grep and regular expressions

Linux Text Processing Three musketeers:

grep: Text Filtering tool

Sed: Text editor

awk: Text Report Generator, awk implementation on Linux Gawk

Grep:

Function: Text Search tool, according to user-specified "pattern" line to search for target text, print the matching line;

Pattern: The filter condition written by metacharacters and text characters of regular expressions;

Metacharacters: A character does not represent its literal meaning, but is used to denote a wildcard or control function;

Divided into two categories:

Basic Regular Expressions: BRE

Extended Regular expression: ERE

grep [OPTIONS] PATTERN [FILE ...] ：

Options:

--color=auto: The matching to the string to do highlighting;

-V: The display mode does not match the row;

-I: ignore character case;

-O: Displays only strings that can be matched to the pattern;

-Q: Silent mode;

-C: Print rows to match

-L; Find rows that do not contain matches

-N: Prints the line containing the match

-E: Using extended regular expressions

The metacharacters of the basic regular expression:

Character Matching:

.: matches any single character;

[]: matches any single character within the specified range;

[^]: matches any single character within the specified range;

[: Alnum:] representing English uppercase and lowercase characters and numbers, i.e. 0-9, A-Z, A-Z

[: Alpha:] on behalf of any English uppercase and lowercase characters, i.e. A-Z, A-Z

[: Upper:] On behalf of uppercase, i.e. A-Z

[: Lower:] for lowercase letters, i.e. A-Z

[^] Outside of the match range

Number of occurrences: used to specify the number of times after the character;

*: any time;

\?:0 or 1 times;

grep "X\?y"

\+:1 or multiple times;

\{m\}: Precisely limited to M times;

\{m,n\}: At least m times, up to n times, [M,n]

\{0,n\}: Up to n times;

\{m,\}: at least m times;

. *: Matches any character of any length;

Location anchoring:

^: Anchor at the beginning of the line, for the leftmost mode;

$: End of line anchoring; for the rightmost side of the pattern;

\< \b: The first anchor of the word; the left side of the pattern used to denote the word;

\> \b: the ending anchor; the right side of the pattern used to represent the word;

^$: blank line;

Group: \ (\)

\ (pattern\) "\ (ab\) *c"

Back reference: The contents of the pattern in the grouped parentheses are recorded by the regular expression engine during execution, and the built-in variables are saved (\1,\2 、、、、 \ n) in the built-in variable can be referenced.

\1: For the referenced content, from left to right in the pattern, with the first opening parenthesis and the contents of the right parenthesis corresponding to it

\2: For the referenced content, the pattern is from left to right, with the second opening parenthesis and the contents of the right parenthesis corresponding to it

Back reference: Use a variable to refer to the character that matches the pattern in the preceding grouping brackets;

An extended regular expression:

The grep family has three commands:

grep: basic Regular expression

-e: extending regular expressions

-F: Regular expressions are not supported

Egrep: Extending Regular expressions

Fgrep: Regular expressions are not supported

Extend the metacharacters of regular expressions:

Character Matching:

.: Any single character

[]:

[^]: outside the range

Number of matches:

*: Any time

?: 0 or 1 times;

+: more than 1 times;

{m}: exact match m times;

{M,n}: At least m times, up to n times;

Anchoring:

^: Anchoring the beginning of the line

$: Anchor Line End

\< \b: with regular regular expressions

\>, \b

Group: ()

Back reference: \1, \2, ...

Or:

A|b

C|cat: does not represent cat or cat, but is C or cat;

To be written (C|c) at

Practice:

1. display the lines in the/etc/passwd file that end with bash

[[email protected] tmp]# grep "bash$"/etc/passwdroot:x:0:0:root:/root:/bin/bashamandabackup:x:33:6:amanda User:/var /lib/amanda:/bin/bashpostgres:x:26:26:postgresql Server:/var/lib/pgsql:/bin/bashcanshan:x:500:500:canshan:/home /canshan:/bin/bashoralce:x:3001:3001::/home/nicai:/bin/bashduanshui:x:3002:3002::/home/duanshui:/bin/ Bashcentos:x:3003:3003::/home/centos:/bin/bashuser1:x:3004:3004::/home/user1:/bin/bash

