Linux grep command

First, the role:

Grep:global serch REgular expression and Print out of the line.

A text search tool that matches the target text line by row according to the user-specified pattern (filter condition);

Second, common options:

grep [OPTIONS] PATTERN [FILE ...]

grep [OPTIONS] [-E PATTERN |-f FILE] [FILE ...]

OPTIONS:

--color=auto: Color The matched text to highlight;

-i:ignorecase, ignoring character case;

-O: Displays only the string that matches to itself;

-V: Reverse display;

-E: Supports the use of extended regular expression metacharacters;

-q,--quiet,--silent: Silent mode, that is, do not output any information;

-A #:after, after # line

-B #:before, front # line

-c #:conext, front and back # lines

Basic regular Expression meta-characters:

Character Matching:

.: matches any single character;

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

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

Number of matches: used to limit the number of occurrences of the preceding character, after the character to specify the number of occurrences, and the default work is in greedy mode;

*: Matches its preceding character any time, 0, 1, multiple times;

. *: Matches any character of any length;

\?: matches the preceding character 0 or 1 times, that is, the preceding character is optional;

\+: Matches the preceding character 1 or more times, that is, the preceding character must appear at least 1 times;

\{m\}: Matches the preceding character m times;

\{m,n\}: Matches its preceding character at least m times, up to n times;

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

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

Location anchoring:

^: Anchor at the beginning of the line; to the left of the pattern; ^root

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

^pattern$: Used for PATTERN to match whole line;

^$: blank line;

^[[:space:]]*$: A blank line or a line containing white space characters;

Word: A continuous character (string) consisting of a non-special character is called a word;

\< or \b: The first anchor of the word, used for the left side of the word pattern;

\> or \b: The ending anchor for the right side of the word pattern;

\<pattern\>: matches complete words;

Grouping and referencing:

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

Note: The contents of the pattern matching in the grouping brackets are automatically recorded in the internal variables by the regular expression engine, and these variables are:

\1: The pattern from the left side, the first opening parenthesis and the matching closing parenthesis, matches the character of the pattern;

\2: The pattern from the left side, the second opening parenthesis, and the matching closing parenthesis to the character;

\3: The pattern from the left side, the third opening parenthesis, and the matching closing parenthesis match the character to the pattern;

...

grep "\ (L.. e\). *\1 "Love.txt behind the \1 can reference \ (l: e\) in the content;

Back reference: References the pattern in the preceding grouping brackets to the character;

grep [OPTIONS] [-E PATTERN |-f FILE] [FILE ...]

Options:-I,-O,-V,-Q,-A,-B,-C

-G: Support for basic regular expressions

Extend the metacharacters of regular expressions:

Character Matching:

Number of matches:

*: Any time, 0,1 or multiple times;

？ : 0 or 1 times, the preceding character is optional;

+: Its preceding character at least 1 times;

{m}: its preceding character m times;

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

{0,n}:

{m,}

Location anchoring:

Grouping and referencing:

(): grouping: Pattern matching in parentheses the hand character is recorded hermetical the internal variables of the expression engine;

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

Or:

A|b:a or B:

C|cat:c or Cat

(c| C) At:cat or cat

Examples of commands:

Find out the number of blank lines in the/etc/fstab file

~]# grep "^$"/etc/fstab | Wc-l

1

Find the user-rooted line in the/etc/passwd file

~]# grep "^root\>"/etc/passwd

Root:x:0:0:root:/root:/bin/bash

Find the line in the/etc/passwd file with the same user name as the shell name

~]# grep--color-e "^ ([^:]+\>). *\1$"/etc/passwd

Sync:x:5:0:sync:/sbin:/bin/sync

Shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown

Halt:x:7:0:halt:/sbin:/sbin/halt

Find the line that starts with M or s in the/proc/meminfo file

~]# grep-e "^ (m| S) "/proc/meminfo

memtotal:808160 KB

memfree:488092 KB

swapcached:0 KB

mlocked:0 KB

swaptotal:2072572 KB

swapfree:2072572 KB

mapped:15460 KB

shmem:236 KB

slab:75108 KB

sreclaimable:16084 KB

sunreclaim:59024 KB

Displays lines beginning with a letter in the/etc/inittab file

~]# grep "^[a-z]"/etc/inittab

Id:3:initdefault:

Find an integer between 1-255 in the ifconfig command result

~]# Ifconfig | Grep-e--color "\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \> "

Eth0 Link encap:ethernet HWaddr 00:0c:29:12:ba:26

inet addr:172.16.249.45 bcast:172.16.255.255 mask:255.255.0.0

Inet6 ADDR:FE80::20C:29FF:FE12:BA26/64 Scope:link

Up broadcast RUNNING multicast mtu:1500 metric:1

RX bytes:23544203 (22.4 MiB) TX bytes:14827318 (14.1 MiB)

inet addr:127.0.0.1 mask:255.0.0.0

Inet6 addr::: 1/128 scope:host

Up LOOPBACK RUNNING mtu:65536 metric:1

Linux grep command