The small theory of LINUX--grep

Linux--grep This is a very important tool, Sed,awk,grep is called the 3 swordsman of Linux!

grep: Searches the text according to the pattern, and displays the line of text that conforms to the pattern.

To cite an example, grammar:

Example 1: #grep-i ' root '/etc/passwd #找出/etc/passwd the root or root line in this file,-I is ignore , ignoring the meaning, ignoring the case.

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Example 2: #grep-V ' root '/etc/passwd #找出/etc/passwd This file, the rows that are processed with root are not displayed

Out, the rest of the lines are shown,-V is invert, so invert

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Example 3: #grep-o ' root '/etc/passwd #找出/etc/passwd This file, matches the word, not the line!

Just show the word root, and you'll know when you knock it all over.

The function of the word. -O, is only, just, only!

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Example 4: #grep--color ' root '/etc/passwd #找出/etc/passwd the root row in this file, using

The color is displayed. --color, color.

Tip: #alias grep= ' grep--color ' #有了这个别名以后, directly with grep, without adding--color, will

The color is displayed directly, but the computer restarts, it will fail.

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Example 5: #grep-A ' model '/proc/cpuinfo #意思是显示出model及其后面的2行,-A, want after

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Example 6: #grep-B ' model '/proc/cpuinfo #意思是显示出model及其前面的2行,-B, want to before

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A little bit of knowledge:

Regular Expressions: (English: REGular expression----REGEXP)

*: Asterisk is any character of any length

？ : The question mark is any single character

[]: The brackets match any single character within the specified range, such as: [0-9] is a number from 0 to 9

Example: #grep--color [0-9]/etc/inittab 0 to 9 of the numbers will be displayed. Knock yourself,

Look at the effect. Only if you knock more, you understand.

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Continue to say:

[^]: brackets with a sharp-pointed number, which matches any single character outside the specified range

Example: #grep--color [^0-9]/etc/inittab you look at the effect, in addition to the 0-9 Arabic numerals,

All the other letters are displayed.

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Character set: [: Alpha:] #意思是所有字母的集合

[:d igit:] #意思是所有数字的集合

[: Upper:] #意思是所有大写字母的集合

[: Lower:] #意思是所有小写字母的集合

[: Alnum:] #意思是所有字母和数字的集合

[:p UNCT:] #意思是所有标点符号的集合

[: Space:] #意思是所有空格的集合

Example: grep ' [[: Upper:]] '/etc/passwd #意思是列成/etc/passwd all the files in this file

Capital.

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Number of matches:

*: The asterisk matches any of the characters in front of it, including 0 times, or countless times, at any time.

. *: Dots and asterisks, which represent any character of any length! Note the comparison with an asterisk alone.

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？ : A single question mark that matches the preceding character once or 0 times.

Example: grep ' a\?b '/etc/passwd #意思是把b, or 1 A in front, then 1 b

The words show up.

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

Example: grep ' a\{1,3\} '/etc/passwd

\{1,\} #最少1次, up to countless times

\{0,3\} #最少0次, up to 3 times, 0 and 3 separated by commas

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^: a bracket character that indicates that the content after this character must appear at the top of the line.

$: Dollar character that indicates that the content before this character must appear at the end of the line.

Example: # grep ' ^a '/etc/passwd means listing a letter must appear at the beginning of the line

# grep ': $ '/etc/passwd means to list the lines that the colon must appear at the end

Tip: ^$: The combination of this character represents a blank line.

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\b or \<: means that any character following it must appear as the header of the word.

\b or \>: means that any character in front of it must appear as the tail of the word.

Compare this symbol to ^ and $ to see what's different.

Comparison:

#grep--color ' \<root '/etc/passwd

#grep--color ' ^root '/etc/passwd

Tip: \<root\> This represents the line for these 1 words, root!

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Group:

\（ \）

#grep ' \ (root\). * \1 '/etc/passwd #命令中, \1 means to refer again to the contents of the preceding parentheses, root

Comparison: sed ' s#\ (root\) #\1r#g '/etc/passwd A comparison of different, better understanding of sed and grep.

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Extensible Regular Expression: (extended regular expression)

What do you mean by extension, for example:

# grep-e ' (root) '/etc/passwd

# grep ' \ (root\) '/etc/passwd

The above 2 commands are to/etc/passwd this file with the root of the line is displayed, but one has a backslash, a no backslash, this is the difference! -e This option plus later, there is no backslash.

#egrep ' (root) '/etc/passwd #注意, egrep = = Grep-e, same function.

The writing is not good, please understand! Can help you a little bit, even if not written in vain.

The small theory of LINUX--grep