Server(feat[if_shell]): add if_shell() wrapping tmux if-shell

why: if-shell enables conditional tmux command execution based on shell
command exit status, useful for scripted environment setup.
what:
- Add Server.if_shell() with shell_command, tmux_command, else_command,
  background (-b), target_pane (-t) parameters
- Add tests for true branch and false-with-else branch

Author: tony

src/libtmux/server.py

@@ -718,6 +718,54 @@ def list_buffers(self) -> list[str]:
 
         return proc.stdout
 
+    def if_shell(
+        self,
+        shell_command: str,
+        tmux_command: str,
+        *,
+        else_command: str | None = None,
+        background: bool | None = None,
+        target_pane: str | None = None,
+    ) -> None:
+        """Execute a tmux command conditionally via ``$ tmux if-shell``.
+
+        Parameters
+        ----------
+        shell_command : str
+            Shell command whose exit status determines which tmux command runs.
+        tmux_command : str
+            Tmux command to run if *shell_command* succeeds (exit 0).
+        else_command : str, optional
+            Tmux command to run if *shell_command* fails (non-zero exit).
+        background : bool, optional
+            Run the shell command in the background (``-b`` flag).
+        target_pane : str, optional
+            Target pane for format expansion (``-t`` flag).
+
+        Examples
+        --------
+        >>> server.if_shell('true', 'set -g @if_test yes')
+        >>> server.cmd('show-options', '-gv', '@if_test').stdout[0]
+        'yes'
+        """
+        tmux_args: tuple[str, ...] = ()
+
+        if background:
+            tmux_args += ("-b",)
+
+        if target_pane is not None:
+            tmux_args += ("-t", target_pane)
+
+        tmux_args += (shell_command, tmux_command)
+
+        if else_command is not None:
+            tmux_args += (else_command,)
+
+        proc = self.cmd("if-shell", *tmux_args)
+
+        if proc.stderr:
+            raise exc.LibTmuxException(proc.stderr)
+
     def source_file(
         self,
         path: StrPath,

tests/test_server.py

@@ -605,6 +605,28 @@ def test_list_buffers(server: Server) -> None:
     assert len(result) >= 2
 
 
+def test_if_shell_true(server: Server) -> None:
+    """Test Server.if_shell() with true condition."""
+    server.new_session(session_name="ifshell_test")
+    server.if_shell("true", "set -g @if_test_true yes")
+
+    result = server.cmd("show-options", "-gv", "@if_test_true")
+    assert result.stdout[0] == "yes"
+
+
+def test_if_shell_false_with_else(server: Server) -> None:
+    """Test Server.if_shell() with false condition and else branch."""
+    server.new_session(session_name="ifshell_else")
+    server.if_shell(
+        "false",
+        "set -g @if_else_test yes",
+        else_command="set -g @if_else_test no",
+    )
+
+    result = server.cmd("show-options", "-gv", "@if_else_test")
+    assert result.stdout[0] == "no"
+
+
 def test_source_file(server: Server, tmp_path: pathlib.Path) -> None:
     """Test Server.source_file() sources a config file."""
     server.new_session(session_name="source_test")