10 examples of grep command in UNIX and Linux

grep command is one of the most frequently used UNIX command stands for "Global Regular Expression Print" like find, chmod or tar command in Unix. grep command in Unix operating system e.g. Linux, Solaris, BSD, Ubuntu or IBM AIX is used to search files with matching patterns, by using grep command in Unix you can search a file which contains a particular word or particular pattern. UNIX grep command also provides several useful command line option which can be used to enhance functionality of grep command e.g. by using grep -v you can list down all files which doesn't contains a word i.e. excluding files which matches a pattern, grep -c will print count of matching pattern in a file etc. One of the popular example of grep command is to find empty files and directories in Unix. This grep command tutorial is not about theory of UNIX grep but practical use of grep command in UNIX and here I am sharing my experience on use of grep command in Linux with an aim that this would serve as 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 large set of data or log files. Also find command in UNIX can be used in place of grep at many places. If you want to leverage full potential of grep, then using Grep pocket reference is not a bad idea, an ideal grep reference for system admin, developers and security professionals

10 ways to use Grep command in Unix - examples

Following examples on grep command in UNIX are based on my experience and I use them on daily basis in my work. Grep command is also part of any beginners UNIX command tutorial as it is an essential command to learn in order to work efficiently in any UNIX environment e..g Redhat Linux, Ubuntu, IBM AIX, Oracle Solaris or BSD. Any way these examples are by no means complete so please contribute your 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 others experience and work efficiently in UNIX or Linux.

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

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

grep Exception logfile.txt | grep -v ERROR

This grep command example will search for word "Exception" in logfile.txt and print them but since we have piped out of first grep command to second grep command which will exclude all lines which match world "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 word UNIX but same time doesn't contain world Linux.

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

Now to exclude all lines which contains 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

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

If you want to count of a particular word in log file you can use grep -c option to count the word. Below example of command will print how many times 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 contains 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

Sometime we are not just interested on matching line 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 example of grep command in UNIX will print 6 lines around matching line of word "successful" in logfile.txt

$ grep --context=6 successful logfile.txt

Show additional six lines after matching very useful to see what is around and to print 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 exception 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 case insensitive search than use -i option from grep command in UNIX. grep -i command will find occurrence of both Error, error and ERROR and quite useful to display any sort of Error from log file.

$ grep -i Error logfile

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

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

$ zgrep -i Error *.gz

Example 7 : How to search whole word in a file using grep command

You can use grep -w command in UNIX to find whole word instead of just pattern, as shown in 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'.

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

Another useful grep command line option is "grep -l" which display only the file names which matches the given pattern. Below command will only display file names which 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'.

Example 9 : grep command option to display lines numbers

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

$ grep -n ERROR log file.

Example 10 : How to do recursive search in a directory using grep in UNIX

If you want to do recursive search using 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 directory or file with name store in current directory and it's all sub-directory.





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



11) grep command in UNIX can show matching patter 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.



12) There are three version of grep command in UNIX `grep, fgrep, egrep'. `fgrep' stands for Fixed `grep', `egrep' Extended `grep'





These examples of grep command in UNIX are something which I use on daily basis; I have seen more sophisticated use of grep with regular expression. I will list some more examples of grep command in UNIX as I come across and find useful to share. As per my experience having good hold on grep and UNIX find command with knowledge of regular expression will be great for you day to day life if you need to look 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 from you guys to share how you are using grep command in UNIX.

Enjoy.



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Source:http://javarevisited.blogspot.com/2011/06/10-examples-of-grep-command-in-unix-and.html