loom@tatooine ~ $ ls
xonsh: subprocess mode: permission denied: ls

loom@tatooine ~ $ /usr/bin/ls
Density_201807  apt-cyg  brdmp_999  setup-x86_64.exe  tmp

loom@tatooine ~ $ which ls
ls --color=auto -v

loom@tatooine ~
$ ls
Density_201807  apt-cyg  brdmp_999  setup-x86_64.exe  tmp

loom@tatooine ~
$ which ls
/usr/bin/ls