The beginner of Linux--regular expression and grep system

grep sed awk is called the text-processing Three Musketeers, this article mainly about grep.

Grep:global search Regular Expression and Print out of the line

The global search is performed using regular expressions and the matching rows are displayed;

To gain an in-depth understanding of grep, first recognize and learn the regular expressions, and then we will introduce the regular expressions.

Regular expressions

Regular expression: is the pattern (pattern) written by a class of characters; Metacharacters: does not represent the meaning of the character itself, and is used for the description of the additional functionality.

Regular expression meta-characters:

Basic Regular Expression BRE

GLOBBING------A simplified version of the regular expression: []? *

Character Matching:

. : Matches any single character

[]: matches any single character within the specified range

^: matches any single character outside the specified range

All character sets can be placed in [] to match a single character

[: Lower:]: All lowercase letters

[: Upper:]: All uppercase letters

[: Alpha:]: All letters

[:d Igit:]: All decimal digits

[: Space:]: white space character

[: Alnum:]: Letters or numbers

[:p UNCT:]: All punctuation

[: Xdigit:]: all 16 binary digits

A-Z: all lowercase letters

0-9: all 10 binary digits

Number of occurrences: the number of occurrences of that character before the type of character

*: The preceding character can appear any time (0,1, multiple times)

\?: The characters in front of it are optional (0/1 times)

\+: The preceding character appears at least once (one or more times)

\{num\}: The preceding character must appear m times

\{m,n\}: The preceding character appears at least m times at most n times;

\{,n\}: The preceding character appears at least 0 times at most occurrences n times;

\{m,\}: The characters in front of it appear at least m times, the more the more;

The means by which any character of any length is represented in a regular expression:. *

Positional anchor characters:

Line anchoring:

Beginning of line anchoring: ^

End of line anchor: $

Word Anchor:

The first anchor:\< \b

Word end anchor: \> \b

\b: Anchoring method in old version, not recommended

For the regular expression engine, the word is a continuous string consisting of non-special characters;

Grouping and referencing characters:

\ (pattern\): All characters matched by this PATTERN are treated as an integral whole

In the regular expression engine, there is a series of built-in variables that hold all the character information within the group for the back reference; These variables are: \1, \2 \3 ....

Pattern1\ (pattern2\) pattern3\ (pattern4\ (pattern5\) \)

\1: The character pattrn matches in the first set of parentheses is pattern2

\2: The character that pattrn matches in the second set of parentheses is PATTERN4

\3: The character that pattrn matches in the third set of parentheses is Pattern5

Or:

\|

Note: \| Treat the strings on the left and right side as a whole

Extending the regular expression ERE

parts that are not the same as the basic regular:

+: match the characters in front of it at least 1 times

?: Match its preceding character 1 or 0 times

{M,n}: matches the preceding character at least m times, up to N times
(): Group
A|b:a or B

Continue learning grep After a preliminary understanding of regular expressions

grep [OPTIONS] PATTERN [FILE ...]

PATTERN: Filter condition consists of regular expression meta-characters and text characters without special meanings;

Metacharacters of regular Expressions: the engine of the regular expression is interpreted as a special meaning;

Pcre-perl: Language's regular expression engine

Basic amount Regular Expression: BRE

Extended Regular expression: ERE

GREP supports only basic regular expressions by default

Egrep only extended regular expressions are supported by default

Fgrep the expression engine is not turned on by default

Text characters: Only those characters that have the meaning of the character surface

Common options:

-I,--ingore-case: Ignores the case of text characters

-V,--invert-match: reverse match; The result is a line that pattern does not match successfully

-C,--count: Count, statistics match all lines of pattern

-O,--only-mathing: Turns off greedy mode, showing only what pattern can match

-Q,--Quiet,--silent: Quiet mode, do not output any matching results

--color [=when],--colour[=when]: Displays the contents of the matching pattern in a highlighted form--color=auto

-e: Extended regular expression, grep-e equivalent to Egrep

-f:grep-f equivalent to Fgrep,

-G: Basic regular expression, egrep-g equivalent to grep

-p; using the Pcre (Perl Common Regular Expression) engine

-A num,--after-context=num: Displays the NUM lines that follow the pattern line while displaying it

-B num,--before-context=num: Displays the NUM lines in front of the matching pattern line

-C num,-num,--context=num: Displays the NUM rows before and after the matching pattern line

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

By default, only one pattern is allowed behind the grep command, and if you want to use the-e option to write multiple pattern in a single grep search, each-e option can use only one pattern as the parameter;

Writes the required pattern to a file, guaranteeing that there is only one pattern per line, and using the-F file method to implement multiple pattrn as parameters;

Other text-processing commands:

Wc:

Wc[option] ... [FILE]

-L: Show only the number of rows

-W: Displays only the number of words

-C: Show only the number of characters

Cut:remove sections from all line of files

Files that can be trimmed by cut are generally text documents with a certain structure or format;

Cut OPTION ... [FILE] ...

-D,--delimiter=delim: Specifies the delimiter to be relied upon when the pruning operation is implemented, by default a white-space character

-F,--fields=list: Specifies the number of the field according to the defined delimiter

How to use address delimitation:

#: Select a single field to be specified

#,#: Discrete number of individual fields that are specified

#-#: Multiple specified fields in a row

--output-delimiter=string: Specifying the output delimiter

grep Practice Questions:

1) display lines in the/etc/passwd file that do not end with bash;

# grep-v ' bash$ '

2) display the default shell for non-/sbin/nologin users in the/etc/passwd file

# grep-v '/sbin/nologin$ '/etc/passwd

3) Find/etc/grub2.cfg (in the CentOS6 is/etc/grub.conf) file, a non-blank line beginning with a blank character;

# grep--color=auto ' ^[[:space:]]\+[^[:space:]] '/etc/grub2.cfg

  4) displays the current system rootx or bash default shell of the user;

     # grep -E ‘^(root|bash):‘ /etc/passwd | cut -d: -f7

 5)找出/etc/rc.d/init.d/functions文件中某单词后跟一组小括号“()”行；

   # grep -E -o ‘\<[[:alnum:]]+\>\(\)‘ /etc/rc.d/init.d/functions 

6) Find all the file paths in the results of the "ldd/usr/bin/vim" command;

# Ldd/usr/bin/vim | Cut-d '-f3

7) Find out all the lines in the/proc/meminfo file that start with uppercase s or lowercase s, how many ways do you implement the task?

# Cat/proc/meminfo | grep ' ^[ss] '

# Cat/proc/meminfo | grep ' ^[s| S] '

# Cat/proc/meminfo | Grep-i ' ^s '

The beginner of Linux--regular expression and grep system