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<@ULVA73B9P> I am interesting in using the Shell i...
# ask-marvin
s
@Marvin I am interesting in using the Shell integration with 
ShellOperation().run()
. The docs say that the run method takes kwargs to pass to Python's 
subprocess.Open
. How can I achieve the following subprocess pattern using 
ShellOperation().run()
:
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import subprocess 

proc = subprocess.Popen("my bash cmds here",
                stdout=subprocess.PIPE, stderr=subprocess.PIPE,
                shell=True)
(stdoutStr, stderrStr) = proc.communicate()
if len(stderrStr) > 0:
    # deal with errors in this section
m
✅ thought for 89.3 seconds
You can get close, but not an exact 
Popen(...).communicate()
equivalent with 
ShellOperation().run()
. What 
ShellOperation().run()
does - executes your commands in a shell script - captures stdout and returns it as a 
list[str]
- captures stderr internally for logging/streaming - raises 
RuntimeError
if the command exits non-zero So unlike 
subprocess.Popen(...).communicate()
, it does not return 
(stdout, stderr)
separately. A basic pattern looks like this:
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python
from prefect_shell import ShellOperation

try:
    stdout_lines = ShellOperation(
        commands=["my bash cmds here"],
        stream_output=True,  # optional: sends stdout/stderr to logs as they arrive
    ).run()

    stdout_str = "\n".join(stdout_lines)

except RuntimeError as exc:
    # non-zero exit code
    # stderr is not returned separately here
    print(f"Command failed: {exc}")
    raise
If you specifically need stderr separately like your subprocess example, the easiest workaround is to redirect stderr yourself in the shell command:
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python
from prefect_shell import ShellOperation
import tempfile
import os

stderr_file = tempfile.NamedTemporaryFile(delete=False)
stderr_path = stderr_file.name
stderr_file.close()

try:
    stdout_lines = ShellOperation(
        commands=[f"""
my bash cmds here 2> "{stderr_path}"
"""]
    ).run()

    stdout_str = "\n".join(stdout_lines)

    with open(stderr_path, "r") as f:
        stderr_str = f.read()

    if len(stderr_str) > 0:
        # deal with errors here
        print("stderr:", stderr_str)

finally:
    os.unlink(stderr_path)
A few important notes: - You do not need to pass `shell=True`; 
ShellOperation
already runs your commands through a shell. - 
run(**kwargs)
does accept kwargs forwarded to 
subprocess.Popen
, but that does not change the fact that 
run()
only returns stdout. - If your main goal is just “fail if there’s an error”, catching 
RuntimeError
is the normal Prefect pattern. So the recommendation is: If you want Prefect-style execution
Copy code
python
try:
    stdout = ShellOperation(commands=["my bash cmds here"]).run()
except RuntimeError:
    # handle command failure
    ...
If you need separate stderr content - redirect 
2>
to a file - then read that file after 
run()
Relevant docs/source: - ShellOperation docs - prefect-shell source If you want, I can also show you a helper function that wraps 
ShellOperation().run()
and returns 
(stdout_str, stderr_str)
in a subprocess-like way.
s
@Marvin
Copy code
If you want, I can also show you a helper function that wraps ShellOperation().run() and returns (stdout_str, stderr_str) in a subprocess-like way.
Sure, show me.
m
✅ thought for 15.5 seconds
Absolutely — here’s a small helper that gives you a more 
subprocess
-like interface on top of 
ShellOperation().run()
.
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python
from prefect_shell import ShellOperation
import tempfile
import os


def run_shell_with_stderr(commands, stream_output=False, **open_kwargs):
    """
    Run shell commands with Prefect ShellOperation and return
    (stdout_str, stderr_str).

