Quickly master grep commands and regular expressions

Built in support of extended regular expressions GNU edition grep Tools , be-all Linux All distributions are installed by default grep ,grep It is used to retrieve text information from any location on a server or workstation , How to be in Linux Systems and classes Unix Using regular expressions in our operating system grep Well ？

Learn about regular expressions quickly

1、 How to match what you're looking for ？

Regular expressions are just patterns that match each input line .

stay ‘/etc/passswd’ To retrieve the ‘vivek’ .

grep vivek /etc/passwd

Output result case ：

vivek:x:1000:1000:Vivek Gite,,,:/home/vivek:/bin/bash
vivekgite:x:1001:1001::/home/vivekgite:/bin/sh
gitevivek:x:1002:1002::/home/gitevivek:/bin/sh

Search in any case ‘vivek’ ( It's not size sensitive )：

grep -i -w vivek /etc/passwd

Case insensitive Retrieval ‘vivek’ and ‘raj’ ：

grep -E -i -w 'vivek|raj' /etc/passwd

In the last example , Using patterns that extend regular expressions .

Fixed location of retrieved content ：
You can use ^ and $ Symbols force a regular expression to match the beginning or end of a line, respectively . The following example shows ‘vivek’ The opening text .

grep ^vivek /etc/passwd

Example of output results ：

vivek:x:1000:1000:Vivek Gite,,,:/home/vivek:/bin/bash
vivekgite:x:1001:1001::/home/vivekgite:/bin/sh

You can just display with vivek The first line of text . For example, it doesn't show vivekgite , vivekg This is the beginning of the word .

grep -w ^vivek /etc/passwd

Search for ‘foo’ The text format at the end ：

grep 'foo$' FILENAME

You can also search for blank lines in the following way ：

grep '^$' FILENAME

2、 How to match specific characters ？

matching ‘Vivek’ or ‘vivek’ ：

grep '[vV]ivek' FILENAME

Or you can ：

grep '[vV][iI][Vv][Ee][kK]' FILENAME

You can match numbers （ For example, match vivek1 or Vivek2 ）：

grep -w '[vV]ivek[0-9]' FILENAME

You can match two digits （ For example, match foo11 , foo12 ）：

grep 'foo[0-9][0-9]' FILENAME

It's not just numbers , You can match letters ：

grep '[A-Za-z]' FILENAME

Show all containing “w” or “n” Text lines of letters ：

grep [wn] FILENAME

In the expression in brackets , stay “ [: ” and “ :] ” The name of the character class attached to ： Represents a list of all characters belonging to the class . Standard character class name ：

[:alnum:] –  Alphanumeric character .
[:alpha:] –  Alphabetical order 
[:blank:] –  Spaces and tabs .
[:digit:] –  Numbers ： ‘0 1 2 3 4 5 6 7 8 9’.
[:lower:] –  Lowercase letters ：‘a b c d e f ‘.
[:space:] –  Special characters ： tabs , A newline , Vertical tabs 、 Change the page , enter , And space .
[:upper:] –  Capital ：‘A B C D E F G H I J K L M N O P Q R S T U V W X Y Z’.

In the following example , Match all capital letters ：

grep '[:upper:]' FILENAME

3、 How to use wildcards ？

You can use it. “.” Instead of a single character . In the following example , Checked all the letters with “b” start 、 Letter “t” The three character word at the end .

grep '\<b.t\>' FILENAME

In the example above :

\<  Match the space string at the beginning of the word 
\>  Match the space string at the end of the word 

Retrieve and output results for all two letters ：

grep '^..$' FILENAME

Retrieve and display all information to ‘.’ And the result at the beginning of the number ：

grep '^\.[0-9]' FILENAME

Escape character ’.’
The following regular expression looks for IP Address 192.168.1.254 The expected results will not be achieved ：

grep '192.168.1.254' /etc/hosts

Three of these points need to be escaped ：

grep '192\.168\.1\.254' /etc/hosts

The following example will match only one address ：

egrep '[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}\.[[:digit:]]{1,3}' FILENAME

The following will match words regardless of case Linux or Unix ：

egrep -i '^(linux|unix)' FILENAME

Explore in depth grep Advanced find mode

1、 How to retrieve a ‘-‘ The pattern of the beginning ？

Use -e Option to search for all matches ‘–test–‘ Result .grep Will try to put ‘–test–‘ As an option, parse ：

grep -e '--test--' FILENAME

2、 How to be in grep Use in OR The logical operation of ？

grep -E 'word1|word2' FILENAME### OR ###egrep 'word1|word2' FILENAME

Or you can do it

grep 'word1\|word2' FILENAME

3、 How to be in grep Use in AND The logical operation of ？

Follow the syntax below to display all the included words ‘word1′ and ‘word2′ Result ：

grep 'word1' FILENAME | grep 'word2'

Or you can ：

grep 'foo.*bar\|word3.*word4' FILENAME

4、 How to test a sequence ？

You can use the following syntax to test the number of times a character repeats in a sequence ：

{
    N}
{
    N,}
{
    min,max}

The match contains two letters v The string result of ：

egrep "v{2}" FILENAME

In the following example, the retrieval file contains “col” and “cool” The string result of ：

egrep 'co{1,2}l' FILENAME

In the following example, the match will contain at least 3 Letters c Result ：

egrep 'c{3,}' FILENAME

The following example will match “91-1234567890″ Mobile phone number in format （ namely “ Two digits - Ten digits ”）

grep "[[:digit:]]\{2\}[ -]\?[[:digit:]]\{10\}" FILENAME

5、 How to make grep Highlight the output of ？

Use the syntax of the following example ：

grep --color regex FILENAME

6、 How to make grep The output of shows only the matching parts, not the whole line ？

Use the syntax of the following example ：

grep -o regex FILENAME

Regular expression operators summary

Regular expressions : The operator meaning

.  Match any single character .
?  Match the previous character 0 Time or 1 Time .
*  Match the previous character ≥0 Time .
+  Match the previous character ≥1 Time .
{
    N}  Match the previous character N Time .
{
    N,}  Match the previous character ≥m Time .
{
    N,M}  Match the previous character  N  To  M Time .
–  If the end point of a list or range in the list , Indicates the range .
^  Start marker , Indicates that an empty string is matched at the beginning . It also represents characters that are not in the range of the list .
$  End mark . Match an empty string .
\b  Word lock . Match an empty string at the edge of a word .
\B  Match an empty string at the non edge of a word .
\<  Matches an empty string starting with a word .
\>  Matches an empty string at the end of a word .

About grep and egrep

egrep namely grep -E , It interprets patterns as an extended regular expression .grep This is defined in the help document ：

 In basic regular expressions the meta-characters ?, +, {
    , |, (, and ) lose their special meaning; instead use the backslashed versions \?, \+, \{
    ,\|, \(, and \).
   Traditional egrep did not support the {
     meta-character, and some egrep implementations support \{
     instead, so portable scripts should avoid  {
      in
   grep -E patterns and should use [{
     ] to match a literal {
    .
   GNU grep -E attempts to support traditional usage by assuming that {
     is not special if it would be the start of an invalid interval specification.
   For example, the command grep -E '{1' searches for the two-character string {
    1 instead of reporting a syntax  error  in  the  regular  expression.
   POSIX.2 allows this behavior as an extension, but portable scripts should avoid it.

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