POSIX 默认字符串的语法差异（冒号）？
POSIX syntax difference (colon) in defaulting a string?
foo=${foo:-default}
bar=${bar-default}
包含冒号 : 和排除冒号在功能上有区别吗？
参见the spec：
$ myvar=
$ echo "${myvar:-default}"
default
$ echo "${myvar-default}"
$ unset myvar
$ echo "${myvar-default}"
default
使用冒号，检查 "if unset or null, use default"。没有冒号，它只是 "if unset, use default".
相关段落（强调我的）：
In the parameter expansions shown previously, use of the in
the format shall result in a test for a parameter that is unset or
null; omission of the shall result in a test for a parameter
that is only unset.
顺便说一句，Bash 也是如此。引用自 manual:
When not performing substring expansion, using the form described
below (e.g., :-), Bash tests for a parameter that is unset or null.
Omitting the colon results in a test only for a parameter that is
unset. Put another way, if the colon is included, the operator tests
for both parameter's existence and that its value is not
null; if the colon is omitted, the operator tests only for existence.
foo=${foo:-default}
bar=${bar-default}
包含冒号 : 和排除冒号在功能上有区别吗？
参见the spec：
$ myvar=
$ echo "${myvar:-default}"
default
$ echo "${myvar-default}"
$ unset myvar
$ echo "${myvar-default}"
default
使用冒号，检查 "if unset or null, use default"。没有冒号，它只是 "if unset, use default".
相关段落（强调我的）：
In the parameter expansions shown previously, use of the in the format shall result in a test for a parameter that is unset or null; omission of the shall result in a test for a parameter that is only unset.
顺便说一句，Bash 也是如此。引用自 manual:
When not performing substring expansion, using the form described below (e.g.,
:-), Bash tests for a parameter that is unset or null. Omitting the colon results in a test only for a parameter that is unset. Put another way, if the colon is included, the operator tests for both parameter's existence and that its value is not null; if the colon is omitted, the operator tests only for existence.