20150827-linux grep text Filtering tool and regular expression

To be perfected .....

Grep:

Linux Text Processing Three musketeers:

grep: Text filtering tool;

Sed: Text editor (line); stream editor

awk: Text report Generator; awk implementation on Linux is gawk

Grep:global search REgular expression and Print out of the line.

Function: Text Search tool, according to user-specified "pattern" line to search for target text, print the matching line;

Pattern: The filter condition written by metacharacters and text characters of regular expressions;

Metacharacters: A character does not represent its literal meaning, but is used to denote a wildcard or control function;

Divided into two categories:

Basic Regular Expressions: BRE

Extended Regular expression: ERE

The regular expression engine;

grep [OPTIONS] PATTERN [FILE ...] ：

Options:

--color=auto: The matching to the string to do highlighting;

-V: The display mode does not match the row;

-I: ignore character case;

-O: Displays only strings that can be matched to the pattern;

-Q: Silent mode;

-E: Using extended regular expressions;

The metacharacters of the basic regular expression:

Character Matching:

.: matches any single character;

[]: matches any single character within the specified range;

[^]: matches any single character within the specified range;

[: Lower:], [: Upper:], ...

Number of occurrences: used to specify the number of times after the character;

*: any time;

Abxy

Xay

Xxxxxxxy

grep "X*y"

\?:0 or 1 times;

grep "X\?y"

\+:1 or multiple times;

\{m\}: Precisely limited to M times;

\{m,n\}: At least m times, up to n times, [M,n]

\{0,n\}: Up to n times;

\{m,\}: at least m times;

. *: Matches any character of any length;

Location anchoring:

^: Anchor at the beginning of the line, for the leftmost mode;

$: End of line anchoring; for the rightmost side of the pattern;

\< \b: The first anchor of the word; the left side of the pattern used to denote the word;

\> \b: the ending anchor; the right side of the pattern used to represent the word;

^$: blank line;

Group: \ (\)

The contents of the pattern in the grouped parentheses are recorded by the regular expression engine during execution, and the built-in variables are saved: The variables are \1, \2, ...

\1: From the left, the first opening parenthesis, and the matching right parenthesis in the middle of the pattern match to the content;

\2:

...

Back reference: Use a variable to refer to the character that matches the pattern in the preceding grouping brackets;

Practice:

1. Display the line in the/etc/passwd file ending with bash;

2. Display the two-digit or three-digit number in the/etc/passwd file;

3, display the ' Netstat-tan ' command results with ' LISTEN ' followed by 0, one or more blank characters end of the line;

4, add user Bash,testbash, basher and Nologin user (Nologin the user's shell is/sbin/nologin), and then find the/etc/passwd file in the user name with the shell name of the line;

An extended regular expression:

The grep family has three commands:

grep: basic Regular expression

-e: extending regular expressions

-F: Regular expressions are not supported

Egrep: Extending Regular expressions

Fgrep: Regular expressions are not supported

Extend the metacharacters of regular expressions:

Character Matching:

.: Any single character

[]:

[^]:

Number of matches:

*

?: 0 or 1 times;

+: more than 1 times;

{m}: exact match m times;

{M,n}: At least m times, up to n times;

Anchoring:

^: Anchoring the beginning of the line

$: Anchor Line End

\<, \b

\>, \b

Group: ()

Back reference: \1, \2, ...

Or:

A|b

C|cat: does not represent cat or cat, but is C or cat;

To be written (C|c) at

Practice:

1. Display the default shell and UID of root, CentOS or User1 user on the current system;

2, find a word in the/etc/rc.d/init.d/functions file (the middle of the word can be underlined) followed by a set of parentheses line;

3, use echo to output a path, and then egrep find its path base name;

Further: Use Egrep to remove its directory name;

4. Find out the number between 1-255 in the result of ifconfig command execution;

Use of the grep command;

[[: Space:]]\{1,\}

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20150827-linux grep text Filtering tool and regular expression