Fix run_before_script() output capturing

CHANGES

@@ -19,6 +19,10 @@ $ pipx install --suffix=@next 'tmuxp' --pip-args '\--pre' --force
 
 <!-- Maintainers, insert changes / features for the next release here -->
 
+### Bug fixes
+
+- `run_before_script()`: Fix output issue (#959)
+
 ## tmuxp 1.52.0 (2025-02-02)
 
 _Maintenance only, no bug fixes or new features_

src/tmuxp/util.py

@@ -28,41 +28,72 @@ def run_before_script(
     script_file: str | pathlib.Path,
     cwd: pathlib.Path | None = None,
 ) -> int:
-    """Execute a shell script, wraps :meth:`subprocess.check_call()` in a try/catch."""
+    """Execute shell script, ``tee``-ing output to both terminal (if TTY) and buffer."""
+    script_cmd = shlex.split(str(script_file))
+
     try:
         proc = subprocess.Popen(
-            shlex.split(str(script_file)),
-            stderr=subprocess.PIPE,
-            stdout=subprocess.PIPE,
+            script_cmd,
             cwd=cwd,
-            text=True,
+            stdout=subprocess.PIPE,
+            stderr=subprocess.PIPE,
+            text=True,  # decode to str
             errors="backslashreplace",
-            encoding="utf-8",
         )
-        if proc.stdout is not None:
-            for line in iter(proc.stdout.readline, ""):
-                sys.stdout.write(line)
-        proc.wait()
-
-        if proc.returncode and proc.stderr is not None:
-            stderr = proc.stderr.read()
-            proc.stderr.close()
-            stderr_strlist = stderr.split("\n")
-            stderr_str = "\n".join(list(filter(None, stderr_strlist)))  # filter empty
-
-            raise exc.BeforeLoadScriptError(
-                proc.returncode,
-                os.path.abspath(script_file),  # NOQA: PTH100
-                stderr_str,
-            )
-    except OSError as e:
-        if e.errno == 2:
-            raise exc.BeforeLoadScriptNotExists(
-                e,
-                os.path.abspath(script_file),  # NOQA: PTH100
-            ) from e
-        raise
-    return proc.returncode
+    except FileNotFoundError as e:
+        raise exc.BeforeLoadScriptNotExists(
+            e,
+            os.path.abspath(script_file),  # NOQA: PTH100
+        ) from e
+
+    out_buffer = []
+    err_buffer = []
+
+    # While process is running, read lines from stdout/stderr
+    # and write them to this process's stdout/stderr if isatty
+    is_out_tty = sys.stdout.isatty()
+    is_err_tty = sys.stderr.isatty()
+
+    # You can do a simple loop reading in real-time:
+    while True:
+        # Use .poll() to check if the child has exited
+        return_code = proc.poll()
+
+        # Read one line from stdout, if available
+        line_out = proc.stdout.readline() if proc.stdout else ""
+
+        # Read one line from stderr, if available
+        line_err = proc.stderr.readline() if proc.stderr else ""
+
+        if line_out:
+            out_buffer.append(line_out)
+            if is_out_tty:
+                sys.stdout.write(line_out)
+                sys.stdout.flush()
+
+        if line_err:
+            err_buffer.append(line_err)
+            if is_err_tty:
+                sys.stderr.write(line_err)
+                sys.stderr.flush()
+
+        # If no more data from pipes and process ended, break
+        if not line_out and not line_err and return_code is not None:
+            break
+
+    # At this point, the process has finished
+    return_code = proc.wait()
+
+    if return_code != 0:
+        # Join captured stderr lines for your exception
+        stderr_str = "".join(err_buffer).strip()
+        raise exc.BeforeLoadScriptError(
+            return_code,
+            os.path.abspath(script_file),  # NOQA: PTH100
+            stderr_str,
+        )
+
+    return return_code
 
 
 def oh_my_zsh_auto_title() -> None:

tests/test_util.py

@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import pathlib
+import sys
 import typing as t
 
 import pytest
@@ -35,8 +37,59 @@ def test_run_before_script_raise_BeforeLoadScriptError_if_retcode() -> None:
         run_before_script(script_file)
 
 
-def test_return_stdout_if_ok(capsys: pytest.CaptureFixture[str]) -> None:
+@pytest.fixture
+def temp_script(tmp_path: pathlib.Path) -> pathlib.Path:
+    """Fixture of an example script that prints "Hello, world!"."""
+    script = tmp_path / "test_script.sh"
+    script.write_text(
+        """#!/bin/sh
+echo "Hello, World!"
+exit 0
+"""
+    )
+    script.chmod(0o755)
+    return script
+
+
+@pytest.mark.parametrize(
+    ["isatty_value", "expected_output"],
+    [
+        (True, "Hello, World!"),  # if stdout is a TTY, output should be passed through
+        (False, ""),  # if not a TTY, output is not written to sys.stdout
+    ],
+)
+def test_run_before_script_isatty(
+    temp_script: pathlib.Path,
+    monkeypatch: pytest.MonkeyPatch,
+    capsys: pytest.CaptureFixture[str],
+    isatty_value: bool,
+    expected_output: str,
+) -> None:
+    """Verify behavior of ``isatty()``, which we mock in `run_before_script()`."""
+    # Mock sys.stdout.isatty() to return the desired value.
+    monkeypatch.setattr(sys.stdout, "isatty", lambda: isatty_value)
+
+    # Run the script.
+    returncode = run_before_script(temp_script)
+
+    # Assert that the script ran successfully.
+    assert returncode == 0
+
+    out, _err = capsys.readouterr()
+
+    # In TTY mode, we expect the output; in non-TTY mode, we expect it to be suppressed.
+    assert expected_output in out
+
+
+def test_return_stdout_if_ok(
+    capsys: pytest.CaptureFixture[str],
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
     """run_before_script() returns stdout if script succeeds."""
+    # Simulate sys.stdout.isatty() + sys.stderr.isatty()
+    monkeypatch.setattr(sys.stdout, "isatty", lambda: True)
+    monkeypatch.setattr(sys.stderr, "isatty", lambda: True)
+
     script_file = FIXTURE_PATH / "script_complete.sh"
 
     run_before_script(script_file)