bash - :- and := in Bash parameter substitution?有什么区别

Question

Bash 参数替换中的 :- 和 := 有什么区别？

他们似乎都设置了默认值？

Answer 1

引用 Bash Reference Manual :

${parameter:-word}

If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.

${parameter:=word}

If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.

区别在于 := 不仅替换了 word，它还将它分配给参数:

var=
echo "$var"               # prints nothing
echo "${var:-foo}"        # prints "foo"
echo "$var"               # $var is still empty, prints nothing
echo "${var:=foo}"        # prints "foo", assigns "foo" to $var
echo "$var"               # prints "foo"

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看到这个很棒 wiki.bash-hackers.org tutorial获取更多信息。