Linuxgrep command usage and Regular Expression

Linuxgrep command usage and Regular Expression

1. Introduction to grep commands and Regular Expressions

(1 ). grep (Global search REgular expression and Print out the line) is a powerful text search tool in Linux, it filters the target text based on the pattern specified by the user and displays the rows matched by the pattern;

(2) Regular Expressions are written by a certain type of characters. Some of them do not represent the literal meaning of a character, but represent the function of control or wildcard.

2. Basic syntax format of the grep command

Grep [OPTION]... 'pattern' FILE...

Common grep options:

-V: reverse returns the matched rows.

-O: Only Matching content is displayed.

-I: case-insensitive characters

-N: Add a row number to the matched row.

-E: use an extended regular expression, equivalent to the egrep command.

-F: does not use regular expressions to search. It is equivalent to the fgrep command.

-A #: it is displayed along with the lower # Rows of matching rows. # It represents any number.

-B #: it is displayed together with the top of the matching row # The row # represents any number.

-C #: it is displayed together with the upper and lower lines matching the row # The row # represents any number.

-- Color = auto: the Matching content is displayed in different colors.

3. Basic usage of grep Regular Expressions

Basic regular expression:

(1) character matching

.: Match any single character

For example, matching rows that start with r and end with t are separated by only two characters

[]: Match any single character in the specified set

Common collection representation methods include:

Pure number: [[: digit:] or [0-9]

Lowercase letter: [[: lower:] or [a-z]

Uppercase letters: [[: upper:] or [A-Z]

Uppercase/lowercase letters: [[: alpha:] or [a-zA-Z]

Numbers plus letters: [[: alnum:] or [0-9a-zA-Z]

Blank character: [[: space:]

Punctuation: [[: punct:]

Example 1: Match rows that contain numbers 0 or 2 (only the first half)

Example 2: match the row containing the letter r or t (only the first half)

Example 3: match the row containing the number 0-9 (only the first half)

[^]: Match any single character outside the specified set

For example, match the rows containing characters other than the 1-9 range (only including the first half)

(2) times matching

*: Match the first character to appear any time, 0, 1 or multiple rows

For example, create a test text that contains the following content:

Match any number of rows with letters x:

\ +: Match the line with the previous character that appears once or multiple times

Example: Match rows with at least one x character

\? : Match the line with the first character 0 or 1

For example, a row that matches letters x 0 or once appears

\ {M \}: The line that matches the character above it appears m times

Example: match the rows with letters x twice

\ {M, n \}: match the first character to appear at least m times, at most n rows, m and n represent a range of m-n

For example, a row that matches letters x at least once and appears at most 3 times

(3) Positioning

^: First line anchored

For example, match the rows with letters x at the beginning of the row.

$: Anchor at the end of a row

For example, the row that matches the letter e at the end of the row

^ $: Matches blank rows

Example: Match blank rows

\ <: Specifies the beginning of a word.

For example, exact match of xy two letters in the first line of a word

\>: Tail anchor

For example, exact match with the line at the end of a word with two xy letters

\ <\>: Match a word

For example, match the row containing the word xy.

(3) Group

\ (\): Grouping and matching a string

For example, match one or zero rows in xy single startup

Backward reference: In the mode, if \ (\) is used to implement grouping, in the check of a line of text, if the \ (\) mode matches a certain content, the mode after this content can be referenced;

The symbols referenced in the previous group are \ 1, \ 2, \ 3.

The Mode starts from left to right and references the # Left brackets and the Pattern Matching content between them;

Back Reference example:

Create a text file with the following content:

Find the rows with the same word before and after:

RegEx metacharacters:

Character match:., [], [^]

Matching times :*,\? , \ +, \ {M \}, \ {m, n \}

Position anchored: ^, $, \ <, \>, \ <\>

Group match :\(\)

4. egrep and extended regular expressions:

Egrep is equivalent to grep-E. egrep can directly use an extended regular expression, while grep needs to add option-E;

Metacharacters of the extended regular expression:

Character match:., [], [^]

Matching times :*,?, +, {M}, {m, n}, {m,}, {0, n}

Positioning: ^, $, \>, \ <

Group match: (), supports backward reference

|: Match rows that meet the condition on the left or right, such as a | B. All rows that contain a or B match;

Example 1: egrep is equivalent to grep-E

Example 2:

5. grep exercise questions:

(1). display the rows starting with uppercase or lowercase s in the/proc/meminfo file;

# Grep-I '^ s'/proc/meminfo

(2). display users whose default shell is not/sbin/nologin in the/etc/passwd file;

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

(3). display the/etc/passwd file whose default shell is/bin/bash.

Further: only the user with the largest ID in the preceding results is displayed.

# Grep '/bin/bash $'/etc/passwd | cut-d:-f1 | sort-n-r | head-1

(4) find one or two digits in the/etc/passwd file;

# Grep '\ <[[: digit:] \ {1, 2 \} \>'/etc/passwd

(5) display at least one line starting with a blank character in/boot/grub. conf

# Grep '^ [[: space:] \ +. *'/boot/grub. conf

(6) display the/etc/rc. d/rc. sysinit file, which starts with #, followed by at least one blank character, and then contains at least one non-blank line;

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

(7) Find the line containing 'listen' in the execution result of the netstat-tan command;

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

(8) Add users bash, testbash, basher, and nologin (SHELL:/sbin/nologin), and find the users with the same username as the default SHELL on the current system;

# Grep '\ (\ <[: alnum:] \ + \> \). * \ 1 $'/etc/passwd

(9). extended question: Create a text file with the following content:

He like his lover.

He love his lover.

He like his liker.

He love his liker.

Find the row where the last word is formed by adding r to the previous word;

# Grep '\ (\ <[: alpha:] \ + \> \). * \ 1r 'grep.txt

(10) display the default shell and User Name of the root, centos, or user1 user on the current system;

# Grep-E '^ (root | centos | user1 \>)'/etc/passwd

(11). Find the line with parentheses '() "after the word in the/etc/rc. d/init. d/functions file;

# Grep-o '\ <[: alpha:] \ + \> ()'/etc/rc. d/init. d/functions

(12). Use echo to output a path, and use egrep to retrieve its base name;

# Echo/etc/rc. d/| grep-o '[^/] \ + /\? $ '| Grep-o' [^/] \ +'