Environments

The Linux shell (or terminal or cmd line) runs in "environments".
There is something like a "global" environment and also a "local" environment.

Environments

Terminals are Temporary Sessions

Every time a terminal/cmd window is open, it is a bash "session".
Each session can "remember" information.

The Host Machine is a Global Session

When your laptop/desktop/other starts up, a "global" session is also "running".
The temporary bash sessions of each terminal window are consumers of the global session contents.
Maybe like rooms in a house - the terminal window is like a room that can have some of its own data inside the house of the machine.

Environments Have Variables

The primary use for environments is storing information in varibales.
These are often referred to as environment variables.

In the web-development javascript space this is maybe most often similar to the differences between how an app might run in dev,qa,staging, production, blue, green, @next, etc. The dev,qa,prod values are often the environment variables. In a node api these might be accessed through process.env.NODE_ENV.

Interesting notes found here and here, starting with variable expansion.

See all variables with printenv

Environment variables are stored with a name and a value, like NAME=value.

ubuntu@primary:~$ printenv
# ...will print all variables

See a single variable with echo

# see the USER variable
# note the '$' prefix
ubuntu@primary:~$ echo $USER
ubuntu

Set a Temporary Variable

# set & use a var in 2 commands
ubuntu@primary:~$ WHAT=is-this
ubuntu@primary:~$ echo $WHAT
is-this

# cant set a var in the same command as the attempted use
ubuntu@primary:~$ ANOTHER_ONE=not-here-yet echo here is $ANOTHER_ONE
here is

# NEED the '&&' to declare 2 commands in one line
ubuntu@primary:~$ ANOTHER_ONE=not-here-yet && echo here is $ANOTHER_ONEhere is not-here-yet

Set System-Wide Permanent Variables

There are a few files on a linux os that store environment variables.

System-Wide with /etc/environment

/etc/environment is a file that stores contents that can be used in any shell any time. Store one var per line in this file, not many-variables in a single line.

System-Wide with /etc/profile.d/*.sh

There's a directory at etc/profile.d. (The .d in the file name might look confusing, it seems out-of-place to me).
This can store *.sh files where more variables can get stored that are available "globally", similar to the /etc/enviorment. This gets run when a shell is opened both as a console (like these blog posts have been about) or even when accessed over ssh.
It seems like files in this /etc/profile.d/custom-file.sh location are for single-purpose use-cases.

Scriptable System-Wide with /etc/profile and /etc/bashrc

/etc/profile looks controversial to edit: a ubuntu "help" doc reads...
"While /etc/profile is often suggested for setting environment variables system-wide, it is a configuration file of the base-files package, so it's not appropriate to edit that file directly. Use a file in /etc/profile.d instead..."

Customizing the Bash Shell with bashrc and bash_profile

The shell can be customized: things like text coloring, git tooling, setting package versions (node, python, etc).

Bash has "startup files", much like the above noted /etc/profile.d and /etc/environment files: ~/.bash_profile and ~/.bashrc are the focus here.

I'm still learning, but the "login shell" seems to be run when starting the machine && not run again after that during the use of the computer.
.bash_profile is for the 'login shell' variables: things that are run 1x when the os starts, and never again.

bashrc for every shell instance

.bashrc, on the other hand, is run on every shell instance, every window or tab. This .bashrc is often the place that "makes the most sense" to edit when customizing the shell.

This "instance" is like a custom config for every time a "local" shell is instantiated.

telling bash_profile to refer to bashrc

It looks like people prefer the .bashrc so much that there's a commin script to make even the .bash_profile look to the .bashrc for how to configure the environment. In the .bash_profile doc:

if [ -f ~/.bashrc ]; then
    source ~/.bashrc
fi

Tags:

linux