The program you’re using, “cmd” is the Windows command prompt. When you
type in a command, it checks from two places whether that command is
The first place is a set of commands that are inside of the program
(such as “dir”, “cd”, et al). These commands let you look superficially
through the filesystem.
The second place is designated by what is called a “PATH” environment
variable. This tells the program “cmd” where to look in the filesystem
for the programs you try to run.
The program “ruby” is not in your “PATH”. That means that the command
prompt cannot find the program. To change this you must first know where
you installed ruby to. In the below instructions, it is assumed you
installed ruby to “c:\ruby”.
Choice 1: Add the ruby binary to your PATH environment variable
To change your path variable, hold down the “windows key” and press the
“pause break” key. It’ll open up the system properties dialog. Now click
on the “Advanced” tab. There should be an “Environment Variables”
button. Click on it and look for “Path”. Then click “Edit”. For the
variable value, add on to the end “;c:\ruby;c:\ruby\bin” without the
quotes. You need the semicolons to separate the two.
Choice 2: Write a batch file (.bat file) to put somewhere in your PATH
to run Ruby without changing your PATH environment variable
If you don’t want to add Ruby to your PATH for whatever reason, you can
just put a batch file in a place that is already in your PATH. The
directory “c:\windows\system32” should be in your PATH already, so go
there and create a text document with notepad.
In the text window put the line “c:\ruby\bin\ruby.exe” without quotes
and save it as “ruby.bat” in the “c:\windows\system32” directory. Make
sure there’s no .txt on the end of “ruby.bat”. Now when you do what
James B. told you above it will work.