20150827-grep and regular Expression learning

Summary of the use of grep and examples of how to display

Basic usage: grep is used as a text search tool to search the target text row by line based on user-specified filters (written by metacharacters and text characters of the regular expression) and display the matching rows

Format: grep [OPTIONS] PATTERN [FILE ...] ：

Common options:

--color=auto: The matching to the string to do highlighting;
-V: The display mode does not match the row;
-I: ignore character case;
-O: Displays only strings that can be matched to the pattern;
-Q: Silent mode;
-E: Using extended regular expressions;

The metacharacters of the basic regular expression:

Character Matching:

.: Matches any single character

[]: matches any single character of the specified range

[^]: does not match any character in the specified range

[: Lower:]: Matching lowercase letters

[: Upper:]: Matching uppercase letters

[[:d Igit:]]: Match numbers

[[: Alnum:]]: Match numbers and uppercase and lowercase letters

[[: Space:]]: matches all whitespace characters

[[:p UNCT:]]: Match punctuation

Number of occurrences: used after the characters that need to be specified;

*: Indicates that the preceding character can appear any time;

\?: Indicates that the preceding character appears 0 or 1 times;

\+: at least once;

\{m\}: The exact number of occurrences is m times;

\{m,n\}: The preceding character at least m times up to n times [M,n]

\{0,n\}: Up to n times;
\{m,\}: at least m times;

. *: Matches any character of any length;

Location anchoring:

^: Anchor at the beginning of the line; for the leftmost side of the pattern (for example, the row is bash: ^bash)

$: End of line anchoring; for the rightmost side of the pattern (for example, the end of the line is bash: bash$)

\< \b: The first anchor of the field; the left side of the pattern used to represent the word;
\> \b: The end of the field anchors; the right side of the pattern used to represent the word;
^$: blank line;

Group: \ (\)

The contents of the pattern in the grouped parentheses are recorded by the regular expression engine during execution, and the built-in variables are saved: The variables are \1, \2, ...
\1: From the left, the first opening parenthesis, and the matching right parenthesis in the middle of the pattern match to the content;
Back reference: Use a variable to refer to the character that matches the pattern in the preceding \b's grouping brackets;

Exercises:

3. Display the lines in the/etc/passwd file that end with bash

grep ' bash$ '/etc/passwd

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M02/72/88/ Wkiom1xlfimsmktmaabz2z86gpw694.jpg "" 444 "height=" "/>

4. Display the two-digit or three-digit number in the/etc/passwd file

Grep–color=auto ' \b[[:d igit:]]\{2,3\}\b '/etc/passwd

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M00/72/88/ Wkiom1xlfjchor1daahzfq5-eha980.jpg "" 461 "height=" 286 "/>

5. Display lines with ' LISTEN ' followed by 0, one or more whitespace characters in the ' Netstat-tan ' command result

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

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M02/72/84/ Wkiol1xlgfjxfhzjaadzsmhexds665.jpg "" 541 "height=" 288 "/>

6. Add user bash, Testbash, basher, and Nologin user (Nologin user's shell is/sbin/nologin), then find the same line in the/etc/passwd file as the user name and its shell name

Useradd bash.  Useradd Testbash. Useradd basher. Useradd-s/sbin/nologin Nologin

grep ' ^\ (\b[[:alnum:]]\+\b\). *\1$ '/etc/passwd

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M00/72/88/ Wkiom1xlfjrz9mltaaet4ymokjw167.jpg "" 606 "height=" 179 "/>

7. Display the default shell and UID of root, CentOS, or User1 user on the current system (please create these users beforehand, if not present)

grep-E ‘^root|^centos|^user1‘ /etc/passwd|cut-d: -f1,3,7

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" Http://s3.51cto.com/wyfs02/M02/72/88/wKiom1Xlfj6D7gWDAABtyYo_ Wvi968.jpg "" 628 "height="/>

8, find a word in the/etc/rc.d/init.d/functions file (the middle of the word can be underlined) followed by a set of parentheses line

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M00/72/88/ Wkiom1xlfkeqi-d1aafzcah9rqu990.jpg "" 658 "height=" 369 "/>

9, use echo to output a path, and then egrep find its path base name; Further use Egrep to remove its directory name

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M01/72/88/ Wkiom1xlfkjw8uhbaabdm4v51c8468.jpg "" 593 "height="/>

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M02/72/88/ Wkiom1xlfkuivs1eaaa9narlwss047.jpg "" 597 "height="/>

10. Find the number between 1-255 in the result of ifconfig command execution

650) this.width=650; "title=" image "style=" border-top:0px; border-right:0px; Background-image:none; border-bottom:0px; padding-top:0px; padding-left:0px; border-left:0px; padding-right:0px "border=" 0 "alt=" image "src=" http://s3.51cto.com/wyfs02/M00/72/84/wKioL1XlgG3Qq_ Xlaac3vvaqrg4242.jpg "" 541 "height=" 103 "/>

20150827-grep and regular Expression learning