busybox/shell/ash_test/ash-redir/redir_script.tests

# Builds a " 3>&- 4>&-" string.
# Note: one of these fds is a directory opened to /proc/self/fd
# for globbing. It is unwanted, but I don't know how to filter it out.
find_fds() {
	fds=""
	for f in /proc/self/fd/*; do
		test "$f" = "/proc/self/fd/0" && continue
		test "$f" = "/proc/self/fd/1" && continue
		test "$f" = "/proc/self/fd/2" && continue
		fds="$fds ${f##*/}>&-"
	done
}

find_fds
fds1="$fds"

# One of the fds is open to the script body
# Close it while executing something.
eval "find_fds $fds"

# Shell should not lose that fd. Did it?
find_fds
test x"$fds1" = x"$fds" \
&& { echo "Ok: script fd is not closed"; exit 0; }

# One legit way to handle it is to move script fd. For example, if we see that fd 10 moved to fd 11:
test x"$fds1" = x" 10>&- 3>&-" && \
test x"$fds"  = x" 11>&- 3>&-" \
&& { echo "Ok: script fd is not closed"; exit 0; }
# or we see that fd 3 moved to fd 10:
test x"$fds1" = x" 3>&- 4>&-" && \
test x"$fds"  = x" 10>&- 3>&-" \
&& { echo "Ok: script fd is not closed"; exit 0; }

echo "Bug: script fd is closed"
echo "fds1:$fds1"
echo "fds2:$fds"
exit 1