預設值替換

${parameter:-word}

如果引數未設定或為 null，則替換單詞的擴充套件。否則，引數的值將被替換。

$ unset var
$ echo "${var:-XX}"     # Parameter is unset -> expansion XX occurs
XX
$ var=""                # Parameter is null -> expansion XX occurs
$ echo "${var:-XX}"
XX
$ var=23                # Parameter is not null -> original expansion occurs
$ echo "${var:-XX}"
23

${parameter:=word}

如果引數未設定或為 null，則將字的擴充套件分配給引數。然後替換引數的值。不能以這種方式分配位置引數和特殊引數。

$ unset var
$ echo "${var:=XX}"     # Parameter is unset -> word is assigned to XX
XX
$ echo "$var"
XX
$ var=""                # Parameter is null -> word is assigned to XX
$ echo "${var:=XX}"
XX
$ echo "$var"
XX
$ var=23                # Parameter is not null -> no assignment occurs
$ echo "${var:=XX}"
23
$ echo "$var"
23