GREP command Examples

GREP is a command line search utility or tool to filter the input given to it. Grep got its name from ed editor as g/re/p (global / regular expression / print).

In this post we will see how to use grep in detail and next post with the help of regular expressions.

1. GREP command syntax 
2. Basic regular expressions 
3. Extended regular expressions

1) GREP command syntax

# grep options search term filename
or
# command | grep options search term

Example1: Search for a word “ada” in file1.txt

grep ‘ada’ file1.txt

Example2: Search for a word which is either capital or small letters(i — case-insensitive )

grep -i ‘ac’ file1.txt

Example3: Inverce your search, search all words other than ace

grep -v ‘ace’ file1.txt

Example4: To get the count of a ared word occurrence.

grep -c ‘ared’ file1.txt

Example5: Find acd characters in a file and display the line numbers where grep finds acd

grep -n ‘acd’ file1.txt

Example6: Find exact word ac ie should find only ac not acd or reacf etc.

grep -w ‘ac’ file1.txt

Example7: Search for a word and print 2 lines Before to ths search

grep -B2 abc file1.txt

Example8: Search for a word and print 2 lines After to ths search

grep -A2 abc file1.txt

Example9: Search for a word and print 2 lines Before and after center to this search term

grep -C2 abc file1.txt

Example10: Search for term recursively in sub directories as well.

grep -r ‘abc’ /etc/

Example11: Search for a term and just list the files which contains it with in a directory.

grep -l ‘abc’ /etc/

2) Basic regular expressions:

^ -- Caret symbol, Match beginning of the line.

$ -- Match End of the line

* -- Match 0 or more occurrence of previous character

. – Match Any single character

[] – Match Range of characters, just single occurrence.

            [a-z] –Match small letters

            [A-Z] –Match cap letters

            [0-9] – Match numerical.

[^] – Match Negate a sequence

 -- Match Escape character.

If we learn any new thing with example, it will be there for long time in our mind.

Now we will see one example each regular expression what we given above.

Example1: Find all the lines which start with “Mr”

grep ‘^Mr’ filename

Example2: Find all the lines which ends with ‘sh’

grep ‘sh$’ filename

Example3: Display all the lines in a file expect empty lines.

grep –v ‘^$’ filename

Note:-v option is used to negate the search term, here ^$ indicates empty line, so our grep –v is filtering empty lines in the output.

Example4: Search for words which are bash, baash, bsh, baaash, baaaaash,

grep ‘ba*s’ filename

This will search for words which has a between b and s zero or more times.

Example5: Search for all words which starts with b and h

grep ‘b.*h’ filename

Example6: Search for a word which is having three letters in it and starts with x and ends with m.

grep ‘x[a-z]m’ filename

This search will return all the 3 letter words which start with x and ends with m.

Example7: Search words do not contain ‘ac’ in a file.

grep ‘[^ac]’ filename

Example8: Search for a ‘[‘ in a file

Note: The “[“is a special character, you cannot search with normal grep we have to use escape character () in order to negate it. So use ‘[‘ to search for [. This is applicable for all the special characters mention above.

grep ‘[’ filename

3) Extended regular expressions:

+ --Match one or more occurrences of previous character.

| -- Match Either character

? – Match 0 or 1 occurrence of previous character.

() –match a group of characters

{number} –Match number of occurrence of a character

{1, 3} –Match a character which is 1 to 3 times repetition

{5, } –Match a repeated character which is repeated 5 or more times.

Note1: In order to use this extended regular expressions we have to use –E option give grep the capability to understand Extended regular expressions.

Note2: egrep is nothing but grep –E, so try to avoid it if grep itself can do the work for you. Why to learn new command?

Example1:Search for a words which contains one or more occurrence of ‘b’ between a and c.

grep –E ‘ab+c’ filename

Example2: Search for a word which contains zero or one occurrence of b between a and c

grep –E ‘ab?c’ filename

Example3: Search for a word which contains either a or b, a and b between d, e characters

grep –E ‘da|be’ filename

Example4: Search for a word which contains either a or b, but not both a and b between d, e characters

grep –E ‘d(a|b)e’ filename

Example5: Search for a word which contains only 2 ‘b’ between a and c character

grep –E ‘ab{2}c’ filename

Example6: Search for a word which contains 3 to 4 ‘b’ between a and c character

grep –E ‘ab{2,4}c’ filename

Example7: Search for a word which contains 3 or more ‘b’ between a and c character

grep –E ‘ab{3, }c’ filename

Note: When we are using {} we have to give a range, but in this example we did not give range we just started the range but did not end the range which indicates infinity.