Regular expression for command summary in CentOS

Regular expressions are classified into two types: basic regular expressions (BRE) and extended regular expressions (ERE ). Metacharacters of the basic regular expression: 1 ,.: match any single character 2, []: match a single character within the specified range 3, [^]: match a single character out of the specified range 4 ,*: match the characters 5 ,\? : Match the first 0 or 1 6 times

Regular expressions are classified into two types: basic regular expressions (BRE) and extended regular expressions (ERE ).

1. The metacharacters of the basic regular expression:

   1.. match any single character

   2. []: match a single character in the specified range

   3. [^]: match a single character out of the specified range

   4. *: match any number of characters above it

   5 ,\? : Match the first 0 or 1 times

   6. \ +: match the first 1 or multiple times

   7. \ {m \}: Exact Match m times

   8. \ {m, n \}: match at least m times, at most n times

   9. *: match any character of any length

   10. ^: The first line is anchored,

   11. $: anchor at the end of the line

   12. \ <: specifies the beginning of a word (\ B can also be used)

   13. \>: The end of the word is anchored (\ B can also be used)

   14. ^ $: Match blank rows

   15. \ (\): used together with \ n (n is a number) to match the reproduction times of a single or multiple character combinations.

II. extend the metacharacters of the regular expression:

1.. match any single character

2. []: match a single character in the specified range

3. [^]: match a single character out of the specified range

4. *: match any number of characters above it

5 ,? : Match the first 0 or 1 times

6. +: match the first 1 or multiple times

7. {m}: Exact Match m times

8. {m, n}: match at least m times, up to n times

9. *: match any character of any length

10. ^: The first line is anchored,

11. $: anchor at the end of the line

12. \ <: specifies the beginning of a word (\ B can also be used)

13. \>: The end of the word is anchored (\ B can also be used)

14. ^ $: Match blank rows

15. (): used together with \ n (n is a number) to match the reproduction times of a single or multiple character combinations.

16. |: Match | character before or after a symbol

3. commands that can match regular expressions include: (here we only use the grep command)

1. grep: grep uses a basic regular expression to match text rows.

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

Grep [OPTIONS] [-e PATTERN |-f FILE] [FILE...]

Option:

-E: use an extended regular expression for matching (equivalent to egrep)

-F: equivalent to fgrep

-I: Case sensitivity is ignored during matching.

-V: displays rows in unmatched mode.

-O: only the matched rows are displayed.

-Q: Silent mode

-- Color = auto: highlight matched characters

IV. example:

1. display the rows ending with bash in the/etc/passwd file;

2. display two or three digits in the/etc/passwd file;

3. display the lines that end with 'listen' followed by 0, 1, or multiple white spaces in the 'netstat-tan' command results

4. add bash, testbash, basher, and nologin users (the shell of nologin users is/sbin/nologin), and find the rows in the/etc/passwd file with the same username as the shell name;

5. display the default shell and UID of the root, centos, or user1 user on the current system;

6. find a word in the/etc/rc. d/init. d/functions file (the word can contain underscores in the middle) followed by a group of parentheses.

7. use echo to output a path, and then egrep finds the base name of the path;

Further: use egrep to retrieve its directory name

8. find the number between 1-in the ifconfig command execution result;