Duct is a library for running child processes. Duct makes it easy to build pipelines and redirect IO like a shell. At the same time, Duct helps you write correct, portable code: whitespace is never significant, errors from child processes get reported by default, and a variety of gotchas, bugs, and platform inconsistencies are handled for you the Right Way™.

• Documentation
• PyPI package
• the same library, in Rust

Run a command without capturing any output. Here "hi" is printed directly to the terminal:

>>> from duct import cmd
>>> cmd("echo", "hi").run()
hi
Output(status=0, stdout=None, stderr=None)

Capture the standard output of a command. Here "hi" is returned as a string:

>>> cmd("echo", "hi").read()
'hi'

Capture the standard output of a pipeline:

>>> cmd("echo", "hi").pipe(cmd("sed", "s/i/o/")).read()
'ho'

Merge standard error into standard output and read both incrementally:

>>> big_cmd = cmd("bash", "-c", "echo out && echo err 1>&2")
>>> reader = big_cmd.stderr_to_stdout().reader()
>>> with reader:
...     reader.readlines()
[b'out\n', b'err\n']

Children that exit with a non-zero status raise an exception by default:

>>> cmd("false").run()
Traceback (most recent call last):
...
duct.StatusError: Expression cmd('false') returned non-zero exit status: Output(status=1, stdout=None, stderr=None)
>>> cmd("false").unchecked().run()
Output(status=1, stdout=None, stderr=None)