Shellingham detects what shell the current Python executable is running in.

>>> import shellingham
>>> shellingham.detect_shell()
('bash', '/bin/bash')

detect_shell pokes around the process's running environment to determine what shell it is run in. It returns a 2-tuple:

• The shell name, always lowercased.
• The command used to run the shell.

ShellDetectionFailure is raised if detect_shell fails to detect the surrounding shell.

• The shell name is always lowercased.
• On Windows, the shell name is the name of the executable, minus the file extension.

Remember, your application's user is not necessarily using a shell. Shellingham raises ShellDetectionFailure if there is no shell to detect, but your application should almost never do this to your user.

A practical approach to this is to wrap detect_shell in a try block, and provide a sane default on failure

try:
    shell = shellingham.detect_shell()
except shellingham.ShellDetectionFailure:
    shell = provide_default()

There are a few choices for you to choose from.

• The POSIX standard mandates the environment variable SHELL to refer to "the user's preferred command language interpreter". This is always available (even if the user is not in an interactive session), and likely the correct choice to launch an interactive sub-shell with.
• A command sh is almost guaranteed to exist, likely at /bin/sh, since several POSIX tools rely on it. This should be suitable if you want to run a (possibly non-interactive) script.
• All versions of DOS and Windows have an environment variable COMSPEC. This can always be used to launch a usable command prompt (e.g. cmd.exe on Windows).

Here's a simple implementation to provide a default shell

import os
def provide_default():
    if os.name == 'posix':
        return os.environ['SHELL']
    elif os.name == 'nt':
        return os.environ['COMSPEC']
    raise NotImplementedError(f'OS {os.name!r} support not available')