To run a subshell interactively, type M-x shell. This creates (or reuses) a buffer named *shell*, and runs a shell subprocess with input coming from and output going to that buffer. That is to say, any terminal output from the subshell goes into the buffer, advancing point, and any terminal input for the subshell comes from text in the buffer. To give input to the subshell, go to the end of the buffer and type the input, terminated by <RET>.
While the subshell is waiting or running a command, you can switch windows or buffers and perform other editing in Emacs. Emacs inserts the output from the subshell into the Shell buffer whenever it has time to process it (e.g., while waiting for keyboard input).
In the Shell buffer, prompts are displayed with the face
comint-highlight-prompt, and submitted input lines are
displayed with the face
comint-highlight-input. This makes it
easier to distinguish input lines from the shell output.
To make multiple subshells, invoke M-x shell with a prefix argument (e.g., C-u M-x shell). Then the command will read a buffer name, and create (or reuse) a subshell in that buffer. You can also rename the *shell* buffer using M-x rename-uniquely, then create a new *shell* buffer using plain M-x shell. Subshells in different buffers run independently and in parallel.
To specify the shell file name used by M-x shell, customize
explicit-shell-file-name. If this is
(the default), Emacs uses the environment variable ESHELL if it
exists. Otherwise, it usually uses the variable
shell-file-name (see Single Shell); but if the default
directory is remote (see Remote Files), it prompts you for the
shell file name.
Emacs sends the new shell the contents of the file ~/.emacs_shellname as input, if it exists, where shellname is the name of the file that the shell was loaded from. For example, if you use bash, the file sent to it is ~/.emacs_bash. If this file is not found, Emacs tries with ~/.emacs.d/init_shellname.sh.
To specify a coding system for the shell, you can use the command C-x <RET> c immediately before M-x shell. You can also change the coding system for a running subshell by typing C-x <RET> p in the shell buffer. See Communication Coding.
Emacs sets the environment variable INSIDE_EMACS in the
subshell to ‘version,comint’, where version is the
Emacs version (e.g., ‘24.1’). Programs can check this variable
to determine whether they are running inside an Emacs subshell. (It
also sets the EMACS environment variable to
t, if that
environment variable is not already defined. However, this
environment variable is deprecated; programs that use it should switch
to using INSIDE_EMACS instead.)