shells.txt

# Shells

## An all-in-one .bash_profile/.bashrc

Bash claims that .bash_profile is executed for "login" shells, and .bashrc
for all "interactive" shells.  The thinking goes that every shell has an
ancestor "login" shell from which it inherits its environment, so we only
need to set environment variables in the "profile", while ".bashrc" covers
the non-inherited (shell-local) settings important mainly for the
command-line UI.

However, in modern environments, many programs execute without inheriting
from a shell, and some of these (e.g. GUI emacs) launch sub-shells, so we
can no longer rely on the "profile" environment settings being present,
even when we are in a shell.

Our solution is to consolidate the environment and shell-local settings into
one file (.bashrc).  It will set environment variables if they have not
already been set.  It will set interactive settings when it is launching an
interactive shell.

Our `.bash_profile` then reduces to one line -- `. .bashrc` -- so whether
`.bash_profile` *or* `.bashrc` is invoked, our shell should have the
appropriate settings.

## Emacs

In Emacs, we set the BASH_ENV environment variable to `.bash_profile` to
ensure that commands invoked via `M-x compile` will execute in the
environment of our .bashrc.  Otherwise, Emacs will invoke a shell without
any of our environment settings.  Why `.bash_profile`?  While we intend
.bashrc to contain all environment settings, sometimes our .bash_profile is
modified without our knowledge -- e.g. by homebrew installations.  This
helps ensure that `compile` invocations will have the same environment as
terminal shells.

`M-x shell`, on the other hand, launches an interactive shell, which sources
.bashrc, no matter what BASH_ENV is set to.