I have recently upgraded from busybox 1.11.1 to 1.13.2 (latest/current stable in gentoo distro!).

After this I have noticed that the "if" statement (ash) doesn't understand any more the negation of empty variables:

if [ ! $variable_that_has_not_yet_been_defined ]; then

This will fail with the error message: "sh: argument expected", whereas in busybox 1.11.1 the command list after "then" would have been executed. As a consequence busybox 1.13.2 is now less portable (standard "bash" accepts the anterior line without complaints).

This behaviour is also not very coherent because empty variables are still accepted, if not negated:

if [ $variable_that_has_not_yet_been_defined ]; then
else
fi

"ash" will execute the else part of this code - without complaint on the error stream.

As the following lines should be executed in the same manner (and old busybox and likewise bash do so) without error message, I think this should be fixed.

if [ ! $variable_that_has_not_yet_been_defined ]; then
if ! [ $variable_that_has_not_yet_been_defined ]; then
if [ ! "$variable_that_has_not_yet_been_defined" ]; then
if ! [ "$variable_that_has_not_yet_been_defined" ]; then

(with and without negation this should work equally!)