If BUFFER exists and shell process is running, just switch to BUFFER.
Program used comes from variable `explicit-shell-file-name',
or (if that is nil) from the ESHELL environment variable,
- or (if that is nil) from `shell-file-name'
- or else from SHELL if there is no ESHELL.
+ or (if that is nil) from `shell-file-name'.
If a file `~/.emacs_SHELLNAME' exists, or `~/.emacs.d/init_SHELLNAME.sh',
it is given as initial input (but this may be lost, due to a timing
error, if the shell discards input when it starts up).