Linux grep command usage and regular expressions

Introduction to 1.GREP commands and regular expressions

(1). grep (Global search REgular expression and print out of the line), which is a powerful text-search tool in the Linux system, that searches the regular expression globally and prints out the matching rows, is based on a user-specified "pattern (pattern) "Filters the target text to show rows that are matched by the pattern;

(2). A regular expression is a pattern written by a class of characters, some of which do not represent the literal meaning of a character, but rather the function of a control or a wildcard.

Basic syntax format for 2.GREP commands

grep [OPTION] ... ' PATTERN ' FILE ...

Common options for grep:

-V: Reverse the matching rows

-O: Show only what matches to

-I : ignore character case

-N: Add line numbers for matched rows

- E: Using an extended regular expression, equivalent to the EGREP command

-F: Do not use regular expression search, equivalent to the FGREP command

-A #: Displayed along with the next # line of the matching line, #代表任意数字

- B #: Displayed along with the # line of the matching line, #代表任意数字

- C #: Displayed along with the top and bottom # lines of the matching row, #代表任意数字

--color=auto: Displays the matching content in a different color

Basic usage of 3.GREP regular expressions

Basic Regular Expressions:

(1) Character matching

. : Matches any single character

Example: Match starts with R, and the end of T is only two characters in the middle of a line

650) this.width=650; "title=" clip_image001 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image001 "src=" http://s3.51cto.com/wyfs02/M01/54/79/ Wkiom1sd2u7q7q9daacg51du5uk180.jpg "width=" 358 "height="/>

[]: matches any single character in the specified collection

Common collection Representations are:

Pure numbers: [[:d Igit:]] or [0-9]

lowercase letters : [[: Lower:]] or [A-z]

Capital letters: [[: Upper:]] or [A-z]

uppercase and lowercase letters: [[: Alpha:]] or [a-za-z]

numbers plus letters: [[: Alnum:]] or [0-9a-za-z]

whitespace characters: [[: Space:]]

punctuation: [[:p UNCT:]]

Example 1: Match a line containing a number 0 or 2 (only the first half)

650) this.width=650; "title=" clip_image002 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image002 "src=" http://s3.51cto.com/wyfs02/M02/54/79/wKiom1SD2u_ Rjow2aaecwnm6lcu413.jpg "width=" 389 "height=" 136 "/>

Example 2: Match a line containing the letter R or T (contains only the first half)

650) this.width=650; "title=" clip_image003 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image003 "src=" http://s3.51cto.com/wyfs02/M00/54/79/ Wkiom1sd2u-hr7ilaaeh75srni8219.jpg "width=" 388 "height=" 137 "/>

Example 3: Match the line containing the number 0-9 (only the first half)

650) this.width=650; "title=" clip_image004 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image004 "src=" http://s3.51cto.com/wyfs02/M01/54/79/ Wkiom1sd2vdzdtj5aaehuruikie767.jpg "width=" 391 "height=" 156 "/>

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

Example: Match a character closest (containing only the first half) that contains a range other than 1-9

650) this.width=650; "title=" clip_image005 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image005 "src=" http://s3.51cto.com/wyfs02/M00/54/79/ Wkiom1sd2vcgxpy1aaeeprykkre997.jpg "width=" 391 "height="/>

(2) Number of matches

*: Matches any of the preceding characters appearing any time, 0, 1, or more lines

Example: Create a Test text that contains the following:

650) this.width=650; "title=" clip_image006 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image006 "src=" http://s3.51cto.com/wyfs02/M00/54/77/ Wkiol1sd24owl7ciaacuzapqjng452.jpg "width=" 391 "height=" 219 "/>

match the X-letter occurrences of the line any time:

650) this.width=650; "title=" clip_image007 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image007 "src=" http://s3.51cto.com/wyfs02/M01/54/77/ Wkiol1sd24ptmzk1aacm7mtyhem823.jpg "width=" 388 "height=" 198 "/>

\+: Matches a line whose preceding character appears 1 or more times

Example: Match the line with x word at least 1 times

650) this.width=650; "title=" clip_image008 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image008 "src=" http://s3.51cto.com/wyfs02/M01/54/77/ Wkiol1sd24stuasdaach6uykbvk410.jpg "width=" 390 "height=" 151 "/>

\? : Matches a line whose preceding character appears 0 or 1 times

Example: Match a row with an X-letter 0 or 1 times

650) this.width=650; "title=" clip_image009 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image009 "src=" http://s3.51cto.com/wyfs02/M02/54/79/ Wkiom1sd2vli9kcaaacx7uxl-tu789.jpg "width=" 391 "height=" 202 "/>

\{m\}: Matches the line whose preceding character appears m times

Example: Match a row with an X letter appearing 2 times

650) this.width=650; "title=" clip_image010 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image010 "src=" http://s3.51cto.com/wyfs02/M01/54/79/ Wkiom1sd2vlqs46-aabrma1lem4477.jpg "width=" 391 "height=" "/> "

\{m,n\}: Matches the preceding character at least m times, at most times the rows with N, and M and n represent a range m-n

Example: Match x letters appear at least 1 times, up to 3 rows

650) this.width=650; "title=" clip_image011 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image011 "src=" http://s3.51cto.com/wyfs02/M01/54/77/ Wkiol1sd24wwdmrkaacgzs5m18y344.jpg "width=" 402 "height="/>

