Shell Regular expression (1)

First, what is the regular

A regular is a way to describe a character or string by combining symbols with special meanings, called regular expressions. Or, the regular is the rule used to describe a class of things.

Second, grep

1. Parameters

-N: Show line numbers
-O: Show only matching content
-Q: Silent mode, no output, you have to use $? To determine the success of the execution, that is, there is no filtering to the desired content

-L: If the match succeeds, only the file name is printed, the failure is not printed, usually-rl together, grep-rl ' root '/etc
-A: If the match is successful, the matching row and the subsequent n rows are printed together
-B: If the match succeeds, the matching row and its first n rows are printed together
-C: If the match succeeds, the matching row and its n rows are printed together
--color
-C: If the match succeeds, the number of rows to match is printed
-e: Equals Egrep, extended
-I: Ignore case

-V: Inverse, mismatch
-W: Match word

2.grep type
Grep
Fgrep
Pgrep
Egrep

3. Regular introduction

^ Beginning of the line
$ End of line
. Any single character other than the line break
* 0 or more of the leading characters
. * All Characters
[] Any character within a group of characters
[^] reverse each character within a character group (does not match each character in a group of characters)
^[^] lines that begin with characters within a non-character group
[A-z] lowercase letter
[A-z] capital letters
[A-z] lowercase and uppercase letters
[0-9] Number
\< words are usually separated by spaces or special characters, and successive strings are treated as words.
\> Word Tail

Extended regular sed plus-r parameter or escape
grep plus-E or egrep or escaped

grep Homework Examples

1. Displays all the rows that contain root:

grep ' Root '/etc/passwd

2. Output any rows that contain bash, and also output the contents of the next two lines, followed by the row:

GREP-C2 ' bash '/etc/passwd

3. Shows how many lines contain nologin.

Grep-c ' Nologin '/etc/passwd

4. Shows that the rows contain root and the line number is output.

Grep-n ' Root '/etc/passwd

6. New user

Abominable

Abominate

Anomie

Atomize

Write regular expressions and match them up

Egrep a.omi.*e/etc/passwd

7. Built four users

Alex213sb

wpq2222b

Yh438pig

egon666

Egon

Filter out the user name consists of a letter + number + letter Line

Egrep ' ^[a-z]+[0-9]+[a-z]+ '/etc/passwd

8. Displays all filenames containing root in/etc directory

Grep-rl ' root '/etc

9. Filter out all comments and all blank lines in/etc/ssh/sshd_config

Grep-v ' ^# '/etc/ssh/sshd_config |grep-v ' ^ *$ '

Shell Regular expression (1)