24 Aug 2016

Notes on Linux: Environment Variables

Environment variables are a pretty fundamental aspect of working with Linux, and are often assumed knowledge in blog posts and tutorials. But they’re also one of those things that are so basic they can get overlooked. I have to admit I’ve been able to survive using Linux for a while now without really knowing how to use and manipulate them. So, this is me taking the time to understand them better.

Environment? Variable?

The environment is the configuration that is set up every time a new session is started on your computer, and defines how the system operates.

A variable is a way of representing or storing information. This is usually achieved using a KEY and a corresponding VALUE. In the case of a traditional dictionary the word or phrase is the key, and it’s meaning is the value. With this we can easily access meanings through their corresponding words.

So, environment variables are pieces of information made available to your computer operating system in order to configure it appropriately.

Linux environment variables are stored in the following format, where it is possible to store multiple values, separating them with a colon.

KEY=value1:value2:value3:...

Accessing Environment Variables

To access our environment variables we can use the env, printenv or set commands. Along with environment variables set displays shell variables, shell functions and local variables. env and printenv are very similar, displaying the same information, though with printenv you are able to access specific variables, and with env you can provide specific variables to a command in order to temporarily modify the environment for that command.

% printenv | less
USER=rob
LANGUAGE=en_GB:en
XDG_SEAT=seat0
XDG_SESSION_TYPE=x11
USER=rob
LANGUAGE=en_GB:en
XDG_SEAT=seat0
XDG_SESSION_TYPE=x11
SSH_AGENT_PID=3928
SHLVL=1
HOME=/home/rob
DESKTOP_SESSION=i3
QT_LINUX_ACCESSIBILITY_ALWAYS_ON=1
GTK_MODULES=gail:atk-bridge
XDG_SEAT_PATH=/org/freedesktop/DisplayManager/Seat0
DBUS_SESSION_BUS_ADDRESS=unix:abstract=/tmp/dbus-w7NbwfF1ee,guid=e8872840b094e30cfe473cae57bd4b7d
QT_QPA_PLATFORMTHEME=appmenu-qt5
MANDATORY_PATH=/usr/share/gconf/i3.mandatory.path
LOGNAME=rob
DEFAULTS_PATH=/usr/share/gconf/i3.default.path
XDG_SESSION_ID=c2
GTK2_MODULES=overlay-scrollbar
PATH=/home/rob/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/go/bin:/home/rob/code/go/bin
GDM_LANG=en_GB
XDG_SESSION_PATH=/org/freedesktop/DisplayManager/Session0
XDG_RUNTIME_DIR=/run/user/1000
DISPLAY=:0
LANG=en_GB.UTF-8
XDG_CURRENT_DESKTOP=i3
DESKTOP_STARTUP_ID=i3/rxvt-unicode/3789-6-agnesmartin_TIME13812728
XDG_SESSION_DESKTOP=i3
XAUTHORITY=/home/rob/.Xauthority
XDG_GREETER_DATA_DIR=/var/lib/lightdm-data/rob
SSH_AUTH_SOCK=/tmp/ssh-rR4FnFfhpAGZ/agent.3926
SHELL=/usr/bin/zsh
NODE_PATH=/usr/lib/nodejs:/usr/lib/node_modules:/usr/share/javascript
QT_ACCESSIBILITY=1
GDMSESSION=i3
:

Above is the output of printenv, piped through less for brevity (the % is from my prompt rather than part of the command). To access specific values we follow printenv with the key we want.

% printenv HOME
/home/rob

And we can temporarily alter our environment with env, placing it in front of the command we want to provide the environment variable for, in this case a simple printenv:

~ 13:35
% printenv HOME
/home/rob

~ 13:35
% env HOME=/another/path printenv HOME
/another/path

~ 13:35
% printenv HOME
/home/rob

(Again, the ~ 13:35 is just the time in my prompt rather than anything to do with the command)

We can also access environment variables with the echo command, though when we do this we have to include a $ to indicate it is a variable rather than the literal word…

~ 13:40
% echo HOME
HOME

~ 13:40
% echo $HOME
/home/rob

Setting & Unsetting Variables

To set an environment variable we can use the same KEY=value format to assign a value to a key…

~ 13:45
% printenv TERM
rxvt-unicode-256color

~ 13:45
% TERM=xterm

~ 13:45
% printenv TERM
xterm

~ 13:45
% export TERM

Setting this only applies to the current shell session, it won’t apply to any other terminal sessions, so we have to export the variable. Setting this overwrites the current value of the variable, which is not always what we want to do. For instance we may need to add a new directory to the PATH variable, a list of all the directory locations the system will look for commands in. To append a new location we can do the following…

~ 13:55
% printenv PATH
/home/rob/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/go/bin:/home/rob/code/go/bin

~ 13:55
% PATH=$PATH:/home/rob/code

~ 13:55
% printenv PATH
/home/rob/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/local/go/bin:/home/rob/code/go/bin:/home/rob/code

We use the current value of PATH as part of the new value, adding on our new value to the end, separating the two with a colon, and assign it back to the PATH variable. We can now see it at the end of our PATH. Again we would need to export this to make it available to all shell sessions.

As well as system environment variables, we can also create and assign user-defined variables…

~ 14:01
% printenv SOME_KEY

~ 14:01 C:1
% SOME_KEY=some_value

~ 14:01 C:1
% export SOME_KEY

~ 14:01
% printenv SOME_KEY
some_value

These can often be created for specific program requirements.

To unset an environment variable we can use the unset command…

~ 14:41
% printenv KEY

~ 14:41 C:1
% export KEY=value

~ 14:41
% printenv KEY
value

~ 14:41
% unset KEY

~ 14:41
% printenv KEY

Persisting Environment Variables

These environment variable changes will not survive a system restart and we don’t want to have to type them in every time we start our system. So we have to include them in a configuration file that is run every time we boot up. On bash this could be ~/.bashrc, ~/.bash_profile, ~/.bash_login or ~/.profile, there’s plenty of discussion online about where to keep environment variables. I use ZSH and have environment variables in both my ~/.zshrc and ~/.zshenv.

In my ./zshrc I set the LANG variable and the TERM variable:

export LANG=en_GB.UTF-8
export TERM=rxvt-unicode-256color

And in ~/.zshenv I set my PATH variable and the value of my GOPATH, a user-defined variable used by the Go programming language.

export PATH="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"
export PATH=$HOME/bin:$PATH

# GOPATH
export GOPATH=$HOME/code/go

I’ve had a few ohhh, that’s why that is like that moments while learning about Linux environment variables. It feels good to properly understand what’s going on. The links below have been super helpful…

Rob Phoenix

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Notes on Linux: Environment Variables

Environment? Variable?

Accessing Environment Variables

Setting & Unsetting Variables

Persisting Environment Variables