Sunday, December 11, 2016

Oracle Linux – short tip #4 - grep invert-match

When working and scripting with Oracle Linux you will use grep at one point in time. Grep searches the input for lines containing a match to the given pattern. By default, grep prints the matching lines.Whenever trying to find something in a file you might want to use a combination of cat and pipe the output of cat to grep to show you only the matching lines. And, as stated grep will show you only the lines that match the given pattern.

Lets assume we have the below file with some random data in it as an example;

[root@localhost tmp]# cat example.txt
#this is a file with some example content

line 1 - abc
line 2 - def

line 3 - ghi
jkl
mno

pqr

#and this is the end of the file
[root@localhost tmp]#

A simple case would be to show you some specific content, for example "abc" and print this to the screen which can be done using the below command;

[root@localhost tmp]# cat example.txt | grep abc
line 1 - abc
[root@localhost tmp]#

Or we could print all lines containing "line" as shown below;

[root@localhost tmp]# cat example.txt | grep line
line 1 - abc
line 2 - def
line 3 - ghi
[root@localhost tmp]#

However, this is only an example showing you how to show lines that match. Something less commonly used is a invert match, showing you all the lines that do NOT match the pattern defined. The invert match can be done using the -v option in grep.

As an example we might want to remove all the lines starting with "#" from the output. This might be very useful when for example trying to quickly read a configuration file which contains a lot of examples. For example, if you try to read the configuration file from the Apache webserver most of the httpd.conf file is examples and comments you might not be interested in and you would like to remove that from the output to quickly see what the actual active configuration is. Below is an example where we use the invert match option from grep to remove those lines from the output;

[root@localhost tmp]# cat example.txt | grep -v '^#'

line 1 - abc
line 2 - def

line 3 - ghi
jkl
mno

pqr

[root@localhost tmp]#

Even though this is already helping a bit, we might also want to remove the empty lines, to make the example more readible we show it in a couple of steps. First step is the cat, second step is the invert match on lines starting with "#" and the third step ie the invert match on empty lines;

[root@localhost tmp]# cat example.txt | grep -v '^#'|  grep -v '^$'
line 1 - abc
line 2 - def
line 3 - ghi
jkl
mno
pqr
[root@localhost tmp]#

As you might already be well up to speed in using grep and using it to match all kinds of output this means that the learning curve for using invert match is nearly zero. It is just a matter of using the -v option in grep to exclude things instead of using the include match option which is the common behavior of grep. the grep command is standard available in Oracle Linux and in almost every other Linux distribution. 
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