[[environment variable]]
Require returns an error if the given variable is not set or is empty.
Log the value of the required variable.
require: log -- "$1=$(eval echo \$$1)"
Check whether it has a value.
require: eval "test -n \"\$$1\""
host obsid lit 'lit=require.md' 'file=require'
To change a variable in the calling script, it needs called in the calling script's environment, by the commands . or source.
. require variable