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Broadcast Commands in tmux


I am a big fan of the Terminator terminal emulator because of its ability to both act as a multiplexer and broadcast a command to all panes simultaneously. This drastically speeds up executing the same task on multiple servers simultaneously (starting a Puppet run on a bunch of servers at once, for example).

However, one problem with Terminator is that it is a somewhat heavyweight application, pulling in piles of dependencies on a lean Linux or BSD install. As far as I know, is not available for macOS either. It also requires X11, which is a problem if running through a VT220. Fortunately, there's tmux, a lightweight terminal multiplexer which can be made to broadcast to multiple panes just like Terminator, but does not have a large dependency chain, and comes in the OpenBSD base system.

The magic is in tmux's "synchronize-panes" option, which can be toggled within a tmux session by typing the prefix combination, ctrl-b, then the colon : , which opens a prompt that can be used to set various options. The following option will toggle broadcasting to all panes:

setw synchronize-panes

Typing that again will turn off pane synchronization, but this is a bit cumbersome. Instead of doing that each time, I made a keybinding shortcut in ~/.tmux.conf that will toggle it for me:

bind a setw synchronize-panes
setw -g window-status-current-format '#{?pane_synchronized,#[bg=red],}#I:#W'
setw -g window-status-format         '#{?pane_synchronized,#[bg=red],}#I:#W'

The top line binds the 'a' key to the synchronize panes option. To toggle it now, just do ctrl-b, then 'a'. The bottom two lines are shamelessly stolen from Jonathan Palardy's excellent blog post about the synchronize panes option in tmux. These lines set the pane borders to red when pane synchronization is on, so that you have a visual indicator of it, saving you from the potential catastrophe of inadvertently broadcasting a dangerous command when SSH'd in to multiple servers.