grep command is used to search files for a specific text. This is incredibly powerful command with lots of options.
Syntax: grep [options] pattern [files]
How can I find all lines matching a specific keyword on a file?
In this example, grep looks for the text John inside /etc/passwd file and displays all the matching lines.
# grep John /etc/passwd jsmith:x:1082:1082:John Smith:/home/jsmith:/bin/bash jdoe:x:1083:1083:John Doe:/home/jdoe:/bin/bash
Option -v, will display all the lines except the match. In the example below, it displays all the records from /etc/password that doesn’t match John.
Note: There are several lines in the /etc/password that doesn’t contain the word John. Only the first line of the output is shown below.
# grep -v John /etc/passwd jbourne:x:1084:1084:Jason Bourne:/home/jbourne:/bin/bash
How many lines matched the text pattern in a particular file?
In the example below, it displays the total number of lines that contains the text John in /etc/passwd file.
# grep -c John /etc/passwd 2
You can also get the total number of lines that did not match the specific pattern by passing option -cv.
# grep -cv John /etc/passwd 39
How to search a text by ignoring the case?
Pass the option -i (ignore case), which will ignore the case while searching.
# grep -i john /etc/passwd jsmith:x:1082:1082:John Smith:/home/jsmith:/bin/bash jdoe:x:1083:1083:John Doe:/home/jdoe:/bin/bash
How do I search all subdirectories for a text matching a specific pattern?
Use option -r (recursive) for this purpose. In the example below, it will search for the text “John” by ignoring the case inside all the subdirectories under /home/users.
This will display the output in the format of “filename: line that matching the pattern”. You can also pass the option -l, which will display only the name of the file that matches the pattern.
# grep -ri john /home/users /home/users/subdir1/letter.txt:John, Thanks for your contribution. /home/users/name_list.txt:John Smith /home/users/name_list.txt:John Doe # grep -ril john /root /home/users/subdir1/letter.txt /home/users/name_list.txt
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Hi,
Another tip ;), grep is very slow on UTF-8. Use “LANG=C grep …” to increase speed.
$ echo $LANG
en_US.UTF-8
$ time grep ‘^….’ /usr/share/dict/words >/dev/null
real 2m16.795s
user 2m10.536s
sys 0m0.087s
$ export LANG=C
$ time grep ‘^….’ /usr/share/dict/words >/dev/null
real 0m0.031s
user 0m0.028s
sys 0m0.003s
Source: http://rg03.wordpress.com/2009/09/09/gnu-grep-is-slow-on-utf-8/
hi ,
what is d dirrerence between
a. grep “John Doe” my-file > /dev/null 2>&1
b. grep “John Doe” my-file 2>&1 > /dev/null
i have a file which contains different combination of numbers
eg :
1 2 3 4 5 6
1 2 3 4
1 2 3 4 5 6 7 8 9
i want to seach 1 2 3 4 5 6, and want to get all the results which contain these 6 numbers and also 1 2 3 4 should show in my results
@Dhanesh:
you need to use ‘egrep’ command, such as:
$ cat /tmp/temp123
1 2 3 4 5 6
1 2 3 4
1 2 3 4 5 6 7 8 9
$ egrep ‘^[1-6 ]*$’ /tmp/temp123
1 2 3 4 5 6
1 2 3 4
‘^[1-6 ]*$’ – it’s mean – grep all lines, where from start (^) to the end ($) of line, only space and numbers from 1 till 6. But this grep is not strict to unique sets of numbers, it will find all lines with repeating numbers, with different formats and so on…
Such as:
1234
111111
1 4