Shell built-in operator-string processing (summary)

First, Judge read the string value

An expression	meaning
${var}	Variable var with the same value as $var
${var-default}	If Var is not declared, then use $default as its value *
${var:-default}	If Var is not declared, or its value is empty, then $default is used as its value *
${var=default}	If Var is not declared, then use $default as its value *
${var:=default}	If Var is not declared, or its value is empty, then $default is used as its value *
${var+other}	If Var is declared, then its value is $other, otherwise it is a null string
${var:+other}	If Var is set, then its value is $other, otherwise it will be a null string
${var? ERR_MSG}	If Var is not declared, print $err_msg *
${var:? ERR_MSG}	If Var is not set, then print $err_msg *
${!varprefix*}	Matches all previously declared variables beginning with Varprefix
${[email protected]}	Matches all previously declared variables beginning with Varprefix

Second, string manipulation (length, interception, substitution)

An expression	meaning
${#string}	Length of $string
${string:position}	In $string, start extracting substrings from position $position
${string:position:length}	In $string, a substring of length $length is extracted starting from position $position
${string#*substring}	Delete the substring of the shortest match $substring from the beginning of the variable $string
${string##*substring}	Delete the substring of the longest matching $substring from the beginning of the variable $string
${string%substring*}	Delete the substring of the shortest match $substring from the end of the variable $string
${string%%substring*}	Delete the substring of the longest matching $substring from the end of the variable $string
${string/substring/replacement}	Use $replacement to replace the first matching $substring
${string//substring/replacement}	Use $replacement instead of all matching $substring
${string/#substring/replacement}	If the $string prefix matches the $substring, then the $replacement is used instead of the matching $substring
${string/%substring/replacement}	If the $string suffix matches the $substring, then the $replacement is used instead of the matching $substring

Shell built-in operator-string processing (summary)