Linux Common Commands-grep,egrep,regexp

grep: Searches for text based on patterns and displays lines of text that conform to the pattern.
Pattern: Text word wildcards regular the expression's meta-character combination to match the condition

grep [Options] PATTERN [FILE ...]

-I (ignoring case)

[[email protected] ~]# grep root/etc/passwd
Root:x:0:0:root:/root:/bin/bash
Operator:x:11:0:operator:/root:/sbin/nologin
[Email protected] ~]#

--color (plus color)

-V: Show rows that are not matched by the pattern

[Email protected] ~]# grep-v root/etc/passwd
Bin:x:1:1:bin:/bin:/sbin/nologin
Daemon:x:2:2:daemon:/sbin:/sbin/nologin
Adm:x:3:4:adm:/var/adm:/sbin/nologin

-O: Displays only strings that are matched by the pattern

[Email protected] ~]# Grep-o root/etc/passwd
Root
Root
Root
Root
[Email protected] ~]#

*: Any character of any length
?: any single character
[]:
[^]:

Regular Expressions: REGular expression, REGEXP
Metacharacters
.: Matches any single character
[]: matches any single character within the specified range
[^]: matches any single character outside the specified range
Character set: [:d igit:], [: Lower:], [: Upper:], [:p UNCT:], [: Space:], [: Alpha:], [: Alnum:]

[[email protected] ~]# grep ' [[:d igit:]]$ '/etc/inittab
# 5-x11
[Email protected] ~]#

Number of matches (greedy mode):
*: Matches any of its preceding characters any time
A, B, AB, AAB, ACB, ADB, AMNB
A*b, A?b
A.*b

[[email protected] ~]# grep ' a*b ' test.txt
B
Ab
AaB
Acb
adb
Acdafb
Asdflkbsdfb
[Email protected] ~]#

. *: Any character of any length

[[email protected] ~]# grep ' a.*b ' test.txt
Ab
AaB
Acb
adb
Acdafb
Asdflkbsdfb
[Email protected] ~]#

\?: Match its preceding character 1 or 0 times

[[email protected] ~]# grep ' a\?b ' test.txt
B
Ab
AaB
Acb
adb
Acdafb
Asdflkbsdfb
[Email protected] ~]#

\{m,n\}: Matches the preceding character at least m times, up to N times
\{1,\}: At least once
\{0,3\}:0-3 Times

[[email protected] ~]# grep ' a\{1,3\}b ' test.txt
Ab
AaB
[Email protected] ~]#

[[email protected] ~]# grep ' a.\{1,3\}b ' test.txt
AaB
Acb
adb
Acdafb
[Email protected] ~]#

Location anchoring:
^: Anchor the beginning of the line, any content after this character must appear at the beginning of the line

[[email protected] ~]# grep ' ^r. T '/etc/passwd
Root:x:0:0:root:/root:/bin/bash
[Root[email protected] ~]#

$: Anchor line end, any content in front of this character must appear at the end of the row

[[email protected] ~]# grep ' login$ '/etc/passwd
Bin:x:1:1:bin:/bin:/sbin/nologin
Daemon:x:2:2:daemon:/sbin:/sbin/nologin
Adm:x:3:4:adm:/var/adm:/sbin/nologin
Lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
Mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
Uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
Operator:x:11:0:operator:/root:/sbin/nologin
Games:x:12:100:games:/usr/games:/sbin/nologin
Gopher:x:13:30:gopher:/var/gopher:/sbin/nologin
Ftp:x:14:50:ftp User:/var/ftp:/sbin/nologin
Nobody:x:99:99:nobody:/:/sbin/nologin

^$: Blank Line

[[email protected] ~]# grep ' ^$ '. BASHRC

[[email protected] ~]# grep ' ^$ '. BASHRC | Wc-l
4
[Email protected] ~]#

\< or \b: Anchor word, any character following it must appear as the first word

[[email protected] ~]# grep ' root\> ' test2.txt
This is root
The user is Mroot
Chroot is a command
Mroot is not a word
[Email protected] ~]#

[[email protected] ~]# grep ' root\b ' test2.txt
This is root
The user is Mroot
Chroot is a command
Mroot is not a word
[Email protected] ~]#

\> or \b: anchors the ending, any character preceding it must appear as the tail of the word

[[email protected] ~]# grep ' \<root ' test2.txt
This is root
Rooter is a dog
[Email protected] ~]#

[[email protected] ~]# grep ' \broot ' test2.txt
This is root
Rooter is a dog
[Email protected] ~]#

\<root>\: Anchoring words

[[email protected] ~]# grep ' \<root\> ' test2.txt
This is root
[Email protected] ~]#

Group:
\(\)
\ (ab\) *
Back to reference
\1: Refers to the first opening parenthesis and all the contents of the corresponding closing parenthesis
[2]
[3]

He love his lover.
She like her liker.
He like his lover.

