Linux--grep and regular expressions

Need for text lookup

Grep,egrep, Fgrep

grep: Search for text based on pattern and display the text lines of the review mode

Pattern: Text Word wildcards regular the expression's meta-character combination to match the condition

grep [Options] PATTERN [FILE ...]

-i:ignore Case ignores capitalization

--color: Matched to the word Fugauliang display

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

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

*: Any character of any length

? : Any single character

[] :

[^]

Regular expression: REGular expression,regexp

Metacharacters

. : Match any single character grep ' R. T '/etc/passwd

[]: 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:]

Number of matches (greedy mode):

*: Matches any of its preceding characters any time

A, B, AB, AAB, ACB, ADB, AMNB

A*b:b before matching any A, can not, the above match is B,ab,aab, ADB,AMNB

A.*b:ab,aab,acb,adb,amnb

. *: Any character of any length

\? : matches the preceding character 1 or 0 times

A\?b:b,ab,aab,acb,amnb

\{m,n\}: Matches the preceding character at least m times, up to N times

Location anchoring:

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

grep ' ^r. T '/etc/passwd

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

grep ' om$ '/etc/passwd

Numbers ending with:

grep ' [[:d igit:]]$ '/etc/inittab

^$: Blank Line

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

\> or \b: anchors the ending, any character in front of which must appear as the tail of the word

\<root\>: Root must appear for a whole word

Group:

\(\)

\ (ab\) *: AB appears together 0 or more times

Back to reference

\1: Apply the first opening parenthesis and all the content that corresponds to the closing parenthesis

[2]

[3]

L.. E

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/meninfo

2. Display the line ending with Nologin in/etc/passwd;

# grep ' nologin$ '/etc/passwd

Remove the default shell for/sbin/nologin user list

# grep '/sbin/nologin$ ' | 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

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\> '
grep ' ^student\> '/etc/passwd | Cut-d:-F3
Id-u Student

Student1
Student2

Exercise: Analyze the characteristics of the first two lines in the following text in the/etc/inittab file (the number must be the same in each row), and write a pattern that can be found exactly like two lines:
L1:1:WAIT:/ETC/RC.D/RC 1
L3:3:WAIT:/ETC/RC.D/RC 3

Linux--grep and regular expressions