(3) Position anchoring

^: Anchor at the beginning of the line

Example: match the X-letter appears at the beginning of the line

650) this.width=650; "title=" clip_image012 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image012 "src=" http://s3.51cto.com/wyfs02/M02/54/77/ Wkiol1sd24xcqvqraab_rvr25cg947.jpg "width=" 389 "height=" 151 "/>

$: End of line anchoring

Example: matches the E letter appears at the end of the line

650) this.width=650; "title=" clip_image013 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image013 "src=" http://s3.51cto.com/wyfs02/M02/54/77/ Wkiol1sd24asggjraacfgrjuzwm036.jpg "width=" 390 "height="/>

^$: Matching Blank lines

Example: matching a blank line

650) this.width=650; "title=" clip_image014 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image014 "src=" http://s3.51cto.com/wyfs02/M00/54/79/wKiom1SD2vSDKET_ Aaa2intmjhs842.jpg "width=" 384 "height="/>

\<: The first anchor of the word

Example: Exact match XY two letters in the first line of a word

650) this.width=650; "title=" clip_image015 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image015 "src=" http://s3.51cto.com/wyfs02/M02/54/79/ Wkiom1sd2vxrfwy5aadqixv2p64649.jpg "width=" 390 "height=" 288 "/>

\>: Ending anchoring

Example: Exact match XY two letters in the ending line of a word

650) this.width=650; "title=" clip_image016 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image016 "src=" http://s3.51cto.com/wyfs02/M02/54/77/ Wkiol1sd24eif9icaacdkbphbh0356.jpg "width=" 390 "height=" 153 "/>

\<\>: Match words

Example: Match a line containing the word xy

650) this.width=650; "title=" clip_image017 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image017 "src=" http://s3.51cto.com/wyfs02/M02/54/79/ Wkiom1sd2vaip8llaadzswzn_x8405.jpg "width=" 390 "height=" 269 "/>

(3) Grouping

\ (\): Group matching of a string

Example: matching XY single start 0 or 1 times of the row

650) this.width=650; "title=" clip_image018 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image018 "src=" http://s3.51cto.com/wyfs02/M00/54/79/ Wkiom1sd2varyiiaaacsuhxdc6i584.jpg "width=" 392 "height=" 216 "/>

Back reference: In the pattern, if you use \ (\) to implement the grouping, in a text check, if \ (\) pattern matches to a certain content, this content can be referenced in the following pattern;

The symbols that refer to the preceding groupings are: \1, \2, \3

The pattern is from left to right, referencing the first # opening parenthesis and matching the pattern between the closing parenthesis and its matching right;

Example of a back reference:

Create a new text file, assuming the following:

650) this.width=650; "title=" clip_image019 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image019 "src=" http://s3.51cto.com/wyfs02/M01/54/77/ Wkiol1sd24igoxo4aabd3nrswf0288.jpg "Width= " "height="/>

find lines with the same words before and after:

650) this.width=650; "title=" clip_image020 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image020 "src=" http://s3.51cto.com/wyfs02/M01/54/79/ Wkiom1sd2vfqzs2baabkw_vv-f0659.jpg "width=" 536 "height=" "/>

Regular expression meta-character summary:

character matching:., [], [^]

Number of matches: *, \?, \+, \{m\}, \{m,n\}

Location Anchoring: ^, $,\<,\>,\<\>

Group matching: \ (\)

4.egrep and extended Regular expressions:

Egrep equivalent to Grep-e,egrep can use extended regular expressions directly, and grep needs to add an option-E;

Extend the metacharacters of regular expressions:

character matching:., [], [^]

number of matches: *,?,+,{m},{m,n},{m,},{0,n}

position Anchor:^,$,\>,\<

Group Matching: (), support for back reference

|: Matches the line on the left or right, such as a|b, with a or B line matching;

example 1:egrep equivalent to Grep-e

650) this.width=650; "title=" clip_image021 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image021 "src=" http://s3.51cto.com/wyfs02/M02/54/79/ Wkiom1sd2vfgw1fqaac35zgoezy298.jpg "width=" 389 "height=" 175 "/>

Example 2:

650) this.width=650; "title=" clip_image022 "style=" border-right-width:0px;border-bottom-width:0px; border-top-width:0px; "border=" 0 "alt=" clip_image022 "src=" http://s3.51cto.com/wyfs02/M00/54/79/ Wkiom1sd2vijhajoaabrbwyinj0018.jpg "width=" 394 "height=" 101 "/>

5.grep Practice Questions:

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

# grep-i ' ^s '/proc/meminfo

(2). Displays the user whose default shell is non-/sbin/nologin in the/etc/passwd file;

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

(3). Displays the user whose default shell is/bin/bash in the/etc/passwd file

Further: Displays only the user whose ID number is the highest in the above results

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

(5). Display lines beginning with at least one whitespace character in/boot/grub/grub.conf

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

(6). Displays the line in the/etc/rc.d/rc.sysinit file, beginning with #, followed by at least one white space character, and then with at least one non-whitespace character;

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

(7). Find the line containing ' LISTEN ' in the result of the Netstat-tan command execution;

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

(8). Add User Bash,testbash,basher,nologin (shell is/sbin/nologin), and find out the same user name and default shell on the current system;

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

(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 consisting of a previous word plus r;

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

(10). Displays 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 after a word in the/etc/rc.d/init.d/functions file followed by a pair of parentheses ' () ";

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

(12). Use Echo to output a path, and use Egrep to remove its base name;

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

Linux grep command usage and regular expressions