有人可以帮我处理这些 shell 脚本行吗？ 我无法理解在这些行中使用 DEBUG 是什么

Question

# Define DEBUG=true on cmdline, if needed DEBUG=${DEBUG:-false} [[ ${DEBUG} = true ]] || DEBUG="false"

Answer 1

Explanation

The first assignment sets the variable DEBUG to false if the contents of DEBUG is empty. Please note that if DEBUG has not been initialized yet, it is also considered as empty and thus will receive the false value.

The second line uses the OR operator using || , it is exactly the same as:

if ! [[ ${DEBUG} = true ]]
then
  DEBUG="false"
fi

In other words, it assigns the value false to your variable if and only DEBUG does NOT equal to true .

In the end, the purpose of this code is to define the DEBUG variable to either true or false and nothing else, using the default value false if the variable does not contain a boolean value.

Improvements

I think the first assignment is pointless because if the variable was empty at the beginning of this code, then the last line will assign the variable to false anyways.

Also, the ${DEBUG} should be protected with quotes, eg [[ "${DEBUG}" = true ]] instead of [[ ${DEBUG} = true ]] because your script may not work properly if the variable contains whitespaces.

A bit overkill maybe, but I encourage checking case sensitivity and use a fallback code.

Let's say the user defines DEBUG=TRUE (uppercase) prior to calling your script, the value of DEBUG will be forced to false . I guess the user would not be very pleased.

The following code solves all those issues:

# Define DEBUG=true on cmdline, if needed

DEBUG=${DEBUG,,} # Force the variable to lowercase
[[ "${DEBUG}" = true ]] || DEBUG=false