Here, -r tells grep to search recursively, meaning it will look inside all subfolders too.

Options in grep Command in Linux

Its name comes from an old editor command: g/re/p, which stands for “globally search for a regular expression and print. Here are some use cases of grep command:

1. Case insensitive search

The -i option enables to search for a string case insensitively in the given file. It matches the words like "UNIX", "Unix", "unix". 

grep -i "UNix" geekfile.txt

Output: 

2. Displaying the Count Matches Using grep

We can find the number of lines that matches the given string/pattern 

grep -c "unix" geekfile.txt

Output: 

3. Display the Matching Filenames Using grep

We can just display the files that contains the given string/pattern. 

grep -l "unix" *

or

grep -l "unix" f1.txt f2.txt f3.xt f4.txt

Output: 

4. Checking Whole Words Using grep

By default, grep matches the given string/pattern even if it is found as a substring in a file. The -w option to grep makes it match only the whole words. 

grep -w "unix" geekfile.txt

Output: 

5. Display Matched Pattern Using grep

By default, grep displays the entire line which has the matched string. We can make the grep to display only the matched string by using the -o option. 

grep -o "unix" geekfile.txt

Output: 

6. Show Line Numbers with grep -n

To show the line number of file with the line matched. 

grep -n "unix" geekfile.txt

Output: 

7. Inverting the Pattern Match Using grep

You can display the lines that are not matched with the specified search string pattern using the -v option. 

grep -v "unix" geekfile.txt

Output: 

8. Match Lines Starting with a String using grep

The ^ regular expression pattern specifies the start of a line. This can be used in grep to match the lines which start with the given string or pattern. 

grep "^unix" geekfile.txt

Output: 

9. Match Lines Ending with a String using grep

The $ regular expression pattern specifies the end of a line. This can be used in grep to match the lines which end with the given string or pattern. 

grep "os$" geekfile.txt

10. Specifies expression with -e option

Can use multiple times : 

grep –e "Agarwal" –e "Aggarwal" –e "Agrawal" geekfile.txt

11. Use -f to Read Patterns from a File

It is used to search for multiple patterns, listed in a separate file, within another target file.

cat pattern.txt

Agarwal
Aggarwal
Agrawal

grep –f pattern.txt  geekfile.txt

12. Print Specific Lines from a File with grep

-A prints the searched line and n lines after the result, -B prints the searched line and n lines before the result, and -C prints the searched line and n lines after and before the result. 

Syntax:

grep -A[NumberOfLines(n)] [search] [file]  
grep -B[NumberOfLines(n)] [search] [file]  
grep -C[NumberOfLines(n)] [search] [file]  

Example:

grep -A1 learn geekfile.txt

Output:  

Conclusion

In this article we discussed the grep command in Linux which is a powerful text-search tool that uses regular expressions to find patterns or text within files. It offers various options like case insensitivity, counting matches, and listing file names. With the ability to search recursively, use regular expression flags, and customize output, grep is a vital tool for Linux users to efficiently handle text-related tasks. Mastering grep enhances your ability to work with text data in the Linux environment.