Environment variables¶

Environment variables store data that is used by the operating system and other programs.

Some are intrinsic to the operating system, some for a specific program/library/programming language, and some are created by the user.

The variables can both be used in scripts and on the command line. Usually you reference them by putting a special symbol in front of or around the variable name. By convention, environment variable names are in UPPER CASE.

Examples:

$HOME Your home directory
$PWD This variable points to your current directory
$LD_LIBRARY_PATH a colon-separated list of directories that the dynamic linker should search for shared objects before searching in any other directories
$OMP_NUM_THREADS Number of OpenMP threads
$PYTHONPATH Path to the directory where your Python libraries and packages are installed

To see the content of an environment variable named ENVIRONMENT-VARIABLE

echo $ENVIRONMENT-VARIABLE

Tip

You will get a long list of all environment variables currently set with the command:

env

You could also use

printenv

In their default state they work the same. The difference is twofold.

You can use printenv to request the value of individual variables:
```
$ printenv VARIABLE
```
You can use env to modify the environment that programs run in by passing a set of variable definitions into a command like this:
```
$ env VARIABLE="value" <command_to_run> <command_options> 
```

Some environment variables need to be exported in order to be used

This is how you set the environment variable VARIABLE to value:

For the bash (and related) shells:

export VARIABLE=value

For csh and related shells:

setenv VARIABLE value

You can create your own variables to use, for instance in scripts.

Creating your own variable

I create a variable called MINE and set it to /usr/bin/gcc

export MINE=/usr/bin/gcc

Check it is set:

echo $MINE

You can now (until you start a new session) use $MINE instead of gcc:

$ $MINE hello.c
$ ls a.out 
a.out
$ ./a.out 
Hello World!
$

Examples¶

See the content of $PATH

$ echo $PATH

Setting the number of OpenMP threads to 8 in bash

export OMP_NUM_THREADS=8

!! note “More useful example of creating your own variable”

Assume you have a script you run in a different directory than where you have the datafiles. You could then use an environment variable to give the path to the data directory instead of each time writing the full path:

```bash 
$ export DATAPATH=/home/bbrydsoe/project/dataset1/
```

Then you can refer to that directory with $DATAPATH in your script.

Adding a new path to $LD_LIBRARY_PATH

export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/your/custom/path/

Warning

The environment variable set this way only retains the value you have set for the duration of the session. When you open a new terminal window or login again, you need to set it again.

To avoid that, add the environment variable to your .bashrc file, but only do so if it should truly be persistent across many sessions (like adding a new directory to search to LD_LIBRARY_PATH for instance).

Quickly add a new directory to LD_LIBRARY_PATH in your .bashrc

echo "export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/your/custom/path/" >> ~/.bashrc

Change /your/custom/path/ to the actual path to the directory for your library.

Exercise¶

Run env to see the environment variables that are set at your computer
Use echo to see the content of $PWD - try change to another directory and run it again.
Create your own variable. Set it to something (export) and use echo to see that it has the right value.
Use printenv to tell you the value of an environment variable.

Summary¶

Keypoints

environment variables are used to store data that is used by the operating system and other programs
Some common environment variables are:
- $HOME Your home directory
- $PWD This variable points to your current directory
- $LD_LIBRARY_PATH a colon-separated list of directories that the dynamic linker should search for shared objects before searching in any other directories
- $OMP_NUM_THREADS Number of OpenMP threads
- $PYTHONPATH Path to the directory where your Python libraries and packages are installed
You can create own environment variables
env is useful
printenv is useful