How can I run Cygwin Bash Shell from within Emacs?

shell-file-name is the variable that controls which shell Emacs uses when it wants to run a shell command.

explicit-shell-file-name is the variable that controls which shell M-x shell starts up.

Ken's answer changes both of those, which you may or may not want.

You can also have a function that starts a different shell by temporarily changing explicit-shell-file-name:

(defun cygwin-shell ()
  "Run cygwin bash in shell mode."
  (interactive)
  (let ((explicit-shell-file-name "C:/cygwin/bin/bash"))
    (call-interactively 'shell)))

You will probably also want to pass the --login argument to bash, because you're starting a new Cygwin session. You can do that by setting explicit-bash-args. (Note that M-x shell uses explicit-PROGRAM-args, where PROGRAM is the filename part of the shell's pathname. This is why you should not include the .exe when setting the shell.

I use XEmacs with Cygwin, and can run bash from XEmacs relatively easily.

Here's the relevant section from init.el

;; Let's use CYGWIN bash...
;;
(setq binary-process-input t) 
(setq w32-quote-process-args ?\") 
(setq shell-file-name "bash") ;; or sh if you rename your bash executable to sh. 
(setenv "SHELL" shell-file-name) 
(setq explicit-shell-file-name shell-file-name) 
(setq explicit-sh-args '("-login" "-i"))

One more important hint on this subject.

If you use Emacs shell mode and want both bash and cmd sometimes, set it up to use bash by default, because you can type cmd at bash and the resulting dos shell works just fine.

If you setup Emacs to use cmd as the shell (which is the way NT emacs installs), then dos shell works fine, but if you type bash or bash -i to run a bash shell, the resulting shell doesn't work right - see answer 0.

But it works fine if bash is the "first" shell emacs invokes.

You can run bash directly from the default Windows command-line shell within your Emacs *shell* buffer:

Microsoft Windows XP [Version 5.1.2600]
(C) Copyright 1985-2001 Microsoft Corp.

C:\temp>bash
bash     

However, no command prompt is visible which can be disorienting resulting in your commands and their output results all blending together.

In addition, for some unknown reason, if you do enter a command and hit return, a return line character (\r) is appended to the end of your command statement causing a bash error:

ls
bash: line 1: $'ls\r': command not found

A workaround is to manually add a comment character (#) at the end of every command which effectively comments out the \r text:

ls #
myfile,txt
foo.bar
anotherfile.txt

This overall approach is far from ideal but might be useful if you want to drop into bash from Windows' native shell to do some quick operations and then exit out to continue working in Windows.

I'm using EmacsW32. C-h a shell$ gives a list of shell launching commands and the commands cmd-shell and cygwin-shell look interesting. Both commands need EmacsW32. They are also found in the menu: Tools > W&32 Shells.

If you run cygwin-shell for the first time, and if you have not setup cygwin path in Emacs, it leads you to the Customization page where you can setup the cygwin path by pressing Find button.