GREP

In this section we will see grep, but the grep “simple” to start. Grep is a very complete utility, which I think is essential to master. the question is not “do I use it every day”, but rather “does an hour go to work without me using it”?

Grep allows us to extract a line from a string, the extraction is done if a pattern match! In other words grep allows us to search in a file! It returns the information to the output standard (stdout).
Here is an example of use:

1
2
3
4
5
6
7
8
9

# simple use of grep
username@hostname:~$ grep --color "bash" /etc/passwd
root:x:0:0:root:/root:/bin/bash
thomas:x:1000:1000:thomas,,,:/home/bob:/bin/bash
user_test:x:1002:1002:,,,:/home/epatino:/bin/bash
postgres:x:118:128:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
guest-QbUTwC:x:126:135:Guest,,,:/tmp/guest-QbUTwC:/bin/bash
tom:x:1003:1003:,,,:/home/tom:/bin/bash

As you can see, it is possible to provide a grep file as argument, but it is able to read from the stdin (0).

1
2
3
4
5
6
7
8

# simple use of grep
username@hostname:~$ cat /etc/passwd |grep --color "bash"
root:x:0:0:root:/root:/bin/bash
thomas:x:1000:1000:thomas,,,:/home/bob:/bin/bash
user_test:x:1002:1002:,,,:/home/epatino:/bin/bash
postgres:x:118:128:PostgreSQL administrator,,,:/var/lib/postgresql:/bin/bash
guest-QbUTwC:x:126:135:Guest,,,:/tmp/guest-QbUTwC:/bin/bash
tom:x:1003:1003:,,,:/home/tom:/bin/bash

Although, in this case, there are only a few differences, it is important to know that grep is optimized for interpreting large files. It may be more optimal at a given moment to provide the file as a parameter instead of transmitting the information via the standard input channel (stdin).

Well, here is the simple grep presentation! To return to our case: I want to manipulate my file to extract the players who play in English clubs. I resume my order and add the grep:

1
2
3
4
5
6
7
8
9
10

# search for players playing in England
username@hostname:~$ cat les_buts.txt | tr -s " " | tr " " ";" | grep ",ang"
Giroud;87;Arsenal,ang ;fr
Loic;29;Newcastle,ang;fr
Lukaku;31;Everton,ang;be
Hazard;78;Chelsea,ang;be
Ozil;55;Arsenal,ang;de
Lyoris;0;Tottenham,ang;fr
Weelbeck;2;ManU,ang;en
Cabaye;65;Newcastle,ang;fr

Is it optimal as a notation?
The file we handle is not too big, so the impact is minimal. However, I advise you to filter your data before handling them with other filters. Here is an “optimized” command whose difference only applies to large files

1
2
3
4
5
6
7
8
9
10

# search days in england
username@hostname:~$ cat les_buts.txt | grep ",ang" | tr -s " " | tr " " ";"
Giroud 87 Arsenal,ang fr
Loic 29 Newcastle,ang fr
Lukaku 31 Everton,ang be
Hazard 78 Chelsea,ang be
Ozil 55 Arsenal,ang de
Lyoris 0 Tottenham,ang fr
Weelbeck 2 ManU,ang en
Cabaye 65 Newcastle,ang fr

Interesting option:

• -v: it is possible to do an inverted search. Here, I’m looking for the lines that do not contain the string bash:

grep reverse search

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

# reverse search. user@hostname:~$ cat /etc/passwd | grep -v bash daemon:x:1:1:daemon:/usr/sbin:/bin/sh bin:x:2:2:bin:/bin:/bin/sh sys:x:3:3:sys:/dev:/bin/sh sync:x:4:65534:sync:/bin:/bin/sync games:x:5:60:games:/usr/games:/bin/sh man:x:6:12:man:/var/cache/man:/bin/sh lp:x:7:7:lp:/var/spool/lpd:/bin/sh mail:x:8:8:mail:/var/mail:/bin/sh news:x:9:9:news:/var/spool/news:/bin/sh uucp:x:10:10:uucp:/var/spool/uucp:/bin/sh proxy:x:13:13:proxy:/bin:/bin/sh www-data:x:33:33:www-data:/var/www:/bin/sh backup:x:34:34:backup:/var/backups:/bin/sh list:x:38:38:Mailing List Manager:/var/list:/bin/sh [[ OUTPUT COUPÉ ]]

We have seen how to extract the list of players who play in an English club by using the grep filter utility.
By the way, how many are they?
It is easy to count 5, 10 lines, but after 20 lines, the risk of error grows which brings us to the wc command.