Shell scripts: undefined -a, -o switch behavior

Question

According to the shellcheck wiki:

-a and -o to mean AND and OR in a [ .. ] test expression is not well defined. They are obsolescent extensions in POSIX and their behavior is almost always undefined.

Any idea what wrong can happen?

Answer 1

See http://pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html#tag_20_128_16 for a discussion about the problems -a and -o can raise. In general, how the expression is parsed becomes dependent on what any parameters expand to. Consider the following test:

test "$1" -a "$2"

Depending on the value of $1, this could be either

# Unspecified behavior
# With three arguments, the first "!", the second argument
# must be a unary primary like -n or -f.
test "!" -a "$2"

or

test "foo" -a "$2"  # Test that "foo" and "$2" are non-zero length strings

If $1 is !, test will treat that as the negation operator, not a string to whose length should be checked.

Also, test is only well-defined for 4 or fewer arguments; -a and -o usually result in 5 or more arguments being passed to test.