shellous 0.29.0

Async Processes and Pipelines

Async Processes and Pipelines

shellous provides a concise API for running subprocesses using asyncio. It is similar to and inspired by sh.

import asyncio
from shellous import sh

async def main():
    result = await sh("echo", "hello")
    print(result)

asyncio.run(main())

Benefits

• Run programs asynchronously in a single line.
• Redirect stdin, stdout and stderr to files, memory buffers or loggers.
• Construct pipelines and use process substitution.
• Set timeouts and reliably cancel running processes.
• Run a program with a pseudo-terminal (pty).
• Runs on Linux, MacOS, FreeBSD and Windows.
• Monitor processes being started and stopped with audit_callback API.

Requirements

• Requires Python 3.9 or later.
• Requires an asyncio event loop.
• Process substitution requires a Unix system with /dev/fd support.
• Pseudo-terminals require a Unix system.

Running a Command

The tutorial in this README uses the asyncio REPL built into Python. In these examples, >>> is the REPL prompt.

Start the asyncio REPL by typing python3 -m asyncio, and import sh from the shellous module:

>>> from shellous import sh

Here's a command that runs echo "hello, world".

>>> await sh("echo", "hello, world")
'hello, world\n'

The first argument to sh is the program name. It is followed by zero or more arguments. Each argument will be converted to a string. If an argument is a list or tuple, it is flattened recursively.

>>> await sh("echo", 1, 2, [3, 4, (5, 6)])
'1 2 3 4 5 6\n'

A command does not run until you await it. When you run a command using await, it returns the value of the standard output interpreted as a UTF-8 string.

Here, we create our own echo command with "-n" to omit the newline. Note, echo("abc") will run the same command as echo -n "abc".

>>> echo = sh("echo", "-n")
>>> await echo("abc")
'abc'

Commands are immutable objects that represent a program invocation: program name, arguments, environment variables, redirection operators and other settings. When you use a method to modify a Command, you are returning a new Command object. The original object is unchanged.

It is often convenient to wrap your commands in a function. This idiom provides better type safety for the command arguments.

>>> async def exclaim(word: str) -> str:
...   return await sh("echo", "-n", f"{word}!!")
... 
>>> await exclaim("Oh")
'Oh!!'

Results

When a command completes successfully, it returns the standard output (or "" if stdout is redirected). For a more detailed response, you can specify that the command should return a Result object by using the .result modifier:

>>> await echo.result("abc")
Result(exit_code=0, output_bytes=b'abc', error_bytes=b'', cancelled=False, encoding='utf-8')

A Result object contains the command's exit_code in addition to its output. A Result is True if the command's exit_code is zero. You can access the string value of the output using the .output property:

if result := await sh.result("cat", "some-file"):
    output = result.output
else:
    print(f"Command failed with exit_code={result.exit_code})

You can retrieve the string value of the standard error using the .error property. (By default, only the first 1024 bytes of standard error is stored.)

ResultError

When a command fails, it raises a ResultError exception:

>>> await sh("cat", "does_not_exist")
Traceback (most recent call last):
  ...
shellous.result.ResultError: Result(exit_code=1, output_bytes=b'', error_bytes=b'cat: does_not_exist: No such file or directory\n', cancelled=False, encoding='utf-8')

The ResultError exception contains a Result object with the exit_code and the first 1024 bytes of standard error.

In some cases, you want to ignore certain exit code values. That is, you want to treat them as if they are normal. To do this, you can set the exit_codes option:

>>> await sh("cat", "does_not_exist").set(exit_codes={0,1})
''

If there is a problem launching a process, shellous can also raise a separate FileNotFoundError or PermissionError.

Async For

Using await to run a command collects the entire output of the command in memory before returning it. You can also iterate over the output lines as they arrive using async for.

>>> [line async for line in echo("hi\n", "there")]
['hi\n', ' there']

Use an async for loop when you want to examine the stream of output from a command, line by line. For example, suppose you want to run tail on a log file.

async for line in sh("tail", "-f", "/var/log/syslog"):
    if "ERROR" in line:
        print(line.rstrip())

Async With

You can use a command as an asynchronous context manager. Use async with when you need byte-by-byte control over the individual process streams: stdin, stdout and stderr. To control a standard stream, you must tell shellous to "capture" it (For more on this, see Redirection.)

cmd = sh("cat").stdin(sh.CAPTURE).stdout(sh.CAPTURE)
async with cmd as run:
    run.stdin.write(b"abc")
    run.stdin.close()
    print(await run.stdout.readline())

result = run.result()

If we didn't specify that stdin/stdout are sh.CAPTURE, the streams run.stdin and run.stdout would be None.

