Asynchronous Execution

sh provides a few methods for running commands and obtaining output in a non-blocking fashion.

Incremental Iteration

You may also create asynchronous commands by iterating over them with the _iter special kwarg. This creates an iterable (specifically, a generator) that you can loop over:

from sh import tail

# runs forever
for line in tail("-f", "/var/log/some_log_file.log", _iter=True):

By default, _iter iterates over STDOUT, but you can change set this specifically by passing either "err" or "out" to _iter (instead of True). Also by default, output is line-buffered, so the body of the loop will only run when your process produces a newline. You can change this by changing the buffer size of the command’s output with _out_bufsize.


If you need a fully non-blocking iterator, use _iter_noblock. If the current iteration would block, errno.EWOULDBLOCK will be returned, otherwise you’ll receive a chunk of output, as normal.

Background Processes

By default, each running command blocks until completion. If you have a long-running command, you can put it in the background with the _bg=True special kwarg:

# blocks
print("...3 seconds later")

# doesn't block
p = sleep(3, _bg=True)
print("prints immediately!")
print("...and 3 seconds later")

You’ll notice that you need to call RunningCommand.wait() in order to exit after your command exits.

Commands launched in the background ignore SIGHUP, meaning that when their controlling process (the session leader, if there is a controlling terminal) exits, they will not be signalled by the kernel. But because sh commands launch their processes in their own sessions by default, meaning they are their own session leaders, ignoring SIGHUP will normally have no impact. So the only time ignoring SIGHUP will do anything is if you use _new_session=False, in which case the controlling process will probably be the shell from which you launched python, and exiting that shell would normally send a SIGHUP to all child processes.

See also

For more information on the exact launch process, see Architecture Overview.

Output Callbacks

In combination with _bg=True, sh can use callbacks to process output incrementally by passing a callable function to _out and/or _err. This callable will be called for each line (or chunk) of data that your command outputs:

    from sh import tail

    def process_output(line):

    p = tail("-f", "/var/log/some_log_file.log", _out=process_output, _bg=True)

To control whether the callback receives a line or a chunk, use _out_bufsize. To “quit” your callback, simply return True. This tells the command not to call your callback anymore.

The line or chunk received by the callback can either be of type str or bytes. If the output could be decoded using the provided encoding, a str will be passed to the callback, otherwise it would be raw bytes.


Returning True does not kill the process, it only keeps the callback from being called again. See Interactive callbacks for how to kill a process from a callback.

Interactive callbacks

Commands may communicate with the underlying process interactively through a specific callback signature Each command launched through sh has an internal STDIN queue.Queue that can be used from callbacks:

    def interact(line, stdin):
        if line == "What... is the air-speed velocity of an unladen swallow?":
            stdin.put("What do you mean? An African or European swallow?")

        elif line == "Huh? I... I don't know that....AAAAGHHHHHH":
            return True

            stdin.put("I don't know....AAGGHHHHH")
            return True

    p = sh.bridgekeeper(_out=interact, _bg=True)


If you use a queue, you can signal the end of the input (EOF) with None

You can also kill or terminate your process (or send any signal, really) from your callback by adding a third argument to receive the process object:

def process_output(line, stdin, process):
    if "ERROR" in line:
        return True

p = tail("-f", "/var/log/some_log_file.log", _out=process_output, _bg=True)

The above code will run, printing lines from some_log_file.log until the word "ERROR" appears in a line, at which point the tail process will be killed and the script will end.


You may also use RunningCommand.terminate() to send a SIGTERM, or RunningCommand.signal() to send a general signal.

Done Callbacks

A done callback called when the process exits, either normally (through a success or error exit code) or through a signal. It is always called.

Here’s an example of using _done to create a multiprocess pool, where sh.your_parallel_command is executed concurrently at no more than 10 at a time:

import sh
from threading import Semaphore

pool = Semaphore(10)

def done(cmd, success, exit_code):

def do_thing(arg):
    return sh.your_parallel_command(arg, _bg=True, _done=done)

procs = []
for arg in range(100):

# essentially a join
[p.wait() for p in procs]