Field Guide to Bash Terminals

23 Sep 2018 Shell Linux

You’re setting up your bash profile. You go to the Bash man page. Damn… you just wanted to know where to stick your config and aliases and this page is written in legalese!

This is an overview of the different kinds of Bash terminal and how they find their config, written in less detail but more to the point.

Terminology used:

• --login, the option that makes Bash a login shell
• --interactive, the option for a terminal that a user can interact with
• -c, the option for running a single command, non-interactively
• ~, your home directory
• file chain, a list of file locations where Bash will look for config files

The 2 File Chains

These file chains are referred to later on and act as a shorthand for particular behaviours of Bash. ‘File chain’ isn’t an official term, I made it up for this post.

Some general notes:

• files in /etc are shared across all users on the system.
• files in ~ are run for you only
• missing files won’t cause an error, they will just be skipped

profile file chain - run once per login

• run whatever is in the file /etc/profile
• run the first match from the following list:
  • ~/.bash_profile
  • ~/.bash_login
  • ~/.profile
• when a terminal is closed, run both of the following files:
  • /etc/bash.bash_logout
  • ~/.bash_logout

bashrc file chain - run once per terminal creation

• run /etc/bash.bashrc (on most distributions of Linux)
• run ~/.bashrc

/etc/bash.bashrc was added so that the bashrc file chain would have its own location for system-wide configuration, like /etc/profile provides. Debian and Ubuntu have adopted this feature.

The 5 Different Kinds of Terminal

--login --interactive

e.g. SSH-ing somewhere, or logging into your physical computer

Runs the profile file chain, as this will be the first terminal to run for a user on a system. When terminals create other terminals, they pass on their environment variables to their children, which is how the profile file chain is shared among all terminals on a system.

Although it is interactive, the property of being a login terminal overrides being an interactive terminal, so a login terminal won’t run the bashrc file chain by default.

It’s useful to make the assumption that login terminals are always interactive - they usually are, and it allows us to:

• always call the bashrc file chain, using a command in a file in the profile file chain
• store config that applies to all terminals in the bashrc file chain (which many distros now do), knowing that it will be run regardless of whether a terminal is login or not

There’s an edge case that contradicts this rule that I’ll cover later.

--interactive

e.g. bash, or opening a GUI terminal program

Runs the bashrc file chain. Doesn’t run profile file chain as it expects to inherit it from a parent login terminal. bashrc file chain is run once per terminal because:

• There is no guarantee --login terminal ran it
• It could contain code that needs to be run once per terminal creation

-c locally

e.g. bash -c 'echo hello world'

Run Bash with a single command rather than interactively. Expects to inherit both the profile and bashrc file chains from a parent terminal. It also has a special environment variable, $BASH_ENV, which can be set to a file path. Bash will run all commands in that file, before running the command set with -c

-c remotely

e.g. ssh user@host 'echo hello world'

Although it doesn’t actually use the -c syntax, ssh user@host <command> achieves a similar effect. It won’t run the remote profile file chain, unlike SSH-ing normally does and it doesn’t use the $BASH_ENV variable, but it will still run the remote system’s bashrc file chain before running the command.

--login, non-interactively?? The Edge Case

e.g. ssh user@host < input-file.txt

99.99% of the time you can assume that all login terminals are also interactive. 0.01% of the time you “need” to pipe a command in a local file to be run on a remote system. In this edge case, the remote profile file chain will be run, then the contents of input-file.txt, and then the SSH connection will terminate. This edge case won’t even have any meaningful side-effects most of the time.

Key Points

• --login should be configured to also run the bashrc file chain as well as profile, as then config common to all terminals can be stored there, rather than have the two file chains be run in a mutually exclusive way (many distros are already doing this but it’s worth checking)
• If it’s just for you it should go in ~, if it’s for everyone on the system it should go in /etc
• Changes to profile file chain will only be applied automatically after your next login.
• Terminals can acquire config by running a file chain or inheriting it from a parent terminal process.