zsh: add multiplexer.zsh

Automatically reattach to existing terminal multiplexer session when
starting zsh and also create new terminal multiplexer session when
connected remotely using ssh.

zsh/multiplexer.zsh

@@ -0,0 +1,41 @@
+zshrc_use_multiplexer=tmux
+
+# Make sure the set multiplexer is available. Otherwise using exec will
+# terminate our session.
+if [[ -n $zshrc_use_multiplexer ]]; then
+    if ! (( $+commands[$zshrc_use_multiplexer] )); then
+        echo "Multiplexer '$zshrc_use_multiplexer' not found." >&2
+        zshrc_use_multiplexer=
+    fi
+fi
+
+# If not already in screen or tmux, reattach to a running session or create a
+# new one when connecting through ssh.
+if [[ $TERM != dumb && $TERM != dialup && $TERM != linux
+        && -z $STY && -z $TMUX ]]; then
+    # Get running detached sessions.
+    if [[ $zshrc_use_multiplexer = screen ]]; then
+        session=$(screen -list | grep 'Detached' | awk '{ print $1; exit }')
+    elif [[ $zshrc_use_multiplexer = tmux ]]; then
+        session=$(tmux list-sessions 2>/dev/null \
+                  | sed '/(attached)$/ d; s/^\([0-9]\{1,\}\).*$/\1/; q')
+    fi
+
+	# Create a new remote session if none is running.
+	if [[ -n $SSH_CONNECTION && -z $session ]]; then
+		if [[ $zshrc_use_multiplexer = screen ]]; then
+			exec screen
+		elif [[ $zshrc_use_multiplexer = tmux ]]; then
+			exec tmux
+		fi
+	fi
+
+    # Reattach to a running session.
+	if [[ -v $session ]]; then
+        if [[ $zshrc_use_multiplexer = screen ]]; then
+            exec screen -r $session
+        elif [[ $zshrc_use_multiplexer = tmux ]]; then
+            exec tmux attach-session -t $session
+        fi
+    fi
+fi