A grep tool

grep is a row filtering tool, used to filter rows based on keywords

Syntax:
grep-option "keyword" file name

1.1 Common options

Options	meaning
-i	not case sensitive
-v	Find rows that do not include keywords, that is, reverse selection
-w	Search by word, match some and only characters
-The	Only display the matched string
-c	Count the number of matches
-n	Show line number
-r	Traverse the directory layer by layer to find
-A n	Display matching lines and the following (after) n lines
-B n	Show matching lines and n lines before (before)
-C n	Display matching lines and n lines before and after
-e	Use regular expressions
-E	Use extended regular expressions
-P	Match numbers, letters or strings

1.2 Option explanation

The test text content is as follows:
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grep -w
command: grep -w'cen' sedtest.txt

can be seen from the figure, when the option -w is not included, as long as the text contains "cen", it will be filtered out, and after -w is required Only if the word exists in the text will be filtered

grep -o
命令： grep -o 'cenjeal' sedtest.txt
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As can be seen from the figure, when -o is not included, a whole line will be output, and after the -o option is used, only the matching string will be displayed

grep -P
command: grep -P'\d' sedtest.txt matches numbers
Command: grep -P'\w' sedtest.txt matches alphanumeric underscores
Command: grep -P'\s' sedtest.txt matches strings
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Two regular expressions

2.1 The meaning of regular expressions

Regular expression (Regular Expression, regex or regexp, abbreviated as RE), also translated into regular expression, regular expression, is a character pattern used to match specified characters in the search process.
Programs that support regular expressions such as: find| vim| grep| sed |awk

2.2 Application scenarios of regular expressions

1. Match email, match ID number, mobile phone number, bank card number, etc.
2. Match some specific strings, do specific processing, etc.

2.3 Regular expression noun explanation

Metacharacter

Refers to those special characters with special meaning in regular expressions, such as: dot (.) star (*) question mark (?), etc.

Leading character

The character before the metacharacter.ab c * aoo o.

2.4 The first type of regular expression

Common metacharacters

Metacharacter	Features	Example
.	Match any single character except newline
*	The leading character appears 0 times or multiple times in a row
.*	Match any length character
^	Start of line (beginning with)	^root
$	End of line (ends with)	root$
^$	Match blank lines	Commonly used
[]	Match any single character or group of single characters in brackets	[abc]
[^]	Match does not contain any single character or a group of single characters in parentheses	[^abc]
^[^]	Match does not start with any single character or a group of single characters in parentheses	^[^abc]

Other commonly used metacharacters

Metacharacter	Features	Example
\<	Take the head of the word	grep '\ <hel' 1.txt
\>	Take the end of the word	grep ‘rld\>’ 1.txt
\< \>	Exact match	grep ‘\<hello\>’ 1.txt
\{n\}	Match the leading character n consecutive times
\{n,\}	Match the leading character at least n times
\{n,m\}	Match the leading character between n and m times
	Save the matched characters	sed ‘s/(10.1.1.).1/\1.254/g’ 1.txt
\d	Match numbers (grep -P)	[0-9]
\w	Match alphanumeric underscore (grep -P)	[a-zA-Z0-9_]
\s	Match spaces, tabs, and form feeds (grep -P)	[\t\r\n]

Commonly used extended metacharacters
grep must add -E or use egrep

sed must add -r

Extended metacharacters	Features	Example
+	Match one or more leading characters
？	Match zero or one leading character
\|	or	Match a or b
()	Group of characters (as a whole)	(my\|your)self: means match yourself or match yourself
{n}	Leading character repeated n times
{n,}	The leading character is repeated at least n times
{n,m}	The leading character is repeated n to m times

2.5 The second type of regular expression

Extended metacharacters	Features	Example
[: Scooping]	Alphanumeric characters	[[Charon]] +
[:alpha:]	Alphabetic characters (including upper and lower case letters)	[[:alpha:]]{4}
[:blank:]	Spaces and tabs	[[:blank:]]*
[:digit:]	digital	[[:digit:]]?
[:lower:]	Lower case letters	[[:lower:]]{4,}
[:upper:]	uppercase letter	[[:upper:]]+
[:point:]	Punctuation	[[:point:]]
[:space:]	All whitespace including line breaks, carriage returns, etc.	[[:space:]]*

Shell programming three swordsman grep and regular expressions