Linux grep usage and grep usage exercises

Grep

Function: Matches the specified row by a specified pattern

Common options:

-E extended regular expression matches the equivalent of the EGREP command

-O get only what matches to

-V matches the opposite of the regular expression

--color=auto to highlight the contents of the search

-A num matches the row to the search and the NUM line below the line

-B num matches the row to the search and the num row above the line

-C num matches the row to the search and the top and bottom num rows

-N Displays the number of rows in the file

-R Recursive directory search for specified content files

-I regular expression content is case insensitive

The basic regular Expression description:

Character Matching:

.: matches any single character;

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

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

[: Lower:] Match lowercase letters

[: Upper:] matches uppercase letters

[: Alpha] matches all letters

[:d Igit:] Match all numbers

[: Alnum:] matches all numeric letters [: Alpha:] and [: Alpha:] and two for one

[: space] represents a space character such as tab, line break, space, etc.

[:p UNCT:] represents punctuation marks such as:'! "# $% & ' () * +,-. / : ; < = >? @ [] ^ _ ' {|}

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

*: any time;

\?:0 or 1 times;

\+: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: \ (\)

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: The variables are \1, \2, ...

\1: From the left, the first opening parenthesis, and the matching right parenthesis in the middle of the pattern match to the content; \2 and so on.

Extend the metacharacters of regular expressions:

Character Matching:

Same as the basic regular expression

Number of Matches

*: 0 or more times

?: 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: 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;

Group:

Same as the basic regular expression

The concept of support or

A|b: Match A or b

Practice:

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

Answer: grep--color=auto ' bash$ '/etc/passwd

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2 display two-digit or three-digit numbers in a/etc/passwd file

Answer: A. Basic regular expression:grep--color=auto ' \b[[:d igit:]]\{2,3\}\b '/etc/passwd

B. Extended regular expression:grep--color=auto-e ' \b[[:d igit:]]{2,3}\b '/etc/passwd

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Because the basic regular expressions are similar to the way they are extended, the following solutions take the form of extending regular expressions

3 Displays the line with ' LISTEN ' followed by 0, one or more white-space characters in the ' netstat-tan ' command result;

Answer: Netstat-tan | grep--color=auto-e ' listen[[:space:]]*$ '

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4 Add user Bash,testbash, basher and Nologin user (Nologin user's shell is/sbin/nologin), and then find the line of user name and shell name in/etc/passwd file;

Answer: 1.useradd bash

2.useradd Testbash

3.useradd basher

4.useradd-s/sbin/nologin Nologin

5.grep--color=auto-e ' ^ ([[: alnum:]]+):. *\1$ '/etc/passwd

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5 Displays the default shell and UID for root, CentOS, or User1 users on the current system;

Answer: egrep ' ^\<root|centos|user1\> '/etc/passwd | cut-d:-f3,7

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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;

Answer: grep-e--color=auto ' \<[0-9a-za-z_]+\>\ (\) '/etc/rc.d/init.d/functions

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7 use Echo to output a path, and then egrep to find its path base name; further: Use Egrep to remove its directory name;

Answer:

Path Base name command: echo/usr/local/nginx/conf/nginx.conf | egrep-o ' ([^/]*) $ '

Path Directory name command: echo/usr/local/nginx/conf/nginx.conf | Egrep-o ' ^ (. *)/'

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8 Find the number between 1-255 in the result of ifconfig command execution;

Answer: ifconfig | egrep--color=auto ' \< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \> '

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Linux grep usage and grep usage exercises