Shell and Shellscript Learning Note III (deletion, substitution and substitution of variable contents)

1 . Deletion and substitution of variable contents

# The representation is deleted from the front, so here it starts / deletes ,* instead of 0 to infinitely more than any character

% Delete variable contents from behind

For example:echo ${path%:* bin} deletes the most a directory, that is, the string from: to the bin and is the shortest

Echo ${path/sbin/sbin}: two slashes in the middle of the old character, followed by the new character

How variables are set	Description
${ variable # keyword }	If the data from the beginning of the variable content matches the "keyword", the shortest data is deleted
${ variable # # keyword }	If the data from the beginning of the variable content matches the "keyword", the shortest data is deleted
${ variable % keyword }	If the variable content from the tail forward data conforms to the "keyword", then the minimum data to be met is deleted
${ variable % keyword }	The longest data to be met is deleted if the contents of the variable content from the tail forward data conform to the "keyword"
${ variable / old string / new string }	If the contents of the variable conform to the old string, the first old string is replaced by the new string
${ variable // old string / new string }	If the contents of the variable conform to the old string, all old strings are replaced by the new string

2. Testing of variables and content substitution

How variables are set	Str is not set	Str is an empty string	STR has set a non-empty string
VAR=${STR-EXPR}	var=expr	Var=	Var= $str
VAR=${STR:-EXPR}	var=expr	var=expr	Var= $str
VAR=${STR+EXPR}	Var=	var=expr	var=expr
VAR=${STR:+EXPR}	Var=	Var=	var=expr
VAR=${STR=EXPR}	str=expr var=expr	Str does not change Var=	Str does not change Var= $str
VAR=${STR:=EXPR}	str=expr var=expr	str=expr var=expr	Str does not change Var= $str
VAR=${STR?EXPR}	Expr output to stderr	Var=	Var=str
VAR=${STR:?EXPR}	Expr output to stderr	Expr output to stderr	Var=str

Shell and Shellscript Learning Note III (deletion, substitution and substitution of variable contents)