Linux Basics (9) Text processing grep of the Three Musketeers

The role of grep: search for text according to the specified pattern, displaying the lines of text that match to

Syntax format: grep "PATTERN" file

Pattern is a regular expression, and grep is generally used in conjunction with regular expressions, so we'll start with a regular expression under the shell. Regular expressions, in simple terms, are matching rules. A line of text matches a regular expression with only two results, either matched or mismatched.

Next we introduce the regular expression in detail

1. Meta-characters:

.: Matches any single character

[]: matches any single character within the specified range

[^]: matches any single character outside the specified range

Character Set:

[:d Igit:] or [0-9] number

[: Lower:] or [A-z] lowercase letter

[: Upper:] or [A-z] capital Letter

[:p UNCT:] Punctuation

[: Space:] Blank

[: Alpha:] [A-za-z] Letter

[: Alnum:] [a-za-z0-9] Letter + number

[^[:d Igit:]] Non-digital

2. Number of matches (greedy mode, always match to last):

*: Matches any of its preceding characters any time

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

A*b, A?b

A.*b

. *: Any character of any length (note that it differs from a wildcard)

\?: match its preceding characters 1 or 0 times don't forget to escape \

\{m,n\}: Match its preceding characters at least m times, up to n times do not forget to escape \

\{1,\} at least 1 times

\{0,3\} 0 to 3 times

3. Location anchoring:

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

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

^$: Blank Line

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

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

4. Group: Don't forget to escape

\(\)

\ (ab\) * AB is seen as a whole

Back to reference

\1: Refers to the first opening parenthesis and all the contents of the corresponding closing parenthesis

[2]

[3]

Light says do not practice false bashi. Here are a few examples

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

7. 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 line), 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

grep ' ^l\ ([0-9]\): \1.*\1$ '/etc/inittab

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Linux Basics (9) Text processing grep of the Three Musketeers