I'd like to display the output of (getenv "HOSTNAME") somewhere in my mode line. My display-time-mode is set to 't', so I'm already displaying time, load level and a mail flag in the mode line. Is there an easy way to get the hostname in there, too?
I'd like to have this because I'm ssh'd into 3 remote machines, all running emacs from a common set of init files, and I'd like some fast easy unobtrusive way to know which machine I'm working on.
You're going to have to futz with mode-line-format to get this to work. This page has an example of doing that, and it even uses the current hostname as part of the example.
To build on Sean Bright's answer, specifically you can do this:
(let ((pos (memq 'mode-line-modes mode-line-format)))
(setcdr pos (cons (getenv "HOSTNAME") (cdr pos))))
This assumes that 'mode-line-modes is a part of your 'mode-line-format, which it is by default. Because you're modifying the list pointed to by the variable 'mode-line-format, you don't have to set the default value. If you were setting the variable itself, you'd have to do something like:
(setq-default mode-line-format (build-list-that-contains-(getenv "HOSTNAME")))