Macro respawn_pane 

macro_rules! respawn_pane {
    (@cmd ($cmd:expr) -k, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -c $start_directory:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -e $environment:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) -t $target_pane:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr) $shell_command:expr, $($tail:tt)*) => { ... };
    (@cmd ($cmd:expr)) => { ... };
    () => { ... };
    (($cmd:expr), $($tail:tt)*) => { ... };
    ($($tail:tt)*) => { ... };
}

Reactivate a pane in which the command has exited

Manual

tmux ^3.0:

respawn-pane [-k] [-c start-directory] [-e environment] [-t target-pane] [shell-command]
(alias: respawnp)

tmux ^2.6:

respawn-pane [-k] [-c start-directory] [-t target-pane] [shell-command]
(alias: respawnp)

tmux ^1.5:

respawn-pane [-k] [-t target-pane] [shell-command]
(alias: respawnp)