L.. E

[[email protected] ~]# grep ' \ (L.. e\). *\1 ' Test3.txt
He love his lover.
She like her liker.
[Email protected] ~]#

Practice:
1. Display the lines in the/proc/meminfo file that begin with a non-differentiated s;
Grep-i ' ^s '/proc/meminfo
grep ' ^[ss] '/proc/meminfo
2. Display the line ending with Nologin in/etc/passwd;
grep ' nologin$ '/etc/passwd

Remove the default shell for/sbin/nologin user list
grep "nologin$ '/etc/passwd | Cut-d:-f1

Remove the user name of the user whose default shell is bash with the lowest user ID number
grep ' bash$ '/etc/passwd | Sort-n-T:-k3 | head-1 | Cut-d:-f1

3, display/etc/inittab in the beginning of #, and followed by one or more white space characters, followed by any non-whitespace character line;
grep "^#[[:space:]]\{1,\}[^[:space:]]"/etc/inittab

4, display/etc/inittab contains: A number: (that is, two colons in the middle of a number) line;
grep ': [0-9]: '/etc/inittab

5. Display lines that start with one or more whitespace characters in the/boot/grub/grub.conf file;
grep ' ^[[:space:]]\{1,\} '/boot/grub/grub.conf

6. Display a line in the/etc/inittab file that begins with a number and ends with a number that is the same as the beginning number;
grep ' ^\ ([0-9]\). *\1$ '/etc/inittab

Practice:
1, find out a file, 1 digits, or 2-digit number;
grep ' [0-9]\{1,2\} '/proc/cpuinfo
grep--color ' \<[0-9]\{1,2\}\> '/proc/cpuinfo

2. Find the integer between 1-255 in the result of ifconfig command;

3. Find information about the account of the user whose name is student (must appear at the beginning of the line) on the current system, the file is/etc/passwd
grep ' ^student\> '/etc/passwd | Cut-d:-F3
Id-u Student

Regexp:regular EXPression
Pattern:

Regular Expressions:
Basic REGEXP: Basics
Extended REGEXP: Extended

Basic Regular Expressions:
.:
[]:
[^]:

Number of matches:
*:
\?: 0 or 1 times
\{m,n\}: At least m times, up to n times;

.*:

Anchoring:
^:
$:
\<, \b:
\>, \b:

\(\)
\1, \2, \3, ...

grep: A command that uses a pattern defined by a basic regular expression to filter text;
-I.
-V
-O
--color
-E: Using extended regular expressions
-A #:

[[email protected] ~]# grep-a 2 ' ^core ID '/proc/cpuinfo
Core id:0
CPU Cores:1
apicid:0
[Email protected] ~]#

-B #:

[[email protected] ~]# grep-b 2 ' ^core ID '/proc/cpuinfo
Physical id:0
Siblings:1
Core id:0

-C #:
[[email protected] ~]# grep-c 2 ' ^core ID '/proc/cpuinfo
Physical id:0
Siblings:1
Core id:0
CPU Cores:1
apicid:0
[Email protected] ~]#

To extend the regular expression:

Character Matching:
.
[]
[^]

Number of matches:
*:
?:
+: Match the characters in front of it at least 1 times
{M,n}

Location anchoring:
^
$
\<
\>

Group:
(): Group
\1, \2, \3, ...

Or
|: OR
C|cat:cat or cat, C or cat

[Email protected] ~]# grep-e ' C|cat ' Test6.text
Cat
Cat
C
China
[Email protected] ~]#

[[email protected] ~]# grep-e ' (c|c) at ' Test6.text
Cat
Cat
[Email protected] ~]#

[Email protected] ~]# grep-e ' ^[[:space:]]+ '/boot/grub/grub.conf
Root (hd0,0)
kernel/vmlinuz-2.6.32-504.el6.x86_64 ro root=uuid=57d85756-7680-4c7c-9125-6ad67dae2c45 Rd_NO_LUKS rd_NO_LVM LANG=en _us. UTF-8 rd_no_md sysfont=latarcyrheb-sun16 crashkernel=auto keyboardtype=pc keytable=us Rd_NO_DM quiet
Initrd/initramfs-2.6.32-504.el6.x86_64.img
[Email protected] ~]#

GREP-E = Egrep

4. Display all files that end with a number and do not contain a blank in the file name;
LS *[^[:space:]]*[0-9]?????????

Find the number between 1-255 in the/boot/grub/grub.conf file;
\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \>

\.

Ifconfig | Egrep ' \< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \>\.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \>\.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \>\.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \> '

Ifconfig | Egrep--color ' (\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \>\.) {3}\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \> '

IPV4:
Class 5: A B C D E
a:1-127
b:128-191
c:192-223

\< ([1-9]|[ 1-9][0-9]|1[0-9]{2}|2[01][0-9]|22[0-3]) \> (\.\< ([0-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-4] \>) {2}\.\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-4]) \>

http://mageedu.blog.51cto.com/

grep, Egrep
Fgrep: Regular expressions are not supported

Linux Common Commands-grep,egrep,regexp