Execute Long-Running Commands Inside Your Sandbox - Fermion Documentation

The run_streaming_command() method is designed for executing long-running or interactive commands inside your sandbox while receiving real-time output streams. Unlike run_command(), this method does not block until completion — it allows you to process live logs or incremental output as the command runs.

`async run_streaming_command(cmd: str, args: Optional[List[str]] = None, stdin: Optional[str] = None, on_stdout: Optional[Callable[[str], None]] = None, on_stderr: Optional[Callable[[str], None]] = None) -> CommandResult`

Description:
Executes a long-running command inside the sandbox and streams output in real time. Callbacks are triggered as new stdout or stderr data chunks arrive.

Parameters:

Parameter	Type	Required	Description
`cmd`	`str`	Yes	Command to execute (e.g., `"npm"`, `"python3"`)
`args`	`List[str]`	No	List of command arguments (defaults to empty list)
`stdin`	`str`	No	Optional standard input to send to the command
`on_stdout`	`Callable[[str], None]`	No	Callback function invoked when stdout data arrives
`on_stderr`	`Callable[[str], None]`	No	Callback function invoked when stderr data arrives

Returns: CommandResult object with the following attributes:

stdout: str - Complete accumulated output
stderr: str - Complete accumulated error output
exit_code: int - Process exit code (0 = success)

Example

import asyncio
from fermion_sandbox import Sandbox

async def main():
    sandbox = Sandbox(api_key="your-api-key")
    await sandbox.create(should_backup_filesystem=False)

    def handle_stdout(data: str):
        print("[stdout]", data.strip())

    def handle_stderr(data: str):
        print("[stderr]", data.strip())

    result = await sandbox.run_streaming_command(
        cmd="npm",
        args=["install", "express"],
        on_stdout=handle_stdout,
        on_stderr=handle_stderr
    )

    print("Exit code:", result.exit_code)

    # With stdin
    result = await sandbox.run_streaming_command(
        cmd="python3",
        args=["-c", 'import sys; [print(line.strip()) for line in sys.stdin]'],
        stdin="Hello from stdin\n"
    )

    print("Output:", result.stdout)

asyncio.run(main())