What are environment variables?

In Linux based operating systems, environment variables are dynamic named values stored within the system which are used by shells or subshells (your terminal) to facilitate functionality. Simply put, they are variables with a name and value which perform a function in how the operating system and applications work.

These variables have a simple format:

KEY=value
KEY="Some other value"
KEY=value1:value2

Important

  • The variable names are case sensitive and by convention they are UPPER CASE.

  • If a variable has multiple values they should be separated by a colon :.

  • Variables do not have spaces around the equals = sign.

Note that environment variables are variables that are available system-wide and are inherited by all spawned child processes and shells where shell variables are variables that apply only to the current shell instance. Each shell such as bash (the default on the clusters), has its own set of internal shell variables.

Listing environment variables

  • env – This command allows you to run another program in a custom environment without modifying the current one. When used without an argument it will print a list of the current environment variables.

  • printenv – This command prints all or the specified environment variables.

  • echo $MYVARIABLE - The command echo when supplied with a variable name prefixed with $ will print that variable. An alternative syntax would be echo ${MYVARIABLE}. Variables can also be utilized in bash scripts in this manner.

Setting environment variables

Manually setting environment variables is trivial and can be accomplished with the commands below.

  • set – The command sets or unsets shell variables. When used without an argument it will print a list of all variables including environment and shell variables, and shell functions.

  • unset – The command deletes shell and environment variables.

  • export – The command sets environment variables.

Caution

Setting or changing environment variables can lead to a corrupted shell environment which can leave you unable to login or run programs. Manually changing values should be avoided in favour of using the modules system.

If you find your shell environment is behaving oddly, programs are no longer available and you suspect you may have corrupted your current shell environment by changing environment variables in the terminal you can simply log out and log back in to clear the problem.