Use 'sed' with pre-search to enable only if present in redis config file.

doc/update/7.2-to-7.3.md

@@ -72,8 +72,9 @@ sudo cp lib/support/init.d/gitlab /etc/init.d/gitlab
     sed 's/^port .*/port 0/' /etc/redis/redis.conf.orig | sudo tee /etc/redis/redis.conf
     # Enable Redis socket for default Debian / Ubuntu path
     echo 'unixsocket /var/run/redis/redis.sock' | sudo tee -a /etc/redis/redis.conf
-    # Be sure redis group can write to the socket.
-    echo 'unixsocketperm 0775' | sudo tee -a /etc/redis/redis.conf
+    # Be sure redis group can write to the socket, enable only if supported (>= redis 2.4.0).
+    sed '/# unixsocketperm/ s/^# unixsocketperm.*/unixsocketperm 0775/' /etc/redis.conf | sudo tee -a /etc/redis/redis.conf
+    echo ' 0775' | sudo tee -a /etc/redis/redis.conf
     # Activate the changes to redis.conf
     sudo service redis-server restart
     # Add git to the redis group