Linux grep command, regular expression

grep command and regular expression

grep command

grep, Egrep, Fgrep

Grep,sed,awk Text Processing Three Musketeers

Grep:global search REgular expression and Print out of the line; finds the regular expression and displays the matching rows;

Regular expressions

Regular expressions are mainly applied to text, so it is applied in various text editor situations, and many programming languages support string manipulation using regular expressions;

The mainstream regular engine is divided into three categories: DFA, the traditional NFA; POSIX NFA;

Dfa:awk,egrep,flex,mysql,procmail, etc.;

Traditional type Nfa:gun emacs,java,ergp,less,. NET language, PCRE Library,perl,php,python,ruby,sed,vi,vim;

POSIX nfa:mawk,mortice Kern System ' Utilities,gun Emacs (can be explicitly specified when used);

DFA/NFA mix: GUN awk,gun grep/egrep,tcl;

grep usage:

grep: External command

Function: Displays the rows that are matched by the pattern from the source data;

Note: grep default mode of operation is greedy mode, that is, each grep pattern matching is done according to as many as possible to complete the matching criteria;

grep [OPTIONS] PATTERN [FILE ...]

grep [OPTIONS] [-e PATTERN] [-F FILE] [file]

Common options:

-A NUM,--After-context=num: The line to which the pattern is matched and the subsequent # line, if not, then does not appear; cannot be used with the-o option;

-B num,-before-context=num: The line that is matched by the pattern and its first # line, which is not displayed if not, and cannot be used in conjunction with the-o option;

-C NUM,--Context=num: The line that is matched by the pattern and the # lines before and after it, it is not displayed before or after it, and cannot be used with the-o option;

--color=auto: Coloring the text to match to the display;

-E pattern,--Regrep=pattren: Implements the matching of multiple patterns in a logical or relationship, with each-e option having only one schema as the parameter;

-E,--extended-regexp: extended Regular expression switches enable grep to use extended regular expressions with egrep;

-I,--ignore-case: ignore character case;

-N,--line-number: Displays the line that matches to and adds the line number to it (the line number in the file)

-o,--only-matching: Displays only the matched string;

-Q,--quiet--silent: Silent mode, do not output any information;

-V,--invert-match: Displays rows that are not matched to;

-w,-word-regexp: Entire line matches whole word;

Exit Status:

If the selected row is successfully found, the status return value is 0; otherwise 1;

Basic regular Expression meta-characters:

1. Character Matching:

. : matches any single character;

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

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

[:d igit:]: denotes all decimal digits equivalent to [0-9];

[: Lower:]: denotes all lowercase letters, equivalent to [A-z];

[: Upper:]: Denotes all uppercase letters, equivalent to [A-z];

[: Alpha:]: denotes all letters, including uppercase and lowercase;

[: alnum:]: means all uppercase and lowercase letters and decimal digits are included;

[: space:]: denotes all whitespace characters;

[:p unct:]: denotes all special characters;

2. Number of matches (used after the character to specify the number of occurrences):

*: Match its preceding character any time;

. *: Matches any character of any length;

\?: matches its preceding character 0 or 1 times;

\+: Matches its preceding character 1 or more times;

\{m\}: Matches its preceding character m times;

\{m,n\}: Matches its preceding character at least m times, up to n times;

\{0,n\}: Up to n times; \{m,0\}: at least m times;

3. Location anchoring:

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

$: line anchor; for the right-hand side of the pattern;

^$: blank line;

^[[:space:]]*$: A blank line or a line containing white space characters;

\< or \b: The first anchor of the word;

\> or \b: final anchoring;

\<pattern\>: matches complete words;

4. Grouping and referencing:

\ (\): Bind one or more characters together and treat as a whole; Example: \ (xy\) *ab

Note: The contents of the pattern matching in the grouping brackets are automatically recorded in the internal variables by the regular expression engine, and these variables are:

\ 1: The pattern from the left side, the first opening parenthesis and the matching closing parenthesis, matches the character of the pattern;

\ 2: ... The second one ....

\ 3: ...

Egrep

An extended regular expression implementation is similar to the grep text filtering feature: GREP-E

Egrep [OPTIONS] pattren [FILE ...]

Options:

-i,-o,-v,-q,-a,-b,-c

-G: Support for basic regular expressions, equivalent to grep;

Extend the metacharacters of regular expressions:

In addition to not having to use the escape character "\" in the number of matches, the other is the same as the metacharacters of the basic regular expression;

Fgrep

Regular expression metacharacters are not supported, and it is better to use fgrep when there is no need to use meta-characters to write patterns;

Application Examples:

1. Remove the base name of the/etc/sysconfig/network-scripts/ifcfg-eno16777736 file

~]# echo/etc/sysconfig/network-scripts/ifcfg-eno16777736 | Grep-o "[^\/]\+\/\?$"

2. Find the line with a parenthesis followed by a word (including an underscore) in the/etc/rc.d/init.d/functions file

~]# grep-o ". *_*.* ()"/etc/init.d/functions

3. Display user account information in the/etc/passwd file with the same user name as the user's default shell name

~]# grep-e "(\<.+\>). *\1$"/etc/passwd

4. Displays the absolute path of the dynamic library file used by the LS command that is seen with the LDD command

~]# Ldd/bin/ls | Egrep-o "/.*lib (+)/[^[:space:]]+"

5.grep can also find matching characters from multiple files, such as:

~]# grep ' ro './*

Linux grep command, regular expression