-Stick to the generally supported options for these programs. For
-example, don't use @samp{mkdir -p}, convenient as it may be, because
-most systems don't support it.
+Generally, stick to the widely-supported (usually
+@sc{posix}-specified) options and features of these programs. For
+example, don't use @samp{mkdir -p}, convenient as it may be, because a
+few systems don't support it at all and with others, it is not safe
+for parallel execution. For a list of known incompatibilities, see
+@ref{Portable Shell, Portable Shell Programming,, autoconf, Autoconf}.
+