grep, Egrep, and Fgrep of the Linux Text Processing tool

first, the basic concept

expression: A regular expression is a logical formula for string manipulation, which is a "rule string" that is used to express a filter logic for a string, using predefined specific characters and combinations of these specific characters.

grep(Global search Regular expression (RE) and print out of the line): A text Search tool that searches the target file based on a user-specified text pattern, Displays the rows that can be matched to the pattern (as a text filter)

Egrep: use extended regular expressions to build patterns, i.e. GREP-E

fgrep: fast, does not parse the regular expression, which is grep–f

Meta-character: does not represent the meaning of the character itself, for the description of the additional functionality

Ii. how grep is used

Format:

grep [OPTIONS] PATTERN [FILE ...]

eg.# grep "Root"/etc/passwd

Common options:

-E: Using extended regular expressions to build patterns

-F: Using non-parsing regular expressions to build patterns

-I: Character case insensitive, ignore-case

-V: Reverse, showing rows that cannot be matched by the pattern

-O: Displays only strings that are matched to the pattern, not the entire row

-A #: match to the line and the # line below it

-B #: Match to the line and the # line above it

-C #: Match to the line and the # line next to it

--color: Specifies the color that is matched by the pattern to the character, the parameter options are never,always and auto

The standard is metacharacters the expression:

1, character matching:

. : Any single character

[[email protected] ~]# grep--color=auto "RO.T"/etc/passwdroot:x:0:0:root:/root:/bin/bashoperator:x:11:0:operator:/ Root:/sbin/nologin

[]: Any single character within the specified range

[0-9], [[:d Igit:]]: Numeric characters
[A-z], [[: Lower:]]: lowercase characters
[A-z], [[: Upper:]]: Uppercase characters
[[: Alpha:]]: uppercase and lowercase characters
[[: Alnum:]]: Numbers and uppercase and lowercase characters
[[: Space:]]: white space characters
[[:p UNCT:]]: punctuation characters

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[^]: Any single character outside the specified range

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2, number of times match: used to specify the number of times to match the preceding characters

*: The preceding character any time

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NOTE: * Any time before the character

. *: matches any character of any length

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\? : The preceding character 0 or 1 times

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Note: The grep pattern uses greedy mode, that is, to match characters as long as possible;

\{m\}: Matches M-Times

\{m,n\}: matches at least m times, up to N times

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

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

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3. Position anchoring: used to specify where characters appear

^: Anchoring the beginning of the line

eg. ^char

$: Anchor Line end

eg. char$

^$: Blank line

\<char: Anchor word head, \bchar

char\>: Anchor ending, char\b

4. Group:

\(\)

eg. \ (ab\) *xy

Reference:

\1: A forward reference that references the first opening parenthesis and the corresponding pattern in the closing parenthesis.

\ 2: a back reference, referring to the previous second opening parenthesis, and to the contents of the pattern in the corresponding closing parenthesis

...

eg. \ (a.b\) xy\1:a6bxya6b

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Note: match the exact match of the reference, and the string is exactly the same

The standard is metacharacters the expression:

Character Matching:

. : Any single character

[]: Any single character within the specified range

[^]: Any single character outside the specified range

Number of matches:

*: Match the characters in front of it any time;

? : matches the preceding character 0 or 1 times;

+: match the characters in front of it at least 1 times

{m}: Matches the preceding character m times;

{m,n}: Matches the preceding character at least m times, up to N times

{m,}: matches the characters preceding it at least m times;

{0,n}: Matches the preceding character up to n times;

Anchoring:

^: Beginning of the line

$: End of line

\<, \b: The head of theword

\>, \b: suffix

Group:

(): Group

| : Or, AC|BC, means AC or BC, or the entire string

eg. Grep-e "Con (c|c) at"

Conc or Cat

Concat or Concat

Iii. Intensive Exercises

Lab Environment:

Hardware: Thinkpad T430 i5-3320m 4GB RAM 500GB HDD

Virtual machine: 10.0.2 build-1744117

Operating system: CentOS 6.5

1. Display the lines in the/proc/meminfo file that begin with the case s;

# grep "^[ss]"/proc/meminfo

2, remove the default shell for non-bash users;

# grep-v "bash$"/etc/passwd | Cut-d:-f1

3. Remove the user whose default shell is bash and its ID number is the largest;

# grep "bash$"/etc/passwd | Sort-n-T:-k3 | Tail-1 | Cut-d:-f1

4, 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:]]\{1,\}[^[:space:]]\{1,\}"/etc/rc.d/rc.sysinit

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

# grep "^[[:space:]]\{1,\}[^[:space:]]\{1,\}"/boot/grub/grub.conf

6, find out the/etc/passwd file in one or two digits;

# grep--color=auto "\<[0-9]\{1,2\}\>"/etc/passwd

7. Find the integer between 1 and 255 in the result of ifconfig command;

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

8. View all the information of the root user on the current system;

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

9, add user bash and Testbash, basher, and then find the current system on its user name and the default shell of the same user;

# grep--color=auto "^\ ([[: Alnum:]]\{1,\}\) \>.*\1$"/etc/passwd

10. Find the line ending with "LISTEN" or "established" in the result of netstat-tan command execution;

# Netstat-tan | Grep-e--color=auto "(listen| Established) [[: space:]]*$]

11, remove all the users of the current system shell, requirements: Each shell is displayed only once, and in ascending order;

# cut-d:-f7/etc/passwd | Sort-u

12. Write a script that counts the number of rows in the/etc/rc.d/rc.sysinit,/etc/init.d/functions, and/etc/fstab files that begin with # and the number of rows in a blank row;

#!/bin/bashfor file In/etc/rc.d/rc.sysinit/etc/init.d/functions/etc/fstab;do bashline= ' grep-e--color=auto "^#" $fi Le | Wc-l ' spaceline= ' grep-e "^#$" $file | Wc-l ' echo ' $file has $bashline, $file have $spaceline "done

13, write a script, respectively copy/etc/rc.d/rc.sysinit,/etc/init.d/functions and/etc/fstab files to the/tmp directory, the file name is formerly known as the current date composition, (for example, the first file after the name of the copy/ tmp/rc.sysinit-2-14-02-16;)

#!/bin/bashfor path In/etc/rc.d/rc.sysinit/etc/init.d/functions/etc/fstab;do filename= ' basename $path ' CP $path/tmp/$fileName ' date +-%y-%m-%d ' done

14. Write a script

Displays the user name, UID, and line number of all the default shell-bash users on the current system in the/etc/passwd file;

#!/bin/bashcat-n/etc/passwd | grep--color=auto "bash$" | Cut-d:-f1,3

Challenge: Write a pattern that matches the IP address in the real sense; (1.0.0.1--223.255.255.254)

#grep-E--color=auto "\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-2][0-3]) \> (.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-5][0-5] \>) {2}.\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-5][0-4]) \> "

Extension: Write a script to determine whether the IP address entered by the user is legitimate

#!/bin/bashread-p "Enter a IP Address:" Ipaddressif [[-N ' echo $ipAddress | Grep-e "\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[0-2][0-3]) \> (.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-5][0-5] \>) {2}.\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-5][0-4]) \> "']];thenecho" This is a valid IP address! " Elseecho "This is a invalid IP address!" Fi

This article is from the "Wind Qing" blog, please be sure to keep this source http://fengqingyang.blog.51cto.com/9185783/1549640

grep, Egrep, and Fgrep of the Linux Text Processing tool