String manipulation in the shell

Shell string manipulation

The bash shell provides several commands for string processing:

• awk command
• Expr command

String length

• ${#..}
• Length of expr
• Length of awk (s)
  Instance:
  string= "Hello World"
  $ {#string}
  Expr Length "$string"
  Note: Double-quotes are required. Because the string has spaces

Matching substrings

Format: Expr match $string $substring
Function: matches the substring at the beginning of the string, returns the matched length, and does not match at the beginning of string substring returns 0,substring can be a regular form
String= "Welcome to our World"

Command	return value
Expr match "$string" w.*	20
Expr match "$string" ou.*	0

Index of common characters

Format: Expr index $string $sunstring
Function: Matches the first occurrence of a character in a string of substring
String= "Welcome to our World"

Command	return value
Expr index "$string" our	5
Expr index "$string" D	20
Expr index "$string" s	0

Perform the discovery. The function of expr index is to look for the first common character between two strings

Intercept sub-strings

• Intercept from left
  
  • ${string:position}
  • ${string:position:length}
• Intercept from right
  
  • ${string:-position} (there is a space after the colon)
  • ${string: (Position)}
  • ${string:-position:length}
  • ${string: (position): length}

Expr substr
Format: Expr substr $string $Position $length
The difference from ${}: ${} position starts from 0 to string, and the position of expr sutstr starts from 1 to the string label
String= "Welcome to our World"

Command	return value
Echo ${string:1:8}	elcome T
Expr substr "$string" 2 8	elcome T

The regular form intercept sub-string
Using a regular table can only extract substrings at the beginning or end of a string.
-Expr Match $string ' \ ($substring\) '
-Expr $string: ' \ ($substring\) '

Command	return value
Expr match "$another" "[0-9]*"	8
Expr match "$another" \ ([0-9]*\)]	20091114
Expr "$another": "\ ([0-9]*\)"	20091114

Note: There are spaces on both sides of the colon

Delete a child string

• ${string#substring}
  Delete the shortest substring that matches the substring at the beginning of a string
• ${string# #substring}
  Delete the oldest string that matches the substring at the beginning of the string
• ${string%substring}
  Delete the shortest substring that matches the substring at the end of a string
• ${string%%substring}
• Delete the oldest string that matches the substring at the end of the string

SUBSTRING is not a regular form of expression.
20091114 Reading Hadoop

Command	Results
echo "${another#2*1}"	Reading Hadoop
echo "${another# #2 *}"	4 Reading Hadoop
echo "${another%a*p}"	20091114 Reading H
echo "${another%%a*p}"	20091114 Re

Replace substring

• ${string/substring/replacement}
  Replace only substrings that match substring for the first time
• ${string//substring/replacement}
  Replace all substrings that match substring
• ${string/#substring/replacement}
  Replace substring with substring at the beginning of string
• ${string/%substring/replacement}
  Replace substring with substring at the end of string

String= "20001020year20050509month"

Command	Results
Echo ${string/200/201}	20101020year20050509month
Echo ${string/200/201}	20101020year20150509month
Echo ${string/r*h/}	20001020yea
echo ${string/#2000/2010}	20101020year20050509month
Echo ${string/%month/month}	20001020year20050509month

String manipulation in the shell