Linux Learning-Regular expressions

Regular Expressions- for system administrators, to process a lot of information every day, there are a lot of useless information, you can filter the information through regular expressions, so-called regular expression, is through the arrangement of some special characters, to search, replace, delete one or more lines of string.

First, text filtering tool grep

Format: grep + [options] + match content + [file]

--color=auto keyword highlighting

- v displays rows that are not matched to

-i ignores case

- n Displays the matching line number cat/etc/passwd |grep-n root

- C Displays the number of matching rows

- o Show only matched strings

- q silent mode, does not display any information

Can be used when we are concerned about whether the execution is successful or not, when we don't care about the results.

grep-q shuguo/etc/group && echo exist | | Echo not exist exists then output exist, not present output not exist

-A + number displays rows that match rows and next to the specified number

cat/etc/passwd |grep-a 3 root

- B + numbers show rows that match rows and numbers that are adjacent up

- C + numbers display rows that match rows and specify numbers up and down

- e Multiple options or relationships

grep-e root-e bin/etc/passwd

- w matches only the entire word

- E or egrep support for regular expressions

grep-e "Root|bin"/etc/passwd

egrep "Root|bin"/etc/passwd

- F or fgrep does not support regular expressions

Second, the regular expression

Character matching

      . match any single character

[] matches any single character in the range

[^] matches any single character outside the range

[ : Alnum:] [: Alpha:] [: Lower:] [: Upper:] [:d igit:] [:p UNCT: ]

[: blank:] spaces and tabs

[: space:] horizontal and vertical whitespace characters (a wide range than [: blank:])

number match is used after the number of characters to be specified

* match the preceding character any time, including 0 times

for example:. * Any character of any length

   \? matches the preceding character 0 or 1 times

\+ matches The preceding character at least 1 times

\{n\} matches the preceding character n times

\{m,n\} matches the preceding character at least m times, up to n times

\{,n\} matches the preceding character up to n times

\{m,\} matches the preceding character at least m times

Position anchor position where it appears, where to start and where to end

^ beginning Anchor

$ line End anchoring

For example: Match starts with #, ends with 0, and the middle content is arbitrary

grep "^#.*0$"

match begins with #, ending with 0

grep "^ #0 $"

^$ empty line

^[[:space:]]*$ blank line

\< the first anchor of the word

\> ending anchoring

Group

\ (\) binds one or more characters together as a whole

For example: grep "[Ab]\{2\}" indicates a or B matches two times

grep "\ (ab\) \{2\}" means ab this whole match two times

The content that is matched within the grouping parentheses is recorded in the variable, and the names of these variables are \1,\2 ...

\ (string1\+\ (string2\) \)

\1 means string1\+\ (string2\)

\2 says string2

\| Or

a\|b A or B

\ (c\|c\) at cat or cat

Third, the case

1. Display lines in the/proc/meminfo file that begin with size S

cat/proc/meminfo |grep "^s\|^s"

cat/proc/meminfo |grep-i ^s

grep ^[ss]/proc/meminfo

grep-e ^s-e ^s/proc/meminfo

2. Display lines in the/etc/passwd file that do not end in/bin/bash

CAT/ETC/PASSWD |grep-v "/bin/bash$"

3. Show user RPC default shell program

grep "^\<rpc\>"/etc/passwd |cut-d:-f7

grep "^rpc\>"/etc/passwd |cut-d:-f7

grep "^\ (rpc\)"/etc/passwd |cut-d:-f7

The first two results are the same as the result of the title request

Note the difference

4. Find out the two-bit or three-digit number in/etc/passwd

cat/etc/passwd |grep-o "[[:d igit:]]\{2,3\}"

cat/etc/passwd |grep-o "[[:d igit:]]\{2\}\| [[:d igit:]]\{3\}]

5. A line that displays at least one whitespace character in the CentOS7/etc/grub2.cfg file, followed by a non-whitespace character

cat/etc/grub2.cfg | grep "^[[:space:]]\+[^[:space:]"

6. Find the line that ends with ' LISTEN ' followed by any number of whitespace characters in the result of the "Netstat-tan" command

Netstat-tan | grep "listen[[:space:]]\+$"

7. Display user name and UID of all system users on CentOS7

cat/etc/passwd |cut-d:-f1,3 | grep "\<[0-9]\{1,3\}\>"

8, add user bash, Testbash, basher, SH, nologin (its shell is/sbin/nologin), find the/etc/passwd user name with the shell name of the line

useradd bash

Useradd Testbash

Useradd basher

Useradd SH

useradd-s/sbin/nologin Nologin

cat/etc/passwd |grep "^\ (. *\) \>.*\<\1$"

9, take advantage of DF and grep, remove the disk partition utilization, and from large to small sort

DF |grep "^/DEV/SD" |grep-o "[[:d igit:]]\{1,3\}%" |tr-d% |SORT-NR

Iv. Extending the regular expression

Egrep = Grep-e

Egrep character matching, number matching, position anchoring, grouping and grep difference is to remove the front of the symbol, in addition to \<,\>,\b unchanged

Case:

1. Display three user root, Mage, Wang's UID and default shell

cat/etc/passwd |cut-d:-f1,3,7 |grep-w "Root\|mage\|wang"

CAT/ETC/PASSWD |cut-d:-f1,3,7 |grep "^\ (root\|mage\|wang\) \>"

2. Find the line at the beginning of the/etc/rc.d/init.d/functions file that has a word (including an underscore) followed by a parenthesis

cat/etc/rc.d/init.d/functions |egrep "^[_[:alpha:]]+\ (\)"

3. Use Egrep to remove its base name in/etc/rc.d/init.d/functions

echo/etc/rc.d/init.d/functions |egrep-o "\<[[:alpha:]]*\>$"

echo/etc/rc.d/init.d/functions |egrep-o "\<[a-z]*\>$"

4. Use Egrep to remove/etc/rc.d/init.d/functions/directory name

echo/etc/rc.d/init.d/functions/|egrep-o ". */." |egrep-o ". */"

5. Count the number of logins per host IP address that was logged in as root in the last command

Last |grep "root" |tr-s "": |cut-d:-f3 |uniq-c |sort-u

6, this string: Welcome to magedu each character in Linux to go to the weight and sort, repeat the number of rows to the front

Echo Welcome to magedu Linux |grep-o. |sort |uniq-c |sort-nr

Linux Learning-Regular expressions