The return value of run.result() is always a Result object. Depending on the command settings, this function may raise a ResultError on a non-zero exit code. If you don't call run.result(), the result is ignored.

When reading or writing individual streams, you are responsible for managing reads and writes so they don't deadlock. The above example is contrived. All streams on the run object are asyncio.StreamReader and asyncio.StreamWriter objects.

You can also use async with to run a server. When you do so, you must tell the server to stop; the context manager normally waits for standard output to close.

async with sh("some-server") as run:
    # Send commands to the server here...
    # Manually signal the server to stop.
    run.cancel()

Redirection

shellous supports the redirection operators | and >>. They work similar to how they work in the unix shell. Shellous does not support use of < or > for redirection. Instead, replace these with |.

To redirect to or from a file, use a pathlib.Path object. Alternatively, you can redirect input/output to a StringIO object, an open file, a Logger, or use a special redirection constant like sh.DEVNULL.

IMPORTANT: When combining the redirect operators with await, you must use parentheses. Remember that await has higher precedence than | and >>.

Redirecting Standard Input

To redirect standard input, use the pipe operator | with the argument on the left-side. Here is an example that passes the string "abc" as standard input.

>>> cmd = "abc" | sh("wc", "-c")
>>> await cmd
'       3\n'

To read input from a file, use a Path object from pathlib.

>>> from pathlib import Path
>>> cmd = Path("LICENSE") | sh("wc", "-l")
>>> await cmd
'     201\n'

Shellous supports different STDIN behavior when using different Python types.

Python Type	Behavior as STDIN
str	Read input from string object.
bytes, bytearray	Read input from bytes object.
Path	Read input from file specified by Path.
File, StringIO, ByteIO	Read input from open file object.
int	Read input from existing file descriptor.
asyncio.StreamReader	Read input from StreamReader.
sh.DEVNULL	Read input from /dev/null.
sh.INHERIT	Read input from existing sys.stdin.
sh.CAPTURE	You will write to stdin interactively.

Redirecting Standard Output

To redirect standard output, use the pipe operator | with the argument on the right-side. Here is an example that writes to a temporary file.

>>> output_file = Path("/tmp/output_file")
>>> cmd = sh("echo", "abc") | output_file
>>> await cmd
''
>>> output_file.read_bytes()
b'abc\n'

To redirect standard output with append, use the >> operator.

>>> cmd = sh("echo", "def") >> output_file
>>> await cmd
''
>>> output_file.read_bytes()
b'abc\ndef\n'

Shellous supports different STDOUT behavior when using different Python types.

Python Type	Behavior as STDOUT/STDERR
Path	Write output to the file path specified by Path.
bytearray	Write output to a mutable byte array.
File, StringIO, ByteIO	Write output to an open file object.
int	Write output to existing file descriptor at its current position. 🔹
logging.Logger	Log each line of output. 🔹
asyncio.StreamWriter	Write output to StreamWriter. 🔹
sh.CAPTURE	Capture output for async with. 🔹
sh.DEVNULL	Write output to /dev/null. 🔹
sh.INHERIT	Write output to existing sys.stdout or sys.stderr. 🔹

🔹 For these types, there is no difference between using | and >>.

Shellous does not support redirecting standard output/error to a plain str or bytes object. If you intend to redirect output to a file, you must use a pathlib.Path object.

Redirecting Standard Error

By default, the first 1024 bytes of standard error is collected into the Result object.

To redirect standard error, use the stderr method. Standard error supports the same Python types as standard output. To append, set append=True in the stderr method.

To redirect stderr to the same place as stdout, use the sh.STDOUT constant. If you also redirect stdout to sh.DEVNULL, you will only receive the standard error.

>>> cmd = sh("cat", "does_not_exist").stderr(sh.STDOUT)
>>> await cmd.set(exit_codes={0,1})
'cat: does_not_exist: No such file or directory\n'

To redirect standard error to the hosting program's sys.stderr, use the sh.INHERIT redirect option.

