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Grep Command In Unix/Linux with 25+ Examples [2020]

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Let’s Learn Grep Command in Unix/Linux with Simple Examples :

New Linux users get anxious when confronted with the prospect of searching for a particular string in a file. Some have no idea what to or where to start. Here is an article to make your work a little easier. grep command in unix/linux is your rescue!

GREP  is a powerful tool Linux System Administrators use to search for specific file patterns. If by any chance your distribution does not have it installed you can use the following command to install it:

On Debian or Ubuntu

On RHEL, CentOS

On Fedora 22 and later

GREP derives its name from a Linux text editor ed, which uses the similar search operation written as g/re/p. Grep also stands for global regular expression print.

Grep command in unix/linux Syntax

The command line syntax used for grep take different forms but here are some of the few command line structures

grep [OPTIONS] PATTERN [FILE…]

The command searches for a particular pattern that matches the expression against a text file or a stream of input. However, it is important to note the three other variants of the GREP command in unix/linux :

• egrep, which also takes the form of grep –E
• fgrep, which takes the form of grep –F
• rgrep, which takes the form of grep –r

Much as the three variations are no longer supported, it allows for applications that use them to continue running without any modifications. The best way to learn about GREP is by using grep command with examples.

1. Search and Find Files

When you are looking for a file or a program that you need to ascertain version, GREP will come to your rescue. The primary usage of the command should give the output in a specified format.

The command displays string Linux if found in the file index.html

2. Searching strings with case insensitive option

If you want to display all string literals with the same name regardless of whether they are case sensitive or not, you add the –i flag as follows

The result will give an output like LINUX, Linux, or Linux.

3. Searching for a string in different files

Grep can also search for a string in multiple files using a single command line. The files in question can have different file extensions. For instance, the command can scan through files with a .txt, .php, or an .html extension

4. Search multiple files using the wildcard

When looking for a search string in multiple files of the same format, the wildcard is used in place of the file name separated by the file extension. For example, instead of using index.html on the command line, we use an asterisk (*) and the file extension.

The extension after the asterisk can be any file format. The above command displays the word Linux found on all html files.

5. Search and filter files

The command can be used to search and output files in a given format. If for instance, you want to troubleshoot your samba configuration file, found on the following path /etc/samba/smb.conf. You can output all the lines in this file by leaving out the commented lined by running

The –v option tells the command to print out all lines in the smb.conf file that do not match the criteria, in our case (print all lines that are not commented)

6. Display the number of lines before and after the search string

Different options are available and can be used in the format –A and –B as options. The two options display the matching line and the number of lines before and after the string.

The command uses the ifconfig command to display the Ethernet cable (eth0) configurations then redirect the result to the grep using the –A option to 4 lines coming after eth0. The second command using the –B option displays 2 lines after eth0.

7. Print the number of lines around a matching string

The grep –C option works the same way as the above command, but it will print the given number of lines before and after the expression.

This command will print out four lines before and after the loopback device lo

8. Count the number of matches to a particular query

The grep’s inbuilt command option allows you to get all the matches that are specific to what you are looking for. If you want to see all the interfaces on your machine use grep in this format

This command displays all IPv4 and IPv6 network interfaces

9. Display the file line number

You can use this command to display line numbers with a matching string when using the –n option. This is useful for debugging compilation errors

This line will show which line number “word” is found in the file.txt file.

10. Recursive search

When you are looking for a specific string in the current directory alongside all its subdirectories, using the –r option allows for recursive search.

The string function in all subdirectory of the current directory will be displayed.

11. Using grep command in unix/linux to search full words only

If you are looking for a specific word that may also be part of another sentence, you can use grep to force a selection that only forms the complete word.

The search will narrow down to the string “bio” and not biochemistry, biology, or biomedicine, etc. if such expressions exist in FileName.

12. Fast Grep (grep -F or fgrep)

This is also known as ‘fast grep’ or fixed string. It’s referred to as fast grep because it boasts of a better performance compared to grep and egrep. It drops regular expressions and looks for a distinct string pattern. It comes in handy when searching for unique static content in a precise and concise manner.

Usage

OR

For instance, file1.txt contains the following lines

To search the pattern “one and only” run

This will display Lines 1 and 3 only because they contain the search string “one and only”

Output

-b flag specifies the block number

Example

Output

-c flag counts the number of lines containing instances of the string pattern

For example

Output

13. Grep -o

Back to our file1.txt, If you want only to display the search string “one and only” run

Output

A similar command that gives the same output is

14. Search a string in Zipped file formats

Sometimes you may be forced to look for strings in a zipped document. Grep uses the following derivatives to handle such cases, the zgrep, zcat, and gzipped. All the derivatives take and use the same options as grep.

