When I start tmux, I get a failure when trying to configure powerline. I have set an environment environment variable with this:
export POWERLINE_CONFIG_COMMAND=`which powerline-config`
My ~/.tmux.conf contains the following:
if-shell "POWERLINE_CONFIG_COMMAND" \
run-shell "$POWERLINE_CONFIG_COMMAND tmux setup"
The error I get is:
unknown command: /path/to/powerline-config
I can run the config command manually after tmux starts with this:
$POWERLINE_CONFIG_COMMAND tmux setup
I don't understand why tmux can't run the command during the startup when it can run just fine afterwards.
I don't understand how you get that error. You should not get any message, and nothing should work.
if-shell "POWERLINE_CONFIG_COMMAND" \
run-shell "$POWERLINE_CONFIG_COMMAND tmux setup"
will fail, because POWERLINE_CONFIG_COMMAND is not a command. Your if-shell should have a $ in front of POWERLINE_CONFIG_COMMAND.
Let's assume that was a typo, and it's correct in your actual .conf. Then, the problem is that run-shell runs against tmux, the way it'd run if you typed <prefix>: in your tmux session.
tmux $POWERLINE_CONFIG_COMMAND tmux setup is not a valid command.
You could instead do
run-shell 'send-keys "$POWERLINE_CONFIG_COMMAND tmux setup" Enter'
If you wanted it run in a single pane.