Rookie's growth record--linux regular expressions and grep

Regular expressions

is a pattern written by a class of characters, many of which do not represent their literal meaning, but rather express control or wildcard functions; a regular expression uses a single string to describe and match a series of strings that conform to a certain syntactic rule. In many text editors, regular expressions are often used to retrieve and replace text that conforms to a pattern.

Metacharacters: does not represent its literal meaning, but is used for additional functional descriptions, below is a list of commonly used metacharacters

Symbol	Meaning
	means match any single character, example A., match AB, AC, AD, ax
*	Span style= "font-family: ' The song Body '; > can appear any time, example a*b, matching AAB, B, ab
\?	means The character in front of it is optional, example A\?b, Match AB, B, Xb
\{m\}	Span style= "font-family: ' The song Body '; > denotes M-Times , example a\{2\}b, matching AAB , Aaaab , Aaaaaab BBB
\{m,n\}	Span style= "font-family: ' The song Body '; > indicates that the character before it appears at least m times, up to n times, example a\{2,5\}b, matching AAB, Aaab,
\{m,\}	Span style= "font-family: ' The song Body '; > indicates that the characters in front of it appear at least m times, example a\{2,\}b, matching AAB, aaaaaaaaaaaab bbbb
\{0,n\}	Span style= "font-family: ' The song Body '; > indicates that the characters in front of it appear up to n times, example a\{0,3\}b, Match B, AAB, Aaaaab bbbb
.*	Span style= "font-family: ' The song Body '; > denotes , greedy mode, as many as possible to match
^	Span style= "font-family: ' The song Body '; > Indicates the beginning of a line anchor, example ^a, representing a row beginning with a
$	Span style= "font-family: ' The song Body '; > represents the end of line anchoring, example $ A, representing the line ending with a
^$	Span style= "font-family: ' The song Body '; > indicates a blank line
\<	Span style= "font-family: ' The song Body '; > Indicates the first anchor of the word, appearing on the left word of the word: a string of consecutive characters that do not contain special characters
\>	Span style= "font-family: ' The song Body '; > represents the ending anchor, appearing on the right side of the word
\#	Span style= "font-family: ' The song Body '; It refers to the content that the nth parenthesis matches to, not the schema itself, example \ (ab\). *\1, matching ab sdfdf ab
+	Indicates that the character before it appears at least once

Linux grep and Egrep

grep (Global search Regular expression (RE) and print out of the line, full search of regular expressions and print out rows) is a powerful text search tool that uses regular expressions to search for text. and print out the matching lines.

Extended Regular expression: Egrep or Grep-e

Basic Regular Expressions: grep

Common parameters:

-V: Reverse Selection

-O: Displays only the matched string, not the line where the string is located

-i:ignore-case, ignoring character case

-E: Supports the use of extended regular expressions

The biggest difference between Egrep and grep: Egrep is the expanded version of grep, which improves the operation of many traditional grep that cannot or is inconvenient. Say:
-Not supported under grep? With + These two kinds of modifier, but Egrep is available.
-grep does not support a|b or (ABC|XYZ) such "or one" pairs, but egrep can.
-grep needs \{and \} When processing {n,m}, but egrep is not required.

Examples

1. Display the lines in the/proc/meminfo file that begin with uppercase or lowercase s;

# grep-i ' ^s '/proc/meminfo

# grep ' ^[ss] '/proc/meminfo

# grep-e ' ^ (s|s) '/proc/meminfo

2. Display the default shell of the/etc/passwd file as a non-/sbin/nologin user;

# grep-v "/sbin/nologin$"/etc/passwd | Cut-d:-f1

3. Display the user whose default shell is/bin/bash in/etc/passwd file;

Further: Display only the user whose ID number is the highest in the above results;

# grep "/bin/bash$"/etc/passwd | SORT-T:-k3-n | Tail-1 | Cut-d:-f1

4. Find out one or two digits in the/etc/passwd file;

# grep "\<[0-9][0-9]\?\>"/etc/passwd

# grep "\<[0-9]\{1,2\}\>"/etc/passwd

5. Display the line beginning with at least one blank character in/boot/grub/grub.conf;

# grep "^[[:space:]]\{1,\}"/boot/grub/grub.conf

6, display/etc/rc.d/rc.sysinit file, start with #, followed by at least one white space character, and then have at least one non-whitespace character line;

# grep "^#[[:space:]]\{1,\}[^[:space:]]\{1,\}"/etc/rc.d/rc.sysinit

7. Find the line ending with ' LISTEN ' in the result of netstat-tan command execution;

# Netstat-tan | grep "listen[[:space:]]*$"

8, add user bash, Testbash, basher, Nologin (shell for/sbin/nologin), and find the current system on its user name and the default shell of the same user;

# grep "^\ ([[: Alnum:]]\{1,\}\):. *\1$"/etc/passwd

10. Display the default shell of root, fedora or User1 user on the current system;

# grep-e "^ (root|fedora|user1):"/etc/passwd | Cut-d:-f7

11, find a word in the/etc/rc.d/init.d/functions file followed by a set of parentheses "()" line;

# Grep-o-E "\<[[:alnum:]]+\>\ (\)"/etc/rc.d/init.d/functions

12. Use the echo command to output a path, and then use grep to remove its base name;

echo "/etc/sysconfig/" | Grep-o-E "[[: alnum:]]+/?]

# echo "/etc/sysconfig/" | Grep-o-E "[^/]+/?$" | Cut-d/-F1

13. Find the number between 1-255 in the result of ifconfig command;

# Ifconfig | Grep-o-E "\< ([1-9]|[ 1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5]) \> "

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