The output here will be the word error on the syslog.2.gz files.

15. Matching regular expression

This powerful feature helps when searching for all patterns that start and end with a particular string. For example to look for a file with a start line “start and ends with “ends” strings with anything in between the two strings use:

The result will be any file with the first and last lines being start and ends. Other options that can be used when looking for matching expressions are as listed below:

• ? The preceding item is optional and matched at most once.
• The preceding item will be matched zero or more times.
• + The preceding item will be matched one or more times.
• {n} The preceding item is matched exactly n times.
• {n,} The preceding item is matched n or more times.
• {,m} The preceding item is matched at most m times.
• {n,m} The preceding item is matched at least n times, but not more than m times.

16. Show the position of the matching string

Grep can be used to show line position where the particular match is found using this format, assuming we already have our file.txt with some content such as ABCDEFGHIJKL

Output

The output above is not the actual line number but the byte offset which counts from 0.

17. Show file names with matching string pattern

We use the grep –l option to show the given pattern. When dealing with multiple files as input with the grep command in unix/linux. This is very useful when looking for some particular notes in your directory.

This command will list all files with “string “on it.

18. Counting the number of matches

When there is a need to count the number of lines matching the string pattern use the –c option.

When you add the option –v to the above command, it will give the number of lines that do not match a given pattern.

Show the number of lines that do not match all given patterns on the command line. For example, a file with the strings ABCDEFGHIJKL when issued with the command below:

This returns BCDFHIJKL
This command can further be modified to give case-insensitive results using the –vi option.

19. Highlight search Strings

The option to highlight search output is made possible when you use the –color option. In some cases, you may have to set the environment variables GREP_OPTIONS as indicated below

The “color” attribute can take any color codes supported by the system.
The command is used as follows:

All the results (Linux in this case) searches will be highlighted in a different color.

20. Display all lines ending with a specific pattern

The $ symbol is used by appending it to the search string on the command line as shown:

All lines in the passwd file ending with “string” will be shown.

21. Display lines starting with a specified string

The ^ symbol is appended to the string to give the desired output

The caret (^) symbol indicates the beginning of the line when used in Linux commands. The output will be all lines starting with the word “string.”

22. Filter specific files

Grep can also be used when you need to get particular files as your output. If for instance, your folder has many images in different formats and you only need a list of all the GIFs from a specified image folder, you can use the find command and channel the output through grep

The above command outputs all gif files piped to grep that will filter out specific images from FolderName and then re-directed through the –vi option to filter out file names “string.”

23. Using Grep and Pipes to Direct Output

If you want to see the devices connected to your machine, you can use grep and pipes in a different format as shown below

The dmesg command displays drive the message or info that matches the drive names given in ‘(s|h)d[a-z]’

24. Display the CPU model name

The cat command is used to create or view files. The  command below uses cat to view cpuinfo and later filter the output using grep where ‘String’ is the search term

# cat /proc/cpuinfo | grep -i ‘String’

For example, to display all attributes in /proc/cpuinfo with ‘cpu’ execute

Output

25.  To confirm if PATH is exported

If you want to confirm if PATH has successfully been exported to ~/.bashrc we use the command export that will search for lines with the string PATH as follows:

# grep export ~/.bashrc | grep ‘\<PATH’

The Common Linux Pipes used by Grep

Grep commands are in most cases used alongside shell pipes used to connect one command output to another without the need of a temporary file. The pipe syntax takes the form:

Command 1 | command 2 or command 1 | command 2 | command N…

In most cases, such redirection is simply interpreted as

“get data” | “verify data” | “process data” | “format data” > OutputData.file

The connection between one command to the next is referred to as a pipe symbolized by a vertical (|) bar. The command direction takes one direction only.

Using GREP Environment Variables

Environmental variables control the GREP behavior. Some of these variables are picked automatically upon installation of the operating system while others have to be defined to suit a particular environment.

GREP_COLOUR
Specifies the color used when the highlight command is invoked. Using GREP_COLORS too also has the same effect that also supports many options

GREP_OPTIONS                                                                                                        This option gives you an option of specifying default options for a particular session or throughout the system. The defined option will override any explicit option.

Conclusion

This article is not the end of grep command in Unix/Linux, we tried as much as possible to include the most common commands you are likely to use. The important thing is to read and understand the command usage and adapt it to serve your purposes. If you are on your command line type grep –-help for more options and usage.

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