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In Linux and Unix Systems Grep, short for “global regular expression print”, is a command used in searching and matching text files contained in the regular expressions. Furthermore, the command comes pre-installed in every Linux distribution. In this guide, we will look at Common grep command usage with a few examples.
Grep command can be used to find or search a regular expression or a string in a text file. To demonstrate this, let’s create a text file welcome.txt and add some content as shown.
Welcome to Linux ! Linux is a free and opensource Operating system that is mostly used by developers and in production servers for hosting crucial components such as web and database servers. Linux has also made a name for itself in PCs. Beginners looking to experiment with Linux can get started with friendlier linux distributions such as Ubuntu, Mint, Fedora and Elementary OS.
Great! Now we are ready to perform a few grep commands and manipulate the output to get the desired results. To search for a string in a file, run the command below Syntax
$ grep "string" file name
$ filename grep "string"
$ grep "Linux" welcome.txt
Output As you can see, grep has not only searched and matched the string “Linux” but has also printed the lines in which the string appears. If the file is located in a different file path, be sure to specify the file path as shown below
$ grep "string" /path/to/file
If you are working on a system that doesn’t display the search string or pattern in a different color from the rest of the text, use the
--color to make your results stand out. Example
$ grep --color "free and opensource" welcome.txt
If you wish to search for a string in your current directory and all other subdirectories, search using the
- r flag as shown
$ grep -r "string-name" *
$ grep -r "linux" *
In the above example, our search results gave us what we wanted because the string “Linux” was specified in Uppercase and also exists in the file in Uppercase. Now let’s try and search for the string in lowercase.
$ grep "linux" file name
Nothing from the output, right? This is because grepping could not find and match the string “linux” since the first letter is Lowercase. To ignore case sensitivity, use the
-i flag and execute the command below
$ grep -i "linux" welcome.txt
Output Awesome isn’t’ it? The
- i is normally used to display strings regardless of their case sensitivity.
To count the total number of lines where the string pattern appears or resides, execute the command below
$ grep -c "Linux" welcome.txt
To invert the Grep output , use the
-v flag. The
-v option instructs grep to print all lines that do not contain or match the expression. The –v option tells grep to invert its output, meaning that instead of printing matching lines, do the opposite and print all of the lines that don’t match the expression. Going back to our file, let us display the line numbers as shown. Hit ESC on Vim editor, type a full colon followed by
Next, press Enter Output Now, to display the lines that don’t contain the string “Linux” run
$ grep -v "Linux" welcome.txt
Output As you can see, grep has displayed the lines that do not contain the search pattern.
To number the lines where the string pattern is matched , use the
-n option as shown
$ grep -n "Linux" welcome.txt
-w flag will search for the line containing the exact matching word as shown
$ grep -w "opensource" welcome.txt
Output However, if you try
$ grep -w "open" welcome.txt
NO results will be returned because we are not searching for a pattern but an exact word!
The grep command can be used together with pipes for getting distinct output. For example, If you want to know if a certain package is installed in Ubuntu system execute
$ dpkg -L | grep "package-name"
For example, to find out if OpenSSH has been installed in your system pipe the
dpkg -l command to grep as shown
$ dpkg -L | grep -i "openssh"
You can use the -A or -B to dislay number of lines that either precede or come after the search string. The -A flag denotes the lines that come after the search string and -B prints the output that appears before the search string. For example
$ ifconfig | grep -A 4 ens3
This command displays the line containing the string plus 4 lines of text after the ens string in the
ifconfig command. Output Conversely, in the example below, the use of the -B flag will display the line containing the search string plus 3 lines of text before the ether string in the
ifconfig command. Output
$ ifconfig | grep -B 4 ether
The term REGEX is an acronym for REGular EXpression. A REGEX is a sequence of characters that is used to match a pattern. Below are a few examples:
^ Matches characters at the beginning of a line $ Matches characters at the end of a line "." Matches any character [a-z] Matches any characters between A and Z [^ ..] Matches anything apart from what is contained in the brackets
Example To print lines beginning with a certain character, the syntax is;
grep ^character file_name
For instance, to display the lines that begin with the letter “d” in our welcome.txt file, we would execute
$ grep ^d welcome.txt
Output To display lines that end with the letter ‘x’ run
$ grep x$ welcome.txt
If you need to learn more on Grep command usage, run the command below to get a sneak preview of other flags or options that you may use together with the command.
$ grep --help
Sample Output We appreciate your time for going through this tutorial. Feel free to try out the commands and let us know how it went.
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