The text-processing tool grep and the initial knowledge regular expression of Linux

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

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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

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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

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04. Find one or two digits in the/etc/passwd file.

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

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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

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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

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07. Find the line that ends with ' LISTEN ' in the Netstat-tan command execution result.

#netstat-tan | grep--color=auto "listen[[:space:]]*$"

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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

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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

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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

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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]) \> "

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The text-processing tool grep and the initial knowledge regular expression of Linux