diff --git a/components/l4d2-host-lib/src/l4d2host/process.py b/components/l4d2-host-lib/src/l4d2host/process.py
new file mode 100644
index 0000000..067e94d
--- /dev/null
+++ b/components/l4d2-host-lib/src/l4d2host/process.py
@@ -0,0 +1,79 @@
+from dataclasses import dataclass
+import subprocess
+import sys
+import threading
+from typing import Callable, Sequence
+
+
+@dataclass(slots=True)
+class CommandResult:
+    returncode: int
+    stdout: str
+    stderr: str
+
+
+def run_command(
+    cmd: Sequence[str],
+    *,
+    on_stdout: Callable[[str], None] | None = None,
+    on_stderr: Callable[[str], None] | None = None,
+    passthrough: bool = False,
+) -> CommandResult:
+    stdout_lines: list[str] = []
+    stderr_lines: list[str] = []
+
+    proc = subprocess.Popen(
+        list(cmd),
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        text=True,
+        bufsize=1,
+    )
+
+    def pump(
+        stream: subprocess.Popen[str].stdout,
+        sink: list[str],
+        callback: Callable[[str], None] | None,
+        output_stream,
+    ) -> None:
+        if stream is None:
+            return
+        for raw in iter(stream.readline, ""):
+            line = raw.rstrip("\n")
+            sink.append(line)
+            if callback is not None:
+                callback(line)
+            if passthrough:
+                print(line, file=output_stream)
+        stream.close()
+
+    stdout_thread = threading.Thread(
+        target=pump,
+        args=(proc.stdout, stdout_lines, on_stdout, sys.stdout),
+        daemon=True,
+    )
+    stderr_thread = threading.Thread(
+        target=pump,
+        args=(proc.stderr, stderr_lines, on_stderr, sys.stderr),
+        daemon=True,
+    )
+    stdout_thread.start()
+    stderr_thread.start()
+
+    returncode = proc.wait()
+    stdout_thread.join()
+    stderr_thread.join()
+
+    result = CommandResult(
+        returncode=returncode,
+        stdout="\n".join(stdout_lines),
+        stderr="\n".join(stderr_lines),
+    )
+    if returncode != 0:
+        raise subprocess.CalledProcessError(
+            returncode=returncode,
+            cmd=list(cmd),
+            output=result.stdout,
+            stderr=result.stderr,
+        )
+    return result
diff --git a/components/l4d2-host-lib/tests/test_process.py b/components/l4d2-host-lib/tests/test_process.py
new file mode 100644
index 0000000..0791034
--- /dev/null
+++ b/components/l4d2-host-lib/tests/test_process.py
@@ -0,0 +1,22 @@
+import subprocess
+
+import pytest
+
+from l4d2host.process import run_command
+
+
+def test_callbacks_receive_lines() -> None:
+    out: list[str] = []
+    err: list[str] = []
+    run_command(
+        ["python3", "-c", "import sys; print('ok'); print('warn', file=sys.stderr)"],
+        on_stdout=out.append,
+        on_stderr=err.append,
+    )
+    assert out == ["ok"]
+    assert err == ["warn"]
+
+
+def test_nonzero_exit_raises() -> None:
+    with pytest.raises(subprocess.CalledProcessError):
+        run_command(["python3", "-c", "import sys; sys.exit(7)"])