The grep of the Linux text processing weapon

First, what is grep?

Grep:global Search Regular expression (RE) and print out of the line, fully searching for regular expressions and printing them out, is a command-line tool that can find strings from text or streams, It can be more flexible by using regular expressions to find a string, that is, through complex patterns to find to meet your needs.

Ii. Classification of regular expressions

1. Basic Regular expression: BRE (basic REGULAR expression)

2, expand the Regular expression: ERE (EXTEND REGULAR expression)

3. Perl Regular Expression: PRE (Perl REGULAR expression)

Iii. features of grep,sed, and awk families

1, grep, egrep, fgrep the characteristics of the regular

1) grep supports BRE, ERE, PRE

The A.GREP directive does not follow any parameters, which means using the BRE

The b.grep instruction is followed by the-e parameter, which indicates the use of Ere

The c.grep instruction followed by the-p parameter indicates the use of the pre

2) egrep support Ere, PRE

The A.EGREP directive does not follow any parameters, which indicates the use of Ere

The b.egrep instruction followed by the-p parameter indicates the use of the pre

3) fgrep Default fixed string mode

A.fgrep default does not support Bre, ERE, only support fixed string mode, faster than Grep,egrep

B.fgrep command followed by-G selection, which means the BRE is supported

2. sed regular expression characteristics

1) SED supports BRE, ERE

a. The BRE is supported by default

The b.sed instruction is followed by the-R parameter, which indicates the use of Ere

C.sed is used to process text files, to curd the contents of a text file, to process it by line

3. awk Regular Expression features

1) awk (gawk) Regular expression features

A. Default support ere

B. As long as the columns of the text file are processed

Iv. Basic use of the grep command

1, the use of grep syntax

1) grep [option] pattern [FILE ...]

2. grep Common options

1)-C: The number of output matching rows

2)-I: do not distinguish between lowercase

3)-R: Recursive lookup (current search directory and its subdirectories)

4)-N: Output matching line and line number

5)-L: Only the file name of the matching string is output when multiple files are found

6)-V: Reverse match, which shows unmatched rows

7)-O: Show only the string that matches to itself

8)-Q: Equivalent to--quiet, silent mode ( can be used to determine whether this search contains this string, suitable for use in the script )

Notice: If there are metacharacters in the regular expression, use quotation marks to expand them, or use double quotes for variables in the pattern, or single quotes.

A.CENTOS7: Using grep aliases, the matching results are highlighted by default

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B.CENTOS6: Default is not used

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3, based on the use of basic regular expression meta-characters

1) Character matching

A.. Match any single character

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B. [] matches a single character within a specified range

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c. [^] characters outside the matching range

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2) Number of matches

A. * match the preceding character any time (0,1 or multiple)

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B. * Any character at any length after a

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C. \+ matches the preceding character at least once

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D. \? Matches the preceding character 0 or 1 times

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E. \{m\} The characters preceding it appear m times and M is a nonnegative integer

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F. \{m,n\} The characters preceding it appear m times and M is a nonnegative integer

\{0,n\} up to n times

\{m,\} at least m times

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3) Position anchoring

Limit the text that the pattern matches to, only where it is in the target text

A. ^ The beginning of the line anchoring, used for the leftmost mode, ^pattern

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B. $ line-tail anchoring, for the right-most mode, $pattern

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C. ^pattern$ matches entire row

^$ to match empty rows

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4) Word anchoring

A. \< or \b The first anchor, used for the left side of the word pattern, in the form of \<pattern,pattern\b

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B. \> or \b Ending anchoring, for the right side of the word pattern, formatted as pattern\>,pattern\b

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5) Grouping and referencing

A. \ (pattern\) handles this matching string as an integral whole

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6) | Or

A. a|b represents a or B, which represents the left or right side of the entire expression

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Five, the expansion of egrep expression

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The grep of the Linux text processing weapon