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This article mainly shows you "how to use colons in shell". The content is simple and clear. I hope it can help you solve your doubts. Let the editor lead you to study and learn this article "how to use colons in shell".

: ${VAR:=DEFAULT}

When the variable VAR is not declared or is NULL, set VAR to the default value DEFAULT. If you don't add a command in front of it, then ${VAR:=DEFAULT} itself will be executed as a command, and an error report is certain.

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