Linux Text processing the grep command of the Three Musketeers

Linux Text processing the Three Musketeers grep

grep: Text filter (Pattern: pattern) Tool

grep, Egrep, fgrep (regular expression search not supported)

Sed:stream Editor, text editing tools

Implementation Gawk on Awk:linux, Text Report Generator

Grep

• Grep:global Search RegularExpression and Print out of the line

Function: The text Search tool, according to the user-specified "mode" to match the target text line by row to check; print matching lines

Patterns: Filter conditions written by regular expression characters and text characters

• grep [OPTIONS] PATTERN [FILE ...]

grep root/etc/passwd

grep "$USER"/etc/passwd

grep ' $USER '/etc/passwd

grep ' WhoAmI '/etc/passwd

• grep command Options

--color=auto: Shading to matching text (centos6 default, 7 default plus)

-V: Displays rows that are not matched by pattern

-I: Ignore character case

-N: Show matching line numbers

-C: Count the number of matching rows

-O: Show only the matching string

-Q: Silent mode, does not output any information

-A #: After, followed by # lines

-B #: Before, Front # line

-c #:context, front and back # lines

-E: Implementing a logical or relationship between multiple options

Grep–e ' Cat '-e ' dog ' file

-W: Matches entire word

The word number underscore cannot be used as a delimiter for a word, others can

-E: Use ere

-F: Equivalent to Fgrep, does not support regular expressions

Regular expressions

Match a string in a file

• REGEXP: A pattern written by a class of special characters and text characters, in which some characters (metacharacters) do not represent literal meanings, but are functions that represent control or a wildcard
• Program support: Grep,sed,awk,vim, less,nginx,varnish, etc.
• Divided into two categories:

1. Basic Regular Expressions: BRE
2. Extended Regular expression: ERE

GREP-E, Egrep

• Regular expression engine:

Using different algorithms to check the software module for processing regular expressions

PCRE (Perl Compatible Regular Expressions)

• Meta-character classification: character matching, number of matches, position anchoring, grouping
• Man 7 regex

Basic Regular Expression metacharacters (different from wildcard characters)

• Character Matching:

. Match any single character

[] matches any single character within the specified range

[^] matches any single character outside the specified range

^ put in [] inside and outside are different concepts

[: Alnum:] Letters and numbers

[: Alpha:] represents any English uppercase and lowercase characters, i.e. A-Z, A-Z

[: Lower:] lowercase letter [: Upper:] Uppercase

[: Blank:] white space characters (spaces and tabs)

[: Space:] Horizontal and vertical white space characters (more than [: blank:] contains a wide range)

[: Cntrl:] non-printable control characters (backspace, delete, alarm ...) ）

[:d igit:] decimal digits [: xdigit:] hexadecimal digits

[: Graph:] printable non-whitespace characters

[:p rint:] printable characters

[:p UNCT:] Punctuation

• Number of matches: used after the number of characters to be specified, to specify the number of occurrences of the preceding character

* match the preceding character any time, including 0 times

Greedy mode: Match as long as possible

. * Any character of any length

\ match its preceding character 0 or 1 times

\+ matches the characters in front of it at least 1 times

\{n\} matches the preceding character n times

\{m,n\} matches the preceding character at least m times, up to N times

\{,n\} matches the preceding character up to n times

\{n,\} matches the preceding character at least n times

A\|bcd=a or BCD

\ (a\|b\) CD=ACD or BCD

• Position anchoring: positioning where it appears

^ Beginning of the line anchor, for the leftmost mode

$ line End anchor for the right side of the pattern

^pattern$ for pattern matching entire row

^$ Empty Line

^[[:space:]]*$ Blank Line

\< or \b The first anchor for the left side of the word pattern

\> or \b ending anchor; for the right side of the word pattern

\<pattern\> matches the entire word =-w

• Group: \ (\) binds one or more characters together and treats them as a whole, such as: \ (root\) \+
• The contents of the pattern in the grouping brackets are recorded in internal variables by the regular expression engine, which are named: \1, \2, \3, ...
• \1 represents the character that matches the pattern between the first opening parenthesis and the matching closing parenthesis from the left
• Example: \ (string1\+\ (string2\) *\)

\1:string1\+\ (string2\) *

\2:string2

• Back reference: References the pattern in the preceding grouping brackets matches the character, not the pattern itself
• Or: \|

Example: A\|b:a or b c\|cat:c or cat \ (c\|c\) At:cat or cat

Egrep and extended Regular expressions

• egrep= GREP-E
• Egrep[options] PATTERN [FILE ...]
• Extend the metacharacters of regular expressions:
• Character Matching:

. Any single character

[] Specify the range of characters

[^] characters not in the specified range

Extending regular Expressions

• Number of matches:

*: matches the preceding character any time

?: 0 or 1 times

+:1 Times or more

{m}: matches M-Times

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

• Location anchoring:

^: Beginning of the line

$: End of line

\<, \b: the first language

\>, \b: The end of the language

• Group:

()

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

• Or:

A|b:a or B

C|cat:c or Cat

(c|c) At:cat or cat

• Appendix:

~] #ls | grep *

Enter LS to list A file that starts with a, and it will have a line, and the default does not enter the directory. You need to add a "a *" to the regular expression

Linux Text processing the grep command of the Three Musketeers