A detailed and regular expression of the grep command on the basics of Linux entry

The grep command is one of the most frequently used commands under Linux, which filters the text according to the user-specified pattern (pattern) and displays the matching rows . Its command format is:

grep [OPTIONS] PATTERN [FILE]

For example: We are looking for the IP address configured in NIC 0 (the file path:/etc/sysconfig/network-scripts/ifcfg-eth0)---grep ' ipaddr '/etc/sysconfig/ Network-scripts/ifcfg-eth0

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(Note: Alias grep= ' grep--color=auto ' allows matching results to be highlighted)

several options (option) commonly used by grep are:

- V: Reverse selection , that is, the remaining rows after the display mode is filtered

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In addition to ipaddr this line is not displayed, other content is displayed to the user.

-I : ignore character case

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The first command does not filter to anything, so the result is not shown, and the matching line is displayed after ignoring the case .

-o: Show only filtered content

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Only content that is filtered according to the specified pattern is displayed.

Pattern represents the user given pattern, why this place we call him a pattern rather than a character, because regular expressions are usually used. The regular expression is a special kind of character, some characters of which usually do not represent the literal meaning, but as a control or globbing function. This type of character is called a meta-character. Regular Expressions are made up of these metacharacters, filtering the text along with grep and using it in later shell programming.

Regular expressions are divided into basic regular expressions and extended regular expressions in two categories.

The meta-character of a basic regular expression has the following characters:

of the filter character . Represents a match to anysinglecharacters. Then A.. B means there are two characters between A and B.

[] represents any single character within the set. A[BC] denotes ab or AC, note: ABC is also compliant with the mode

[^] denotes any single character outside of the matching set .

Special expression: [0-9] can be expressed by [[:d Igit:]]

[A-z] can be represented by [[: Lower:]]

[A-z] can be represented by [[: Upper:]]

[A-za-z] That is, all letters can be represented by [[: Alpha:]]]

[0-9a-za-z] That is, all letters and numbers can be represented by [[: Alnum:]]

Blank (Empty Characters, tab tab, etc.) denoted by [[: Space:]]

All punctuation marks are denoted by [[:p UNCT:]]

number of matches (qualifying for the number of occurrences of the preceding character)

* indicates a match * The preceding character appears any time 0,1,2 ..., then a*b indicates that a appears any time.

\? Indicates a match? The preceding character appears 0 or 1 times (\ Represents the escaped function only), then a\?b indicates that a appears 0 or 1 times

\{m\} indicates that the match {m} precedes the character M, then a\{2\} indicates a appears 2 times

\{m,n\} indicates that the preceding character of the match {m} appears at least n times, extending \{m,\} at least m times, \{0,n\} up to n times. then a\{1,2\} indicates that a appears at least 1 times up to 2 times.

\+ can also be expressed as a minimum of 1 occurrences

anchor Position (the character is positioned, the writing format is simple to remember: first left, tail right)

^ indicates that the beginning of a line is anchored, then ^a indicates that a must appear at the beginning of a row

$ means the end of the line is anchored, then a$ indicates that a row ends with a

\< indicates the first anchor of the word, then \<a indicates that a word begins with a

\> represents the ending anchor, then \>a indicates that a word ends with a

The grouping character divides the specified characters into groups, which are matched in the form of a group.

\ (\), then \ (ab\) indicates that AB is a group of

support back reference, \1,\2,\3 ... Represents the content from left to right that refers to its first # (start, #) ends directly.

An extended regular expression is used in the same way as a basic regular expression, except that there is no need to write an escape character on the writing format, which is more than the basic regular representation of a conditional selector:| That is, AB|CD represents AB or CD

I. Matches the character's have . []   [^]

Ii. number of Matches * ? +   {m} {m,n} {m,} {0,n}

III. The anchor has ^ $ \< or \b \> or \b

Iv. group character (with ) |

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Regular expression Exercises

1. Display the lines in the/proc/meminfo file that begin with uppercase or lowercase s;

Grep-i ' ^s '/proc/meminfo

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2. Display the default shell of the/etc/passwd file as a non-/sbin/nologin user;

Grep-v '/sbin/nologin$ '/etc/passwd | Cut-d:-f1

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3. Display the user whose default shell is/bin/bash in the/etc/passwd file; further: Display only the user whose ID number is the largest in the above results;

grep '/bin/bash$ '/etc/passwd | cut-d:-f1

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grep '/bin/bash$ '/etc/passwd | SORT-T:-k3-n | Tail-1 | Grep-o ' ^[[:alnum:]]\+ '

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grep ' \<[[:d igit:]]\{1,2\}\> '/etc/passwd

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5. Display the line beginning with at least one blank character in/boot/grub/grub.conf;

grep ' ^[[:space:]]\+ '/boot/grub/grub.conf

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6, display/etc/rc.d/rc.sysinit file, start with #, followed by at least one white space character, and then have at least one non-whitespace character line;

grep ' ^#[[:space:]]\+[^[:space:]]\+ '/etc/rc.d/rc.sysinit

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7, find the Netstat-tan command execution results with ' LISTEN ' (can have white space characters) end of the line;

Netstat-tan | grep ' listen[[:space:]]*$ '

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8, add user bash, Testbash, basher, Nologin (shell for/sbin/nologin), and find the current system on its user name and the default shell of the same user;

grep ' ^\ ([[: alnum:]]\+\). *\1$ '/etc/passwd

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9. Extension question: Create a new text file, assuming the following:

He like his lover.

He love his lover.

He like his liker.

He love his liker.

Find out that the last word is a line made up of a previous word plus r.

# Nano Blog.text

# grep ' \ (\<[[:alpha:]]\+\>\). *\1r './blog.text

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Use Egrep to complete the following exercises

Exercise 10: Display the default shell and user name for root, CentOS, or User1 users on the current system;

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Exercise 11: Find the line after a word in the/etc/rc.d/init.d/functions file followed by a pair of parentheses "()";

Egrep ' \<[[:alpha:]]+\>\ (\) '/etc/rc.d/init.d/functions | Cut-d '-f1

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Exercise 12: Use echo to output a path, and use Egrep to remove its base name;

Echo/tmp/abc-/-s | Egrep-o ' [^/]+$ '

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Exercise 13: Find the value between 1-255 in the ifconfig command result;    

ifconfig | egrep ' \< ([1-9]|[ 1-9][0-9]|1[0-9]\+|2[0-4][0-9]|25[0-5]) \>'

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Exercise 14: Find the IP address in the ifconfig command result (hint: 1.0.0.1-223.255.255.254)

Ifconfig | Egrep-o ' (\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \>\.\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \>\.\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \>\.\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-4][0-9]|25[0-5]) \>) '

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Experience: It's easier to understand the text content you want to search all as a single character.

Extension: There is also a quick search tool fgrep, but he does not support regular expressions and is therefore seldom used.

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A detailed and regular expression of the grep command on the basics of Linux entry