A detailed description of the search tool and basic regular expressions for grep file content in Linux

A detailed description of the search tool and basic regular expressions for grep file content in Linux

grep command : Search for text based on patterns (matching criteria for text characters and meta-character combinations of basic Regular Expressions )

and displays the line of text that conforms to the pattern.

Format: grep [option] Match condition text name

Option:-I: Ignore case

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

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

--color: Search out files High brightness display

-A: Matches the next few lines of the specified line

-B: Matches the last few rows of the specified row

-C: matches the upper and lower lines of the specified row

-e: Extended regular expressions are equivalent to Egrep

Regular Expressions : Regular expression, abbreviated RegExp

Note: \ is a caret that prevents conflicts with bash and takes off its original meaning

Meta characters :

.: Matches any single character

[]: matches any single character within the specified range

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

For example: special character set: [:d igit:], [: Lower:], [: Upper:], [:p UNCT:], [: Space:], [: Alpha:], [: Alnum:]

number of matches :

*: Matches any of its preceding characters any time

. *: Any character of any length

\?: Match its preceding character 1 or 0 times

\{a,d\}: Matches the preceding characters at least a time, up to D times

For example: \{1,\} indicates that the preceding character matches at least 1 times and is not limited to

\{0,6\} means: The preceding character matches at least 0 times, up to 6 times

Example: Display lines that start with one or more whitespace characters in a/boot/grub/grub.conf file

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

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The position is locked or called anchored :

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

Example: Displaying lines beginning with R in the/etc/passwd file

grep ' ^r '/etc/passwd

$: Locks the end of the line, any content that precedes this character must appear at the end of the row

Example: Show the line in the/etc/passwd file that the bash ends

grep ' bash$ '/etc/passwd

^$: Indicates a 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

Example: Displays a line in the/etc/passwd file that begins with Root and ends with the root word

grep ' ^root\> '/etc/passwd

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group : \ (\)

For example: \ (mn\) * means matching any MN combination character

Back to reference

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

\2: Refers to the second opening parenthesis and all the contents of the corresponding closing parenthesis

\ n: references the nth opening parenthesis and all the contents of the corresponding closing parenthesis

Example: Displays a line in a/etc/inittab file that starts with a number and ends with the same number at the beginning

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

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This article is from the "Xavier Willow" blog, please be sure to keep this source http://willow.blog.51cto.com/6574604/1762878

The grep file Content search tool and basic regular expression in Linux