A), the three major text processing of Linux grep:
grep (Global search REgular expression and print out of the line, fully search for regular expressions and print them out): After giving a list of files or standard output, grep searches for text that matches one or more regular expressions , and only the lines or text that match (or do not match) are output. ----Excerpt from Wikimedia encyclopedia
The grep family includes:
Grep:
Egrep: grep that supports extended regular expressions, equivalent to GREP-E.
fgrep:fixed grep or fast grep, regular expressions are not supported.
b), syntax format:
grep [option] ... ' PATTERN ' FILE ...
Note: the string is searched by line.
Common Command options:
-V: Reverses the selection, which shows the rows that do not match.
-O: Displays only the matched string, not the line where the string is located.
-i:ignore-case, ignoring the character case.
-E: Supports the use of extended regular expressions.
-A #: Displays the matching line and the next # line.
-B #: Displays matching lines and upper # lines.
-C #: Displays matching lines and up and down # lines.
--color=auto: Match to the string display color.
c), Regular expression:
Regular Expression, often abbreviated in code as a regex, regexp, or RE, is a concept of computer science. A regular expression uses a single string to describe and match a series of strings that conform to a certain syntactic rule. In many text editors, regular expressions are often used to retrieve and replace text that conforms to a pattern. ----Excerpt from Wikimedia encyclopedia
Four), meta-characters:
1), Character matching:
.: Matches any single character
[]: Matches a character within a specified range
[^]: matches a character that is not within the specified range
[:d igit:]: numeric character, i.e. [0-9]
[: Lower:]: lowercase characters, i.e. [A-z]
[: Upper:]: Uppercase characters, i.e. [A-z]
[: Alpha:]: literal character, i.e. [a-z,a-z]
[: Alnum:]: literal numeric character, i.e. [a-z,a-z,0-9]
[: Xdigit:]: hexadecimal number, i.e. [0-9,a-f,a-f]
[:p UNCT:]: Punctuation
[: Space:]: white space character
[: Graph:]: non-whitespace characters
[: Cntrl:]: Control character
[:p rint:]: Non-null characters (including spaces)
2), the number of times matching:
*: 0 or more previous characters.
. *: Any character at any time
\?: 0 or one-time previous character.
\{m\}: The previous character repeats M times.
\{m,n\}: The previous character M to n times.
X\{m,\}: The previous character is at least m times.
X\{0,n\}: The previous character is up to n times.
3), Position anchoring:
^: Anchor at the beginning of the line.
$: End of line anchoring.
^$: Blank line.
\<,/b: Anchors the first word, appearing on the left side of the word.
\>,\b: Anchors the ending, appearing on the right side of the word.
\<, .... \>,\b. \b: The word lock character.
4), grouping and referencing:
\(.. \):
The patterns in the grouping match to the content that can be remembered in memory by the regular expression engine and then referenced.
\#:
Refers to the content that the nth parenthesis matches to, not the schema itself.
V), extended regular expression
1), Character matching:
Is the same as the basic regular expression.
2), number of times matching
Is the same as the basic regular expression and does not need to be translated.
+: One or more previous characters.
A|B: Branch, A or B,
Con (c|c) at:concat or concat
Conc|cat:conc or Cat
3), Position anchoring:
Is the same as the basic regular expression.
4), grouping and referencing
As with the basic regular expression, the grouping does not need to be translated.
VI), example description:
01. Displays lines in the/proc/meminfo file that begin with uppercase or lowercase s.
#grep--color=auto-i ' ^s '/proc/meminfo#grep--color=auto-e ' ^ (s| S) '/proc/meminfo
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431959526140463.jpg "title= "1431959526140463.jpg" alt= "1.jpg"/>
02. Displays the user whose default shell is non-/sbin/nologin in the/etc/passwd file.
#grep--color=auto-v "/sbin/nologin$"/etc/passwd | Cut-d:-f1
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431959858169135.jpg "title= "1431959858169135.jpg" alt= "2.jpg"/>
03. Display the user whose default shell is/bin/bash in/etc/passwd file;
Further: Displays only the user whose ID number is the highest in the above results.
#grep--color=auto "/bin/bash$"/etc/passwd | SORT-T:-k3-n | Tail-n 1 | Cut-d:-f1,3,7
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431960190164226.jpg "title= "1431960190164226.jpg" alt= "3.jpg"/>
04. Find one or two digits in the/etc/passwd file.
#grep--color=auto "\<[[:d igit:]]\{1,2\}\>"/etc/passwd
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431960515911997.jpg "title= "1431960515911997.jpg" alt= "4.jpg"/>
05. Displays the lines in/boot/grub/grub.conf that begin with at least one whitespace character.
#grep--color=auto "^[[:space:]]\{1,}"/boot/grub/grub.conf
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431962706890926.jpg "title= "1431962706890926.jpg" alt= "5.jpg"/>
06. Display the/etc/rc.d/rc.sysinit file, starting with #, followed by at least one white-space character, and then with at least one non-whitespace character line.
#grep--color=auto "^#[[:space:]]\{1,\}[^[:space:]]\{1,\}"/etc/rc.d/rc.sysinit
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431960981386967.jpg "title= "1431960981386967.jpg" alt= "6.jpg"/>
07. Find the line that ends with ' LISTEN ' in the Netstat-tan command execution result.
#netstat-tan | grep--color=auto "listen[[:space:]]*$"
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431961263115812.jpg "title= "1431961263115812.jpg" alt= "7.jpg"/>
08, 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--color=auto "^\ ([[: Alpha:]]\{1,\}\):. */\1$"/etc/passwd
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431962021113971.jpg "title= "1431962021113971.jpg" alt= "8.jpg"/>
09. Displays the default shell for root, Rhel, or CentOS users on the current system.
#grep--color=auto-e "^ (Root|rhel|centos):. *"/ETC/PASSWD | Cut-d:-f1,7
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431962937111020.jpg "title= "1431962937111020.jpg" alt= "9.jpg"/>
10. Find a word followed by a set of parentheses "()" line in the/etc/rc.d/init.d/functions file
#grep--color=auto-e "\<[[:alpha:]]+\>\ (\)"/etc/rc.d/init.d/functions
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150518/1431963348651501.jpg "title= "1431963348651501.jpg" alt= "10.jpg"/>
11. Find the number between 1-255 in the result of ifconfig command;
#ifconfig | grep--color=auto-e "\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \> "
650) this.width=650; "src=" Http://www.178linux.com/ueditor/php/upload/image/20150519/1431965057125437.jpg "title= "1431965057125437.jpg" alt= "11.jpg"/>
This article is from the "Pony Learning Record" blog, make sure to keep this source http://masachencer.blog.51cto.com/8683770/1652621
The text-processing tool grep and the initial knowledge regular expression of Linux