dirname

 dirname /usr/sbin/letsencrypt-auto

/usr/sbin

basename

basename /usr/sbin/letsencrypt-auto

letsencrypt-auto

realpath

ln -s /usr/src/letsencrypt/letsencrypt-auto /usr/sbin/

realpath /usr/sbin/letsencrypt-auto

/usr/src/letsencrypt/letsencrypt-auto