Test for function's existence that can work on both bash and zsh?

If you want to check that there's a currently defined (or at least potentially marked for autoloading) function by the name foo regardless of whether a builtin/executable/keyword/alias may also be available by that name, you could do:

if typeset -f foo > /dev/null; then
  echo there is a foo function
fi

Though note that if there's a keyword or alias called foo as well, it would take precedence over the function (when not quoted).

The above should work in ksh (where it comes from), zsh and bash.

This is pure POSIX, so it should work on all POSIX shells.

foo()
{
    echo "bar"
}

if type 'foo' 2>/dev/null | grep -q 'function'
then
   echo 'function exists'
fi