GREP command is one of the most frequently used UNIX
command stands for "Global Regular Expression Print” .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.
Example 1
Finding relevant word and
exclusion of irrelevant word is the most feature of grep command. 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 operating system
UNIX operating system
UNIX and Linux operating system
Linux operating 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
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
UNIX operating system
Example 2
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
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
egrep 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
If you want to do case insensitive search than use -i option from
grep command in UNIX. Grep -i 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
Use grep -w command in
UNIX if you find whole word instead of just pattern.
grep -w ERROR logfile
Example 7
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 8
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.