>>> cmd = sh("cat", "does_not_exist").stderr(sh.INHERIT)
>>> await cmd
cat: does_not_exist: No such file or directory
Traceback (most recent call last):
  ...
shellous.result.ResultError: Result(exit_code=1, output_bytes=b'', error_bytes=b'', cancelled=False, encoding='utf-8')

If you redirect stderr, it will no longer be stored in the Result object.

Default Redirections

For regular commands, the default redirections are:

• Standard input is read from the empty string ("").
• Standard out is buffered and stored in the Result object (BUFFER).
• First 1024 bytes of standard error is buffered and stored in the Result object (BUFFER).

However, the default redirections are adjusted when using a pseudo-terminal (pty):

• Standard input is captured and ignored (CAPTURE).
• Standard out is buffered and stored in the Result object (BUFFER).
• Standard error is redirected to standard output (STDOUT).

Pipelines

You can create a pipeline by combining commands using the | operator.

>>> pipe = sh("ls") | sh("grep", "README")
>>> await pipe
'README.md\n'

A pipeline returns a Result if the last command in the pipeline has the .result modifier.

>>> pipe = sh("ls") | sh("grep", "README").result
>>> await pipe
Result(exit_code=0, output_bytes=b'README.md\n', error_bytes=b'', cancelled=False, encoding='utf-8')

Process Substitution (Unix Only)

You can pass a shell command as an argument to another. Here is the shellous equivalent to the bash command: grep README <(ls).

>>> cmd = sh("grep", "README", sh("ls"))
>>> await cmd
'README.md\n'

Use .writable to write to a command instead.

>>> buf = bytearray()
>>> cmd = sh("ls") | sh("tee", sh("grep", "README").writable | buf) | sh.DEVNULL
>>> await cmd
''
>>> buf
bytearray(b'README.md\n')

The above example is equivalent to ls | tee >(grep README > buf) > /dev/null.

Timeouts

You can specify a timeout using the timeout option. If the timeout expires, shellous will raise a TimeoutError.

>>> await sh("sleep", 60).set(timeout=0.1)
Traceback (most recent call last):
  ...
TimeoutError

Timeouts are just a special case of cancellation. When a command is cancelled, shellous terminates the running process and raises a CancelledError.

>>> t = asyncio.create_task(sh("sleep", 60).coro())
>>> t.cancel()
True
>>> await t
Traceback (most recent call last):
  ...
CancelledError

By default, shellous will send a SIGTERM signal to the process to tell it to exit. If the process does not exit within 3 seconds, shellous will send a SIGKILL signal. You can change these defaults with the cancel_signal and cancel_timeout settings. A command is not considered fully cancelled until the process exits.

Pseudo-Terminal Support (Unix Only)

To run a command through a pseudo-terminal, set the pty option to True.

>>> await sh("echo", "in a pty").set(pty=True)
'in a pty\r\n'

Alternatively, you can pass a pty function to configure the tty mode and size.

>>> ls = sh("ls").set(pty=shellous.cooked(cols=40, rows=10, echo=False))
>>> await ls("README.md", "CHANGELOG.md")
'CHANGELOG.md\tREADME.md\r\n'

Shellous provides three built-in helper functions: shellous.cooked(), shellous.raw() and shellous.cbreak().

Context Objects

You can store shared command settings in an immutable context object. To create a new context object, specify your changes to the default context sh:

>>> auditor = lambda phase, info: print(phase, info["runner"].name)
>>> sh_audit = sh.set(audit_callback=auditor)

Now all commands created with sh_audit will log their progress using the audit callback.

>>> await sh_audit("echo", "goodbye")
start echo
stop echo
'goodbye\n'

You can also create a context object that specifies all return values are Result objects.

>>> rsh = sh.result
>>> await rsh("echo", "whatever")
Result(exit_code=0, output_bytes=b'whatever\n', error_bytes=b'', cancelled=False, encoding='utf-8')

Type Hints

Shellous fully supports PEP 484 type hints.

Commands

Commands are generic on the return type, either str or Result. You will type a command object as Command[str] or Command[Result].

from shellous import sh, Command, Result

cmd1: Command[str] = sh("echo", "abc")
# When you `await cmd1`, the result is a `str` object.

cmd2: Command[Result] = sh.result("echo", "abc")
# When you `await cmd2`, the result is a `Result` object.