grep and regular expressions

grep and regular expressions

Grep:
The Three musketeers of text processing on Linux
grep: Text filter (Pattern: pattern) tool;
grep, Egrep, Fgrep
Sed:stream Editor, text editing tools;
Awk:linux on the implementation of Gawk, Text Report generator;

Grep:global search REgular expression and Print out of the line.
Function: Text Search tool, according to user-specified "mode" to match the target text line by row to check; print matching lines;
Pattern: The filter condition written by the regular expression character and the text character;
REGEXP: A pattern written by a class of special characters and text characters, some of which do not represent the literal meaning of a character, but are a function of control or wildcard;
Divided into two categories:
Basic Regular Expressions: BRE
Extended Regular expression: ERE
GREP-E, Egrep

Regular expression engine

grep [OPTIONS] PATTERN [FILE ...]

Options:
--color=auto: Coloring the text to match to the display;
-V: Displays rows that cannot be matched to pattern;
-I: ignore character case;
-O: Displays only the matching string;
-Q: Silent mode, do not output any information;
-A #:after, after # line
-B #: Before, Front # line
-c #:context, front and back # lines

-E: Use ere;

[[email protected] ~]# grep root/etc/passwdroot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/root:/sbin/ Nologin

[Email protected] ~]# grep--color=auto root/etc/passwdroot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/ Root:/sbin/nologin

[Email protected] ~]# grep-v root/etc/passwdbin:x:1:1:bin:/bin:/sbin/nologindaemon:x:2:2:daemon:/sbin:/sbin/ nologinadm:x:3:4:adm:/var/adm:/sbin/nologinlp:x:4:7:lp:/var/spool/lpd:/sbin/nologinsync:x:5:0:sync:/sbin:/bin/ syncshutdown:x:6:0:shutdown:/sbin:/sbin/shutdownhalt:x:7:0:halt:/sbin:/sbin/haltmail:x:8:12:mail:/var/spool/ mail:/sbin/nologinuucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologingames:x:12:100:games:/usr/games:/sbin/ Nologingopher:x:13:30:gopher:/var/gopher:/sbin/nologinftp:x:14:50:ftp user:/var/ftp:/sbin/nologinnobody:x:99:99 : Nobody:/:/sbin/nologinvcsa:x:69:69:virtual Console memory owner:/dev:/sbin/nologinsaslauth:x:499:499: "SASLAUTHD User ":/var/empty/saslauth:/sbin/nologinpostfix:x:89:89::/var/spool/postfix:/sbin/nologinsshd:x:74:74: Privilege-separated Ssh:/var/empty/sshd:/sbin/nologin

-I ignores case

-O displays itself only

[Email protected] ~]# Grep-o root/etc/passwdrootrootrootroot

[Email protected] ~]# grep-q root/etc/passwd[[email protected] ~]# echo $?0

[Email protected] ~]# grep-q xxxxxxxxxx/etc/passwd[[email protected] ~]# echo $?1

A match to, a no match to

-A 2 indicates that the matched row and the following 2 rows are displayed

[Email protected] ~]# grep-a 2 root/etc/passwdroot:x:0:0:root:/root:/bin/bashbin:x:1:1:bin:/bin:/sbin/ NOLOGINDAEMON:X:2:2:DAEMON:/SBIN:/SBIN/NOLOGIN--OPERATOR:X:11:0:OPERATOR:/ROOT:/SBIN/NOLOGINGAMES:X:12:100: Games:/usr/games:/sbin/nologingopher:x:13:30:gopher:/var/gopher:/sbin/nologin

-B 2 indicates matching rows and first 2 rows

[Email protected] ~]# Grep-b 2 root/etc/passwdroot:x:0:0:root:/root:/bin/bash--mail:x:8:12:mail:/var/spool/mail:/ Sbin/nologinuucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologinoperator:x:11:0:operator:/root:/sbin/nologin

-C 2 represents the matched rows and their preceding and after 2 rows

[Email protected] ~]# grep-c 2 root/etc/passwdroot:x:0:0:root:/root:/bin/bashbin:x:1:1:bin:/bin:/sbin/ NOLOGINDAEMON:X:2:2:DAEMON:/SBIN:/SBIN/NOLOGIN--MAIL:X:8:12:MAIL:/VAR/SPOOL/MAIL:/SBIN/NOLOGINUUCP:X:10:14: Uucp:/var/spool/uucp:/sbin/nologinoperator:x:11:0:operator:/root:/sbin/nologingames:x:12:100:games:/usr/games :/sbin/nologingopher:x:13:30:gopher:/var/gopher:/sbin/nologin

         Basic Regular expression meta-characters:
             Character matching:
                .   :   match any single character;
                []  : Matches any single character within the specified range
                [^]  : matches any single character outside the specified range
                    [ :d Igit:], [: Lower:], [: Upper:], [: Alpha:], [: Alnum:], [:p UNCT:], [: space:]
                     
means to find items with an S start, N end, and 2 single-character characters in the middle

