This post serves as a reference for various Linux startup, login and init scripts, environment variables and the order in which they are loaded or executed.

I use Arch Linux with Bash, but the general concepts should be applicable to any distro or shell. I also use Bash as my default login shell. You can confirm which login shell you’re running (sh or bash) by using echo $SHELL. The default user shell can be changed using the chsh command.

/etc/environment

• Loaded by Linux PAM (Pluggable Authentication Modules)
• More specifically by pam_env.so which is loaded by login shells as well as the Display Manager (for my setup - LXQT with SDDM - it is loaded by /etc/pam.d/sddm-greeter)
• Is NOT a shell script, i.e. existing variables or bash script commands can’t be used (e.g. echo $VAR will not work)
• A good place for setting system wide environment variables –> also works for SSH
• Environment variables can be assigned, one per line, as: FOO = Hi, I'm a variable & my name is FOO
• But if Bash commands need to be executed (e.g. setting terminal colors or modification of $PATH), /etc/bash.bashrc or /etc/profile (login shells only) are good options for doing it for all users. For user specific script commands ~/.bashrc can be used for non-login shells or either of .bash_profile, .bash_login or .profile for login shells.

~/.bashrc

• Bash executes commands in ~/.bashrc (if it exists) for interactive non-login shells (i.e. it is not executed when using SSH)
• Good for user specific environment variables and commands when Bash is running locally

$BASH_ENV

• For non-interactive shells, if $BASH_ENV is set, Bash executes the script specified in $BASH_ENV
• I usually set $BASH_ENV in /etc/environment to ~/.bash_env, this way, each user can have their own .bash_env

Note: SSH is an interactive login shell BUT it starts non-interactively. This is how I confirmed this:

• If any echo statements present in my $BASH_ENV script are executed while logging in via SSH, then this is proof enough that Bash is executing the $BASH_ENV script, which is only done for non-interactive shells. But more specifically, I used echo $-.
• $- is a special environment variable containing current shell options, if it contains i, it is an interactive shell:

$ echo $-
himBH

• If you add echo $- to your $BASH_ENV file, logging in via SSH will show something like hBc confirming script execution and its non-interactive nature. All other scripts are executed in interactive mode.

~/.bash_profile, ~/.bash_login, ~/.profile

For login shells, Bash searches for & executes whichever script it finds in the following order of priority:

• .bash_profile
• .bash_login
• .profile

/etc/profile

• Executed by login shells and the Display Manager
• Sources /etc/bash.bashrc and all *.sh files in the /etc/profile.d/ folder
• For login shells, /etc/profile can be used for executing Bash commands or setting system-wide environment variables. But directly editing etc/profile is generally not recommended as it is a configuration file for system packages; it is a better idea to create a new script with an .sh extension in the /etc/profile.d/ folder

/etc/bash.bashrc

• Loaded by all interactive (login AND non-login shells)

Execution/Loading Order

• /etc/environment: Environment variables added here are always available in all types of sessions

:

• /etc/bash.bashrc
• ~/.bashrc

:

• /etc/profile
• /etc/profile.d/*.sh
• /etc/bash.bashrc
• ~/.bash_profile: or ~/.bash_login or ~/.profile (whichever you prefer)

:

• $BASH_ENV
• /etc/profile
• /etc/profile.d/*.sh
• /etc/bash.bashrc
• ~/.bash_profile: or ~/.bash_login or ~/.profile (whichever you prefer)

References

1. https://help.ubuntu.com/community/EnvironmentVariables#System-wide_environment_variables
2. https://wiki.archlinux.org/index.php/Bash
3. https://bugs.launchpad.net/ubuntu/+source/manpages/+bug/18574/comments/13
4. http://askubuntu.com/questions/98433/run-a-script-on-login-using-bash-login
5. http://linux.die.net/man/8/pam_env
6. http://serverfault.com/questions/593472/where-is-bash-env-usually-set
7. http://stackoverflow.com/questions/3327013/how-to-determine-the-current-shell-im-working-on
8. http://www.chainsawonatireswing.com/2012/02/02/find-out-what-your-unix-shells-flags-are-then-change-them//?from=@#fn:curly-brace-expansion
9. http://capistranorb.com/documentation/faq/why-does-something-work-in-my-ssh-session-but-not-in-capistrano/