SC2157 – ShellCheck Wiki

Argument to implicit `-n` is always true due to literal strings.

(Or: Argument to -z is always false due to literal strings. )

Problematic code:

if [ "$foo " ]
then
  echo "this is always true because of the trailing space"
fi

Correct code:

if [ "$foo" ]
then
  echo "correctly checks value"
fi

Rationale:

Since [ str ] and [ -n str ] check that the string is non-empty, any literal characters in the string -- including a space character like in the example -- will cause the test to always be true.

Equivalently, since [ -z str ] checks that the string is empty, any literal character in the string will cause the test to always be false.

Double check the string: you may have added trailing characters, or bad quotes or syntax. Some examples include:

[ "$foo " ] like in the example, where the space becomes part of the string
[ "{$foo}" ] instead of [ "${foo}" ], where the { becomes part of the string
[ "$foo -gt 0" ] instead of [ "$foo" -gt "0" ], where the -gt becomes part of the string

Exceptions:

None.