    Args:
        commands: str or list[str]
        stream_output: whether to stream output to Prefect logs
        **open_kwargs: kwargs forwarded to ShellOperation.run()

    Returns:
        tuple[str, str]: (stdout_str, stderr_str)

    Raises:
        RuntimeError: if the shell command exits non-zero
    """
    if isinstance(commands, str):
        commands = [commands]

    stderr_file = tempfile.NamedTemporaryFile(delete=False)
    stderr_path = stderr_file.name
    stderr_file.close()

    try:
        redirected_commands = [
            f"{{ {cmd}; }} 2>> \"{stderr_path}\"" for cmd in commands
        ]

        stdout_lines = ShellOperation(
            commands=redirected_commands,
            stream_output=stream_output,
        ).run(**open_kwargs)

        stdout_str = "\n".join(stdout_lines)

        with open(stderr_path, "r") as f:
            stderr_str = f.read()

        return stdout_str, stderr_str

    finally:
        if os.path.exists(stderr_path):
            os.unlink(stderr_path)
Usage:
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python
stdout_str, stderr_str = run_shell_with_stderr(
    "echo hello && echo warning >&2"
)

print("STDOUT:")
print(stdout_str)

print("STDERR:")
print(stderr_str)
And with error handling:
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python
try:
    stdout_str, stderr_str = run_shell_with_stderr(
        "echo hello && ls /does-not-exist"
    )
except RuntimeError as exc:
    print(f"Command failed: {exc}")
One subtle thing here: if 
ShellOperation().run()
raises 
RuntimeError
, you may still want access to stderr. In that case, use this version instead:
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python
from prefect_shell import ShellOperation
import tempfile
import os


def run_shell_with_stderr(commands, stream_output=False, **open_kwargs):
    if isinstance(commands, str):
        commands = [commands]

    stderr_file = tempfile.NamedTemporaryFile(delete=False)
    stderr_path = stderr_file.name
    stderr_file.close()

    stdout_str = ""
    stderr_str = ""

    try:
        redirected_commands = [
            f"{{ {cmd}; }} 2>> \"{stderr_path}\"" for cmd in commands
        ]

        try:
            stdout_lines = ShellOperation(
                commands=redirected_commands,
                stream_output=stream_output,
            ).run(**open_kwargs)
            stdout_str = "\n".join(stdout_lines)
        except RuntimeError:
            with open(stderr_path, "r") as f:
                stderr_str = f.read()
            raise

        with open(stderr_path, "r") as f:
            stderr_str = f.read()

        return stdout_str, stderr_str

    finally:
        if os.path.exists(stderr_path):
            os.unlink(stderr_path)
If you want to preserve the stderr on failure, you can wrap the exception: ```python from prefect_shell import ShellOperation import tempfile import os class ShellRunError(RuntimeError): def __init__(self, message, stdout="", stderr=""): super().__init__(message) self.stdout = stdout self.stderr = stderr def run_shell_with_stderr(commands, stream_output=False, **open_kwargs): if isinstance(commands, str): commands = [commands] stderr_file = tempfile.NamedTemporaryFile(delete=False) stderr_path = stderr_file.name stderr_file.close() stdout_str = "" try: redirected_commands = [ f"{{ {cmd}; }} 2>> \"{stderr_path}\"" for cmd in commands ] try: stdout_lines = ShellOperation( commands=redirected_commands, stream_output=stream_output, ).run(**open_kwargs) stdout_str = "\n".join(stdout_lines)
except RuntimeError as exc: with open(stderr_path, "r") as f: stderr_str = f.read() raise ShellRunError(str(exc), stdout=stdout_str, stderr=stderr_str) from exc with open(stderr_path, "r") as f: stderr_str = f.read() return stdout_str, stderr_str finally: if os.path.exists(stderr_path): os.unlink(stderr_path)
Copy code
Then:

```python
try:
    stdout_str, stderr_str = run_shell_with_stderr(
        "echo hello && ls /does-not-exist"
    )
except ShellRunError as exc:
    print("stdout:", exc.stdout)
    print("stderr:", exc.stderr)
If you want, I can also give you: - a version for multiple commands - a version that returns exit code + stdout + stderr - or a 
@task
-wrapped Prefect version
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