Also, I suspect the most common issue is going to be that people won't have these binaries installed. We can probably check for that more explicitly:

```
# stdlibrary function to find in PATH
cmd = distutils.spawn.find_executable('shiftx2.py')
if cmd is None:
    raise OSError('shiftx2.py: command not found.\nchemical_shifts_shiftx2 requires the external program SHIFTX2, available at http://www.shiftx2.ca/')

try:
    output = subprocess.check_output([path, '-b', 'trj*.pdb'] , stderr=subprocess.STDOUT, shell=True)
except subprocess.CalledProcessError:
    print(output, file=sys.stderr)
    raise
```