[[email protected] ~]# grep  ' s. N '  /etc/passwdbin:x:1:1:bin:/bin:/sbin/nologindaemon:x:2:2:daemon:/sbin:/sbin/nologinadm:x:3:4:adm:/var/ adm:/sbin/nologinlp:x:4:7:lp:/var/spool/lpd:/sbin/nologinsync:x:5:0:sync:/sbin:/bin/syncshutdown:x:6:0: shutdown:/sbin:/sbin/shutdownhalt:x:7:0:halt:/sbin:/sbin/haltmail:x:8:12:mail:/var/spool/mail:/sbin/ Nologinuucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologinoperator:x:11:0:operator:/root:/sbin/nologingames:x : 12:100:games:/usr/games:/sbin/nologingopher:x:13:30:gopher:/var/gopher:/sbin/nologinftp:x:14:50:ftp user :/var/ftp:/sbin/nologinnobody:x:99:99:nobody:/:/sbin/nologinvcsa:x:69:69:virtual console memory  owner:/dev:/sbin/nologinsaslauth:x:499:499: "Saslauthd user":/var/empty/saslauth:/sbin/ nologinpostfix:x:89:89::/var/spool/postfix:/sbin/nologinsshd:x:74:74:privilege-separated ssh:/var/empty/ Sshd:/sbin/nologin 

Number of matches: used after the number of characters to be specified, to specify the number of occurrences of the preceding character;
*: matches the preceding character any time; can be 0,1,2,3 .... any
For example: grep "X*y"
Abxy
Xay
Xxxxxxy

[email protected] ~]# cat Aa.txt abxyxayxxxyxxaayaaaa

[Email protected] ~]# grep x*y aa.txt abxyxayxxxyxxaay

Greedy mode
. *: Any character of any length;

[email protected] ~]# cat Aa.txt abxyxayxxxyxxaayaaaa

[Email protected] ~]# grep x.*y aa.txt abxyxayxxxyxxaay

\?: matches the preceding character 0 or 1 times, i.e. the preceding is optional;
\+: Matches its preceding character at least 1 times;
\{m\}: Matches the preceding character m times;
\{m,n\}: Matches the preceding character at least m times, up to n times;
\{0,n\}: Matches the preceding character up to n times;
\{m,\}: Matches the preceding character at least m times;

[[email protected] ~]# grep--color=auto x*y aa.txt abxyxayxxxyxxaay

Location anchoring:
^: Anchor at the beginning of the line, for the leftmost mode;
$: End of line anchoring; for the rightmost side of the pattern;
^pattern$: Used for pattern matching whole line;
^$: Empty line;
^[[:space:]]*$

\< or \b: The first anchor of the word, the left side of the word pattern;
\> or \b: the ending anchor; for the right side of the word pattern;
\<pattern\>: matches the whole word;

Group:
\ (\): Bind one or more characters together and treat them as a whole;
\ (xy\) *ab

Note: The contents of the pattern in the grouping brackets are recorded in internal variables by the regular expression engine, which are named: \1, \2, \3, ...
\1: From the left, the first opening parenthesis and the matching closing parenthesis to match the pattern between the characters;
\ (ab\+\ (xy\) *\):
\1:ab\+\ (xy\) *
\2:xy

Back reference: References the pattern in the preceding grouping brackets to match the character (not the pattern itself)

Practice:
1. Display the line in the/proc/meminfo file that begins with the size S; (Requires: use two ways)
Grep-i ' ^s '/proc/meminfo

2. Display the lines in the/etc/passwd file that do not end in/bin/bash;
grep '/bin/bash$ '/etc/passwd
Grep-v '/bin/bash$ '/etc/passwd

3. Display the user name of the user with the largest ID number in the/etc/passwd file;
SORT-T:-k3-n/etc/passwd | Tail-l | Cut-d:-f1

4, if the user root exists, display its default shell program;
# ID Root &>/dev/null && grep "^root\>"/etc/passwd | Cut-d:-f7

5. Find out the number of two or three digits in/etc/passwd;
# grep "\<[0-9]\{2,3\}\>"/etc/passwd

6. Display a line in the/etc/rc.d/rc.sysinit file that begins with at least one white space character and that is followed by a non-whitespace character;
# grep "^[[:space:]]\+[^[:space:]]"/etc/grub2.cfg

7. Find the line ending with ' LISTEN ' followed by 0, 1, or more whitespace characters in the result of the "Netstat-tan" command;
# Netstat-tan | grep "listen[[:space:]]*$"

8, add user bash, Testbash, basher and Nologin (its shell is/sbin/nologin), and then find the/etc/passwd file user name with the shell name of the line;
Useradd-s/sbin/nologin Nologin
# grep "^\ ([[: alnum:]]\+\>\). *\1$"/etc/passwd


Practice:
1, write a script to achieve the following functions
If the User1 user exists, it is shown to exist, otherwise added;
Displays information such as the ID number of the added user;
#!/bin/bash
ID user1 &>/dev/null && echo "user1 exists." | | Useradd user1
ID User1

2, write a script to complete the following functions
If the root user is logged in to the current system, the root user is shown online, otherwise it is not logged in;
W | grep "^root\>" &>/dev/null && echo "root logged" | | echo "No Loggin"

This article is from the "Liang blog" blog, make sure to keep this source http://7038006.blog.51cto.com/7028006/1827208

grep and regular expressions