grep

grep

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Count Occurrence: -c

$ cat example.txt | grep -c good
2

Get Context: -C

Set --context=0 to print that line alone

$ cat example.txt | grep --context=0 "good luck"
good luck

Set --context=1 to print 1 line below and 1 line above

$ cat example.txt | grep --context=1 "good luck"
hello world
good luck
good day

or use -C 1

$ cat example.txt | grep -C 1 "good luck"
hello world
good luck
good day

Ignore: -v

Use -v to exclude some lines(i.e. NOT)

$ cat example.txt | grep good | grep -v day
good luck

Case Insensitive: -i

$ cat example.txt | grep GOOD

$ cat example.txt | grep -i GOOD
good luck
good day

Show Match in Color: --color

$ cat example.txt | grep good --color

Show Matched Line Number: -n

$ cat example.txt | grep good -n
2:good luck
3:good day

Show Matched File Name: -l

grep is not limited to searching a single file, compare the results below

$ grep good example.txt
good luck
good day

to search from multiple files:

$ grep good *
example.txt:good luck
example.txt:good day

filename will be shown along with the matched lines; to show the filename only:

$ grep -l good *
example.txt

what happens to the "pipe" version?

$ cat example.txt | grep good -l
(standard input)

Search for Whole Words Only: -w

$ grep -w goo example.txt

this returns nothing since goo is a pattern though not a whole word

$ grep goo example.txt
good luck
good day

Recursive grep: -R

This will search all the directory and sub-directories recursively

$ grep -R pattern *

Set Maximum Matches: -m

$ cat example.txt | grep -m 1 good
good luck

Match

Show ssh processes

$ ps | grep ssh

Specify max count by -m:

$ cat foo.log | grep -m 10 ERROR

Show file name

grep -H

grep vs egrap vs fgrep

grep, egrep and fgrep are used to match patterns in files, here are the differences:

• grep: basic regular expressions
• egrep: extended regular expressions(?, +, |), equivalent to grep -E
• fgrep: fixed patterns, no regular expression; faster than grep and egrep; equivalent to grep -F

Checkout the Regular_expression wikipedia page for the definitions of POSIX basic and extended regular expressions

Assume there's a examle.txt file containing 4 lines:

$ cat example.txt
hello world
good luck
good day
linux

grep vs fgrep

fgrep does not support regular expression at all, this will return nothing

$ cat example.txt | fgrep g..d

use grep instead

$ cat example.txt | grep g..d
good luck
good day

grep vs egrep

grep does not support |, so this will return nothing

$ cat example.txt | grep "good|linux"

however egrep can recognize | as OR

$ cat example.txt | egrep "good|linux"
good luck
good day
linux

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