What does ${2:-${1}} mean in Bash?

Question

What does the following bash snippet do exactly? ${2:-${1}}

Answer 1

“使用第二个参数，但如果没有，那么第一个参数”。

Answer 2

${var:-default} evaluates to the value of $var , unless $var isn't set in which case it evaluates to the text "default" . $1 , $2 , et al are the command-line arguments to your program (or function). Putting the two together it means to return $2 if there were two arguments passed, otherwise return $1 .

Answer 3

It means "Use the second argument if the first is undefined or empty, else use the first". The form "${2-${1}}" (no ':') means "Use the second if the first is not defined (but if the first is defined as empty, use it)".

Answer 4

如果定义，它会给出$ {2}的值，或者默认为$ {1} http://jaduks.livejournal.com/7934.html