10 examples of grep command in UNIX and Linux (original) (raw)

Hello guys, if you want to learn the grep command in Linux and looking for some awesome, hands-on, easy to use tutorial then you have come to the right place. Earlier, I have shared many examples based tutorials to learn essential Linux commands like find, lsof, curl, sed, and chmod command and in this article, I am going to share 10 examples of grep command in Linux. The grep command is one of the most frequently used UNIX command stands for "Global Regular Expression Print" along with find, chmod, and tar command in UNIX. grep command in Unix operating systems like Linux, Solaris, BSD, Ubuntu, or IBM AIX is used to search files for matching patterns.

By using the grep command in Unix you can search a file that contains a particular word or particular pattern. UNIX grep command also provides several useful command-line options that can be used to enhance the functionality of the grep command.

Similarly, by using grep -v you can list down all files which don't contain a word I mean excluding files which match a pattern, grep -c will print the count of matching pattern in a file, etc. One of the popular examples of the grep command is to find empty files and directories in Unix.

This grep command tutorial is not about the theory of UNIX grep but the practical use of grep command in UNIX and here I am sharing my experience on the use of grep command in Linux with an aim that this would serve as a quick guide or tutorial for using grep in UNIX for new beginners and help them to understand the grep command better and its thoughtful usage in UNIX or Linux.

Many people use grep just for finding words in a file and missed the real potential of grep by not using all its powerful command-line options and its regular expression capability which could not only save a lot of time but also works as a great and powerful tool while analyzing a large set of data or log files.

By the way, if you are new to Linux then I also suggest you go through a comprehensive Linux course to learn some basics commands and fundamentals like Linux file system, permissions, and other basic things.

10 ways to use Grep command in Unix - examples

The following examples of grep command in UNIX are based on my experience and I use them on a daily basis in my work. Grep command is also part of any beginner's UNIX command tutorial as it is an essential command to learn in order to work efficiently in any UNIX environment like Redhat Linux, Ubuntu, IBM AIX, Oracle Solaris, or BSD.

Anyway, these examples are by no means complete so please contribute you're own grep command tips or how you are using grep in Linux to make it more useful and allow all of us to benefit from each other's experience and work efficiently in UNIX or Linux.

Example 1: How to ignore some words while doing a search using grep in UNIX

Finding relevant words and exclusion of irrelevant words. Most of the time I look for Exception and Errors in log files and sometimes I know certain Exception I can ignore so I use the grep -v option to exclude those Exceptions

grep Exception logfile.txt | grep -v ERROR

This grep command example will search for the word "Exception" in logfile.txt and print them but since we have piped out of the first grep command to the second grep command which will exclude all lines which match the word "ERROR".

To make this grep example more concrete let's see another example, here we have a file which contains three lines as shown below:

$ cat example.txt UNIX operating system UNIX and Linux operating system Linux operation system

Now we want to search all lines in file example.txt which contains the word UNIX but at the same time doesn't contain the word Linux.

$ grep UNIX example.txt UNIX operating system UNIX and Linux operating system

Now to exclude all lines which contain Linux we will apply another grep command in this output with option -v to exclude matching word as shown in below grep command:

$ grep UNIX example.txt | grep -v Linux UNIX operating system

You can see how useful the grep -v command option can be.

Example 2: How to count the occurrence of a word in a file using the grep command

If you want to count on a particular word in the log file you can use the grep -c option to count the word. Below an example of a command will print how many times the word "Error" has appeared in logfile.txt.

$ grep -c "Error" logfile.txt

If we apply this grep command on our example file to find how many lines contain word e.g. UNIX has occurred in the file:

$ grep -c UNIX example.txt 2

Example 3: printing lines before and after of matching word using grep

Sometimes we are not just interested in matching lines but also on lines around matching lines particularly useful to see what happens before any Error or Exception. grep --context option allows us to print lines around matching pattern.

Below an example of grep command in UNIX will print 6 lines around the matching line of word "successful" in logfile.txt

$ grep --context=6 successful logfile.txt

Show an additional six lines after matching very useful to see what is around and to print the whole message if it splits around multiple lines. You can also use command-line option "C" instead of "--context" for example

$ grep -C 2 'hello' *

Prints two lines of context around each matching line.

Example 4: How to search pattern using egrep and regular expression

stands for extended grep and it is more powerful than grep command in Unix and allows more regular expression like you can use "|" option to search for either Error or Exception by executing just one command.

$ egrep 'Error|Exception' logfile.txt

Example 5: How to do case-insensitive searching using grep in Linux

If you want to do a case insensitive search then use -i option from the grep command in UNIX. grep -i command will find any occurrence of both Error, error, and ERROR and quite useful to display any sort of Error from the log file.

$ grep -i Error logfile

Example 6: How to search patterns in gzip files using the zgrep command

zgrep is another great version of grep command in Unix which is used to perform the same operation as grep does but with .gz files. Many times we gzip the old file to reduce the size and later want to look or find something on those files. zgrep is your man for those days.

The below grep command will print all files which have "Error" on them.

$ zgrep -i Error *.gz

Example 7: How to do a recursive search in a directory using grep in UNIX

If you want to do a recursive search using the grep command in Unix there are two options either use "-R" command-line option or increase directory one by one as shown below.

$ grep -R store *

This command will search for a directory or file with the name stored in the current directory and it's all sub-directory. If you want to learn more you can also see this recursive grep example from my other article.

Example 8: UNIX command to display files names which contain given word

Another useful grep command-line option is "grep -l" which displays only the file names which match the given pattern. Below command will only display file names that have ERROR?

$ grep -l ERROR *.log

grep -l 'main' *.java will list the names of all Java files in the current directory whose contents mention `main'.

Also, find command in UNIX can be used in place of grep in many places.

Example 9: grep command option to display lines numbers

If you want to see line number of matching lines you can use the option "grep -n" below command will show on which lines Error has appeared.

$ grep -n ERROR log file.

Example 10: How to search the whole word in a file using the grep command

You can use the grep -w command in UNIX to find the whole word instead of a just pattern, as shown in the following example. This example will only print lines from logfile.txt which contains full word ERROR.

$ grep -w ERROR logfile.txt

Above grep command in UNIX searches only for instances of 'ERROR' that are entire words; it does not match `SysERROR'.

For more control, use `\<' and `\>' to match the start and end of words. For example:

$ grep 'ERROR>' *

Searches only for words ending in 'ERROR', so it matches the word `SysERROR'.

Bonus GREP Examples

Now I have two bonus examples of grep command in UNIX:

1. grep command in UNIX can show a matching pattern in color which is quite useful to highlight the matching section, to see matching pattern in color use below command.

$ grep Exception today.log --color

You can also create alias grep='grep --color' in your bash_profile file to avoid typing --color every time.

1. There are three versions of grep command in UNIX "grep, fgrep, and egrep". `fgrep' stands for Fixed `grep', `egrep' Extended `grep'

And, here is a nice summary of all the grep commands you have learned so far. You can take the print out of this slide to keep these grep commands handy.

These examples of grep command in UNIX are something that I use on a daily basis; I have seen the most sophisticated use of grep with a regular expression. I will list some more examples of grep command in UNIX as I come across and find it useful to share.

As per my experience having a good hold on grep and UNIX find command with knowledge of regular expression will be great for your day to day life if you need to look at log files or config files or need to do production support on electronic trading systems or any other kind of system which is running on UNIX.

This list of grep command in UNIX is by no means complete and I look forward to you guys sharing how you are using the grep command in UNIX.