Emacs Lisp: Determine OS, Emacs Version, Machine Host Name

The following are useful for setting up emacs init file on different machines.

Find Operating System Type

Here's how to find out what operating system emacs is running on.

(cond
 ((string-equal system-type "windows-nt")
  (progn
    (message "Microsoft Windows")))
 ((string-equal system-type "darwin") ;  macOS
  (progn
    (message "Mac OS X")))
 ((string-equal system-type "gnu/linux")
  (progn
    (message "Linux"))))

Find Emacs Version

The following variables returns emacs version number.

emacs-version

Value is a dotted string. Sample value: "24.5.1"

emacs-major-version

Value is a integer, emacs major version number. Sample value: 28.

(if (< emacs-major-version 28)
    (progn
      (message "not emacs 28"))
  (progn
    (message "yes emacs 28 or later")))

emacs-minor-version

Value is a integer, emacs minor version number. Sample value: 5

Compare Version String

version<

Compares emacs version string.

(if (version< emacs-version "27.1")
    (message "too old")
  (message "good"))

version<=

Compares emacs version string.

version=

Compares emacs version string.

Get Host Name, User Name, Init File Path

Here's some other useful elisp function or variable to determine machine name (host name), user name, emacs init path.

system-name

Returns the host name.

user-login-name

Returns user login name.

user-emacs-directory

Init file dir.

user-init-file

Init file path.

For detail, see:

• (info "(elisp) User Identification")
• (info "(elisp) System Environment")

Check If a Package is Loaded

Emacs Lisp: Check If a function/variable is Defined

• 
  Buy Xah Emacs Tutorial