How to use grep command in Unix and Linux with examples

grep command Means – globally search regular expression. It is very useful while searching for strings in Unix  and Linux operating system. Here we would be taking  a look on grep command in Unix with examples,Linux grep examples,grep command options,egrep command in Unix

How to use grep command in Unix

The grep utility searches text file.txt for a pattern and prints all lines that contain that pattern.

Syntax: grep [ -options ] limited-regular-expression [filename … ]

grep options or grep command options

Unix grep command examples

(1) To find all uses of the word “top” (in any case) in the multiples file like x*, and write with line numbers(“grep -n”)

grep -i -n top x*
 
 

(2) search ‘tmpfile’ for ‘CAT’ anywhere in a line

grep CAT tmpfile
 
 

(3) grep case insensitive search. By default grep command is case sensitive. You can use the option “grep -i” to make it case insensitive. 

grep -i Ten test.txt

(4) find ‘run time’ or ‘run-time’ in all txt in file.txt

grep run[- ]time *.txt

(5) pipe who to grep, look for applmgr

who | grep applmgr

(6) grep recursive option(“grep -r”). Its searches for oracle string in current directory files and all the files in a subdirectory

grep -r "oracle" *
 
 

  (7) Grep exclude option (“grep -v”) . We can use grep -v to exclude the search item. It will not show the lines which have oracle string in it

ps -ef|grep -v oracle

(8) We can use the “grep -w” option for searching the specific work, not the sub-string. The below example  searches adpatch.log for word failure in any case

grep -w failure adpatch.log

(9) You can search multiple words in a file using “grep -e”

grep -e tom -e bob  -e bill file.txt

This can also be done like this also

grep 'tom\|bob\|bill\|'  file.txt

Understanding Regular Expressions:

grep regex examples

(1)search file.txt for lines with ‘kite’

grep kite file.txt

(2) ‘kite’ at the start of a line

grep '^kite' file.txt
 
 

(3) ‘kite’ at the end of line

grep 'kite$' 
 

(4)lines containing only ‘kite’

grep '^kite$'

(5)lines starting with ‘^s’, “\” escapes the ^

grep '\^s' file.txt

(6)Search for ‘kite’ or ‘Kite’

grep '[Kk]ite' file.txt
 

(7)search for TOM, Tom, TOm, or ToM

grep 'T[oO][mM]' file.txt
 

(8)search for blank lines

grep '^$'

(9)search for pairs of numeric digits

grep '[0-9][0-9]' file
 

(10)list your mail

grep '^From: ' /usr/mail/$USER

(11)any line with at least one letter

grep '[a-zA-Z]' 1.txt
 

(12)anything, not a letter or number

grep '[^a-zA-Z0-9]'
 

(13)lines with exactly one character

grep '^.$' 

(14) ‘kite’ within double quotes

grep '"kite"'

(15) ‘kite’, with or without quotes

grep '"*kite"*'

(16)any line that starts with a Period “.”

grep '^\.' 

(17) line starts with “.” and 2 lower case letters} letters

 grep '^\.[a-z][a-z]'
 

egrep command in Unix

If you want to search multiple words in the same grep command, then use the egrep command in UNIX
It searches all the three words in the file

egrep  'cat|bad|sat'  file.txt

It discarded all the lines having any of these three words from the output of ps -ef

ps -ef|  egrep  -v  'cat|bad|sat'    :

grep with pipe command

pipe command in Linux lets u input the output of one command to the other command.

Example

ps -ef|grep python

Here the output of the “ps -ef” command is input for the grep command

Some more Important Grep commands

(1) Sometimes we just want the grep to show out only the file names which matched the given pattern then we use the -l (lower-case L) option. if multiple files are there. This will simply print all the file names. Example of “grep -l”

grep -l ORA-0600 *.trc

(2) Suppose you want to count how many lines matches the given pattern/string, then use the option -c (“grep -c”)

grep -c "TOM" 1.txt

 (3) When you are searching error using grep on a huge file, it may be useful to see some lines around the match.

Lines before the match 

grep -A 10 "TOM" 1.txt 

Lines after the match 

grep -B 10 "TOM" 1.txt 

Lines around the match

 grep -C 10 "TOM" 1.txt 

(4) When we want to show the line number of the matched pattern within the file. we can use “grep -n”

grep -n "ORA-0600" alert.log

(5) Grep excludes directory in recursive search. Sometimes we want to exclude one directory from grep recursive search

 grep -r  --exclude-dir=log "TOM" *
 

Conclusion

grep is a very useful command for search word, expression in the Unix operating system. Hope you like this post on  grep command in Unix with examples

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