What are the functional differences between .profile .bash_profile and .bashrc

.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.

.profile

.profile is for things that are not specifically related to Bash, like environment variables PATH and friends, and should be available anytime.

For example, .profile should also be loaded when starting a graphical desktop session.

.bashrc

.bashrc is for the configuring the interactive Bash usage, like Bash aliases, setting your favorite editor, setting the Bash prompt, etc.

.bash_profile

.bash_profile is for making sure that both the things in .profile and .bashrc are loaded for login shells.

For example, .bash_profile could be something simple like

. ~/.profile
. ~/.bashrc

If you were to omit .bashrc, only .profile would be loaded.

According to the bash man page, .bash_profile is executed for login shells, while .bashrc is executed for interactive non-login shells.

So, what is a login or non-login shell?

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) inside Gnome or KDE, 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.

An exception to the terminal window guidelines is Mac OS X’s Terminal.app, which runs a login shell by default for each new terminal window, calling .bash_profile instead of .bashrc. Other GUI terminal emulators may do the same, but most tend not to.

More at: .bash_profile vs .bashrc - by Josh Staiger.

I resorted to the man page for this answer (man bash).

The three files are all used by bash in different scenarios.

Scenario 1: interactive login shell (interactive means you can do user input. Note this does NOT include cron jobs! A login shell is the first shell that is spawned when you login, but does not include sub-shells) So basically this is the command prompt you get right when you login. What will bash do? Bash reads /etc/profile every time. Then, it looks for ~/.bash_profile. If it doesn't exist, it looks for ~/.bash_login. If that doesn't exist, then it looks for ~/.profile. But it will only read one of .bash_profile, .bash_login, and .profile. So it will quit after it finds one and reads it.

Scenario 2: interactive non-login (This is a shell that is started after the first login shell. This shell is one that allows user input). When an interactive non-login shell is started: Bash just reads /etc/bash.bashrc and ~/.bashrc.

Scenario 3: non-interactive shell (for example cronjob) When non-interactive, it doesn't load any of the files at all. But, it does look for an environment variable called BASH_ENV which is supposed to contain the path to a file that will be executed. So in this case, it doesn't read any of the ~/.profile or ~/.bash_profile files unless you explicitly set the BASH_ENV to an absolute path. I believe this is why you should always specify the absolute path in any cronjobs because none of the config files get executed by default.

Here is my source

From bash(1) - Linux man page