How to use grep commands in Linux

grep (abbreviated from globally search a Regular expression and print) is a powerful text-search tool that can use regular expressions to search for text and print matching lines. The grep family includes grep, Egrep, and Fgrep.

grep, Egrep, fgrep-print lines matching a pattern

How to use: grep [OPTIONS] PATTERN [FILE ...]

grep [OPTIONS] [-E PATTERN |-f file] [FILE ...]

Options:

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

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

-I: Ignore character size;

-E: Write using extended regular expressions;

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

Demonstrate:

The matching string is highlighted;

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Inverse, displaying rows that are not matched;

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Ignore letter case;

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Using an extended regular expression

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Show only strings that are matched to the pattern itself

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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;

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

*: any time;

\?:0 or 1 times;

\+:1 or multiple times;

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

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

\{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;

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:

*: any time;

?: 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;

Exercises:

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

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

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

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4. Add user bash, Testbash, basher, and Nologin user (Nologin user's shell is/sbin/nologin), then find the same line in the/etc/passwd file as the user name and its shell name

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5. Display the default shell and UID of root, CentOS, or User1 user on the current system (please create these users beforehand, if not present)

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6, 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

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7, use echo to output a path, and then egrep find its path base name; Further use Egrep to remove its directory name

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8. Find the number between 1-255 in the result of ifconfig command execution

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How to use grep commands in Linux