The main difference with shell config files is that some are only read by "login" shells (eg. when you login from another host, or login at the text console of a local unix machine). these are the ones called, say, .login or .profile or .zlogin (depending on which shell you're using).

Then you have config files that are read by "interactive" shells (as in, ones connected to a terminal (or pseudo-terminal in the case of, say, a terminal emulator running under a windowing system). these are the ones with names like .bashrc, .tcshrc, .zshrc, etc.

bash complicates this in that .bashrc is only read by a shell that's both interactive and non-login, so you'll find most people end up telling their .bash_profile to also read .bashrc with something like

[[ -r ~/.bashrc ]] && . ~/.bashrc

Other shells behave differently - eg with zsh, .zshrc is always read for an interactive shell, whether it's a login one or not.

The manual page for bash explains the circumstances under which each file is read. Yes, behaviour is generally consistent between machines.

.profile is simply the login script filename originally used by /bin/sh. bash, being generally backwards-compatible with /bin/sh, will read .profile if one exists.

TL;DR:

• ~/.bash_profile should be super-simple and just load .profile and .bashrc (in that order)

• ~/.profile has the stuff NOT specifically related to bash, such as environment variables (PATH and friends)

• ~/.bashrc has anything you'd want at an interactive command line. Command prompt, EDITOR variable, bash aliases for my use

A few other notes:

• Anything that should be available to graphical applications OR to sh (or bash invoked as sh) MUST be in ~/.profile

• ~/.bashrc must not output anything

• Anything that should be available only to login shells should go in ~/.profile

• Ensure that ~/.bash_login does not exist.

.bash_profile is executed for login shells, while .bashrc is executed for interactive non-login shells.

When you login (type username and password) via console, either sitting at the machine, or remotely via ssh: .bash_profile is executed to configure your shell before the initial command prompt.

But, if you’ve already logged into your machine and open a new terminal window (xterm) then .bashrc is executed before the window command prompt. .bashrc is also run when you start a new bash instance by typing /bin/bash in a terminal.

On OS X, Terminal by default runs a login shell every time, so this is a little different to most other systems, but you can configure that in the preferences.

It helps to understand which files get sourced, when, and why.

• .profile is sourced by a login shell on startup. Typically, the only login shell you start is the one started when you log in, but you can run a login shell at any time with bash -l. (Also, on macOS, there is no initial login shell, so terminal emulators tend to run a login shell for each new window.)

• .profile is an ideal place to set environment variables that can be inherited by any program started from the login shell.

• .bashrc, on the other hand, is sourced by non-login interactive shells, such as those started by terminal windows (most configs also source .bashrc in interactive login shells though). This is where you set things specific to your interactive shell that aren't otherwise inherited from the parent process. For example, PS1 is set here because only interactive shells care about its value, and any interactive shell will source .bashrc anyway, so there is no need to define and export PS1 from .profile.

• And though you didn't ask, it's worth pointing out the difference between .profile and .bash_profile here. .profile is "shared" by all POSIX shells (such as dash), so don't put anything bash-specific here. .bash_profile, though, is only used by bash, so you can use bash extensions in it. If .bash_profile is present, .profile will be ignored, so if for whatever reason you want to use both, you can add . ~/.profile to the top of your .bash_profile.

.bash_profile and .bashrc are specific to bash, whereas .profile is read by many shells in the absence of their own shell-specific config files. (.profile was used by the original Bourne shell.) .bash_profile or .profile is read by login shells, along with .bashrc; subshells read only .bashrc. (Between job control and modern windowing systems, .bashrc by itself doesn't get used much. If you use screen or tmux, screens/windows usually run subshells instead of login shells.)

The idea behind this was that one-time setup was done by .profile (or shell-specific version thereof), and per-shell stuff by .bashrc. For example, you generally only want to load environment variables once per session instead of getting them whacked any time you launch a subshell within a session, whereas you always want your aliases (which aren't propagated automatically like environment variables are).

Other notable shell config files:

/etc/bash_profile (fallback /etc/profile) is read before the user's .profile for system-wide configuration, and likewise /etc/bashrc in subshells (no fallback for this one). Many systems including Ubuntu also use an /etc/profile.d directory containing shell scriptlets, which are . (source)-ed from /etc/profile; the fragments here are per-shell, with *.sh applying to all Bourne/POSIX compatible shells and other extensions applying to that particular shell.

Only .bashrc is run on non-login shells, while only .bash_profile is run on login shells.

.bashrc should typically contain things you want to set in every shell you open, like aliases, functions, etc. These are per shell session items that are not inherited from environment.

.bash_profile should contain things that need to be defined at login time only, like PATH and other environment variables, startup programs, etc. You just need things once, not in every shell you open. In most cases, you also need the things from .bashrc in your login shell. That's why .bash_profile sources .bashrc as well, but .bashrc doesn't usually source .bash_profile.

/etc/bashrc and /etc/profile are system wide settings made by the sys admin or the package manager. /etc/profile is sourced automatically in each login shell, before ~/.bash_profile. /etc/bashrc is not sourced, so it needs to be sourced from ~/.bashrc when required.

Now, you can club the two into a single file and link the other file to the first one. But you have to make sure that PATH and other variables are not relative defined (like PATH=$HOME/bin:$PATH) otherwise they will just keep becoming needlessly bigger. Also, you have to be careful of starting programs repeatedly. It's just easier to have these two separate.

Relevant section from the bash man page:

When bash is invoked as an interactive login shell, or as a non-interactive shell with the --login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The --noprofile option may be used when the shell is started to inhibit this behavior.

When a login shell exits, bash reads and executes commands from the file ~/.bash_logout, if it exists.

When an interactive shell that is not a login shell is started, bash reads and executes commands from ~/.bashrc, if that file exists. This may be inhibited by using the --norc option. The --rcfile file option will force bash to read and execute commands from file instead of ~/.bashrc.