With the exclusion of psql, all of the commands we use in this checklist are present on almost every UNIX system. They do nothing more than provide a very general idea of the system's health.
If we can ping a server, that doesn't mean it is running. The network service is one of the first things that the operating system starts and one of the last things it stops. The server can be stuck somewhere in its boot process or equally frozen in a shutdown. It does indicate, however, that something is available for further checks.
The next thing we try is to ssh to the server. If this command hangs indefinitely or returns with any kind of error, the server is effectively unusable. At this point, we would request the infrastructure ...
