grep Family and regular expressions

1.the grep family concept

1.grep Family Concept

grep : Supports the use of basic extension expressions

Egrep : Supports the use of extended regular expressions

Fgrep : Using regular expressions is not supported

function: A powerful text search tool that matches the target text line-by-row with a user-specified "Pattern(filter)" To print out qualifying rows

Pattern: Filter conditions written by metacharacters and text characters of regular expressions

2.grep command Explanation

Usage: grep [OPTIONS] PATTERN [FILE] ...

Common options:--color=auto: Color-coded after matching to text-I: case insensitive to Characters-O: Only the text that matches to is displayed itself-V,--invert-match: Reverse match-e: Regular expressions that support extensions-Q,--quiet,-- Silient: Silent mode, does not output any information-F: Supports the use of fixed strings, does not support regular expressions, equivalent to FGREP-G,--basic-regexp: Supports the use of basic regular Expressions-P,--perl-regexp: Support for using the PCRE regular expression-e PATTERN,--regexp=pattern: Multi-mode mechanism-f file,--file=file:file for each line contains a PATTERN of text files, and grep script-a NUM,-- After-context=num: Display matches to the next few rows-B num,--before-context=num: Display matches to the previous lines-C num,-num,--context=num: Shows the front and back lines that match to

Examples of Use:

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Because grep and egrep use regular expressions, they are later combined with regular expressions.

3. The function of regular expressions and meta-characters

Regular Expressions a pattern written by a class of special characters and text characters, where some characters do not represent their literal meanings, but are used to denote control or the function of a wildcard.

Regular expression engine: A program that parses a given text using the regular expression pattern.

And the regular expression is divided into two categories:

(1) Basic Regular expressions

(2) Extended Regular expression

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note that the basic regular expressions and extended regular expressions are different!

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Application of 4.grep command combined with regular expression

(1) Character matching

Find the line with Nologin in the/etc/passwd file

grep "[Nologin]"/etc/passwd

(2) Number of matches

Find the three-bit or four-digit number in the/etc/passwd file

Grep-o "\<[0-9]\{3,4\}\>"/etc/passwdegrep-o "\<[0-9]{3,4}\>"/etc/passwd

(3) Position anchoring

Displays lines in the/etc/passwd file that do not end with bash

Grep-v "bash$"/etc/passwd

(4) Group application

In/etc/passwd find the user whose user name is the same as the shell name

Egrep "^ ([a-z0-9]+) \>.*\1$"/etc/passwd

(5) Comprehensive application

Find the IP address in the ifconfig command result

Ifconfig | Egrep "\<inet[[:space:]]+.*[0-9]\>"

Find the/etc/grub2.cfg file, starting with at least one whitespace character, followed by a line with non-whitespace characters

Egrep "^[[:space:]]+[^[:space:]]*"/etc/grub2.cfg

Find all lines in the/proc/meminfo file that begin with uppercase or lowercase c; At least three ways to implement

Egrep "^ (c| C) "/tmp/meminfogrep" ^[cc] "/tmp/meminfogrep-i" ^c "/tmp/meminfo

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grep Family and regular expressions