String manipulation in the shell

Recently summed up some of the shell's operations on strings, hoping to help you better understand the shell. Strings can also be manipulated in the shell through external commands such as awk, SED, but invoking these external commands is very different from the performance of the shell built-in operators, so it is recommended that you use built-in operators and functions to complete the operation of the string. Needless to say, let's take a look at the string operators and functions built into the shell.

1. Judge the value of reading the shell variable   

An expression	Meaning	Instance
${var}	variable var value, same as $var	[Email protected]:~$ var= "test" [Email protected]:~$ Echo ${var} Test
${var-default}		[email protected]:~$ unset var [email  protected]:~$ Echo ${var-default} Default [email protected]:~$ echo $var [email  protected]:~$ var= "" [email protected]:~$ Echo ${var-default} [email  protected]:~$
${var:-default}		[email protected]:~$ unset var [email protected]:~$ Echo ${var:-default} Default [email protected]:~$ Echo $var [email protected]:~$ var= "" [email protected]:~$ Echo ${ Var-default} [email protected]:~$
${var=default}	If Var is not declared, then default is used as the value of ${var-default}, and Var is assigned to default	[Email protected]:~$ unset var [Email protected]:~$ Echo ${var=default} Default [Email protected]:~$ Echo $var Default [Email protected]:~$ var= "] [Email protected]:~$ Echo ${var=default} [Email protected]:~$
${var:=default}	If Var is not declared, or its value is empty, then default is used as the value of ${var-default}, and Var is assigned to default	[Email protected]:~$ var= "] [Email protected]:~$ Echo ${var:=default} Default [Email protected]:~$ Echo $var Default [Email protected]:~$ var= "] [Email protected]:~$ Echo ${var=default} Default [Email protected]:~$
${var+default}	If Var is declared, then its value is default, otherwise it is a null string	[Email protected]:~$ var= [Email protected]:~$ Echo ${var+default} Default [Email protected]:~$ Echo ${var:+default} [Email protected]:~$ Echo $var [Email protected]:~$ var= "test" [Email protected]:~$ Echo ${var+default} Default [Email protected]:~$ Echo ${var:+default} Default [Email protected]:~$ Echo $var Test
${var:+default}	If Var is set, then its value is default, otherwise it is a null string	Ditto
${!test*} ${[email protected]}	Matches all previous variables declared with the start of test	[Email protected]:~$ test1= [Email protected]:~$ test2= [Email protected]:~$ test3= [Email protected]:~$ echo ${[email protected]} Test Test1 test2 Test3 [Email protected]:~$ Echo ${!test*} Test Test1 test2 Test3 [Email protected]:~$

2. String manipulation

An expression	Defined	Instance
${#var}	Length of $var	[Email protected]:~$ var= "Hello" [Email protected]:~$ echo ${#var} 5 [Email protected]:~$
${VAR:X}	Extracting substrings starting from position X	[Email protected]:~$ Echo ${var:2} Llo
${var:x:y}	Extracts a substring of length y starting from position x	[Email protected]:~$ Echo ${var:2:2} ll
${VAR#*STR}	From the beginning of the variable $var, Delete the shortest matched substring of str	[Email protected]:~$ Echo ${var#*l} Lo
${VAR##*STR}	Delete the longest substring matching str from the beginning of the variable $var	[Email protected]:~$ Echo ${var##*l} O
${var%str*}	Delete the shortest matched substring of str from the end of the variable $var	[Email protected]:~$ Echo ${var%l*} Hel
${var%%str*}	Delete the longest substring matching str from the end of the variable $var	[Email protected]:~$ Echo ${var%l*} He
${VAR/SRC/DST}	Use DST instead of the first one to match the SRC	[Email protected]:~$ Echo ${var/l/o} Heolo
${VAR//SRC/DST}	Use DST instead of all matching SRC	[Email protected]:~$ Echo ${var//l/o} Heooo
${var/#src/DST}	If the var prefix matches src, then DST is used instead of the matching SRC	[Email protected]:~$ echo ${var/#l/A} Hello [Email protected]:~$ echo ${var/#h/A} Aell
${VAR/%SRC/DST}	If the suffix of var matches src, then DST is used instead of the matching SRC	[Email protected]:~$ Echo ${var/%o/a} Hella

PS: "str and SRC" can be regular expressions

This article is from the IT Worker blog, so be sure to keep this source http://sunhualong.blog.51cto.com/7175959/1760210

String manipulation in the shell