Merge pull request 2899 from krytarowski/netbsd-support-1

Merge pull request 2899 from krytarowski/netbsd-support-1

Improve shell portability on NetBSD

The "test" command, as well as the "[" command, are not required to know
the "==" operator.

Comparing with empty strings ("") might lead to unspecified results.

Replace unportable shell loops:
    for I in {2..6}; do ..; done
with POSIX ones:
    I=2; while [ $I -le 6 ]; do ..; I=$((I + 1)); done