2. display the two-digit or three-digit number in the/etc/passwd file

[[email protected] tmp]# grep  "[[:d igit:]]\{2,3\}"  passwd 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/nologinftp:x:14:50 : Ftp user:/var/ftp:/sbin/nologinnobody:x:99:99:nobody:/:/sbin/nologindbus:x:81:81:system message  bus:/:/sbin/nologinusbmuxd:x:113:113:usbmuxd user:/:/sbin/nologinrpc:x:32:32:rpcbind daemon:/ var/cache/rpcbind:/sbin/nologinabrt:x:173:173::/etc/abrt:/sbin/nologinvcsa:x:69:69:virtual console  memory owner:/dev:/sbin/nologinrtkit:x:499:497:realtimekit:/proc:/sbin/nologinavahi-autoipd:x:170:170: avahi ipv4ll stack:/var/lib/avahi-autoipd:/sbin/nologinapache:x:48:48:apache:/var/www:/sbin/ Nologinsaslauth:x:498:76:saslauthd user:/var/empty/saslauth:/sbin/nologinrpcuser:x:29:29:rpc service  user:/var/lib/nfs:/sbin/nologinnfsnobody:x:65534:65534:anonymous nfs user:/var/lib/nfs:/sbin/nologinpostfix:x:89:89::/var/ Spool/postfix:/sbin/nologinricci:x:140:140:ricci daemon user:/var/lib/ricci:/sbin/nologinhaldaemon:x : 68:68:hal daemon:/:/sbin/nologingdm:x:42:42::/var/lib/gdm:/sbin/nologinntp:x:38:38::/etc/ntp:/sbin/ nologintomcat:x:91:91:apache tomcat:/usr/share/tomcat6:/sbin/nologinmemcached:x:497:495:memcached  Daemon:/var/run/memcached:/sbin/nologinamandabackup:x:33:6:amanda user:/var/lib/amanda:/bin/bashpulse:x : 496:494:pulseaudio system daemon:/var/run/pulse:/sbin/nologinpiranha:x:60:60::/etc/sysconfig/ha:/ Sbin/nologinsshd:x:74:74:privilege-separated ssh:/var/empty/sshd:/sbin/nologinpostgres:x:26:26:postgresql  Server:/var/lib/pgsql:/bin/bashluci:x:141:141:luci high availability management  application:/var/lib/luci:/sbin/nologindovecot:x:97:97:dovecot imap server:/usr/libexec/dovecot:/ sbin/nologindovenull:x:495:491:dovecot ' s unauthorized user:/usr/libexec/dovecot:/sbin/nologintcpdump:x:72:72::/:/sbin/ Nologincanshan:x:500:500:canshan:/home/canshan:/bin/bashoralce:x:3001:3001::/home/nicai:/bin/bashduanshui:x : 3002:3002::/home/duanshui:/bin/bashcentos:x:3003:3003::/home/centos:/bin/bashuser1:x:3004:3004::/home/user1:/ Bin/bash

3, line

that displays the result of ' Netstat-tan ' command ending with ' LISTEN ' followed by 0, one, or more whitespace characters

[[email protected] tmp]# netstat -tan |grep -e  ' LISTEN[[:space:]]*$ ' TCP         0      0 0.0.0.0:40355                0.0.0.0:*                     listen      tcp        0       0 0.0.0.0:111                  0.0.0.0:*                    LISTEN      tcp         0      0 0.0.0.0:22                   0.0.0.0:*                    LISTEN       tcp        0      0 127.0.0.1:631                0.0.0.0:*                     listen      tcp        0       0 127.0.0.1:25                 0.0.0.0:*                    LISTEN      tcp         0      0 127.0.0.1:6010               0.0.0.0:*                    LISTEN      tcp         0      0 127.0.0.1:6011               0.0.0.0:*                    LISTEN       tcp        0       0 :::111                       :::*                          listen      tcp         0      0 :::22                        :: :*                          LISTEN      tcp         0      0 :::44054                     :::*                           LISTEN      tcp         0      0 ::1:631                      :::*                          LISTEN       tcp        0       0 ::1:25                       :::*                          LISTEN       tcp        0      0  ::1:6010                     :::*                          LISTEN      tcp         0      0 ::1:6011                     :::*                           listen

4,

[[email protected] tmp]# useradd bash[[email protected] tmp]# useradd testbash[[email protected] tmp]# useradd Basher[[ema Il protected] tmp]# useradd-s/sbin/nologin nologin[[email protected] tmp]# grep "^\ (\<[[:alnum:]].*\>\). *\1$" Pas swdsync:x:5:0:sync:/sbin:/bin/syncshutdown:x:6:0:shutdown:/sbin:/sbin/shutdownhalt:x:7:0:halt:/sbin:/sbin/ Haltbash:x:3005:3005::/home/bash:/bin/bashnologin:x:3008:3008::/home/nologin:/sbin/nologin

5. Display the default shell and UID of root, CentOS, or User1 user on the current system (please create these users beforehand, if not present)

Note: I do not understand why to use Egrep

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6, find a word in the/etc/rc.d/init.d/functions file (the middle of the word can be underlined) followed by a set of parentheses line

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7, use echo to output a path, and then egrep find its path base name; Further use Egrep to remove its directory \

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Note: This is not too clear, there are references to other classmates

10. Find the number between 1-255 in the result of ifconfig command execution

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grep and regular expressions