Environment variables in the shell

Written Explanation

Recap: what an environment variable is

An environment variable is a name and a text value, such as COURSE with the value java. Microsoft's PowerShell documentation describes them as always strings, and as inherited by child processes: when a program starts another program, the child receives a copy of the parent's variables at that moment.

You do not have to remember the technical words. Remember the effect: set it in a terminal, and programs started from that terminal can see it.

Reading a variable

In Bash, the $ in front of a name means "the value of". In PowerShell, $env: is the prefix for environment variables. If the variable does not exist, Bash prints an empty line and PowerShell prints nothing.

Variable names are case-sensitive in Bash and, according to Microsoft, in PowerShell on macOS and Linux, so PATH and path differ there. On Windows they do not. Convention uses capitals for environment variables, and you should too.

Setting a variable

Bash

$ export COURSE=java
$ echo $COURSE
java

Rules: no spaces around =. export COURSE = java fails. Quote a value that contains spaces: export TITLE="Java for backend".

PowerShell

PS> $env:COURSE = "java"
PS> $env:COURSE
java

In PowerShell, spaces around = are fine and quotes are normal.

What export does in Bash

In Bash, you can create a plain shell variable with COURSE=java (no export). Your shell can use it, but programs it starts will not see it. Adding export marks the variable to be passed on to child processes, making it an environment variable.

You can prove it with a child shell:

$ COURSE=java
$ bash -c 'echo "child sees: $COURSE"'
child sees:
$ export COURSE
$ bash -c 'echo "child sees: $COURSE"'
child sees: java

The single quotes matter. They stop your current shell from expanding $COURSE itself, so the child does the reading. This behavior is long-standing Bash practice; the manual page I read mentions that export assigns an attribute to variables, but I did not verify the child-process detail in a source in this session, so try the demo on your own computer.

PowerShell has no separate step: $env:COURSE = "java" writes straight into the environment, so child programs started from that session see it. You can check with a child PowerShell (the program is pwsh for PowerShell 7 and powershell for Windows PowerShell):

PS> pwsh -NoProfile -Command '$env:COURSE'
java

Single quotes keep $env:COURSE for the child to evaluate.

Listing and removing variables

• Bash: printenv lists all environment variables. printenv COURSE prints one value. unset COURSE removes the variable from the current shell.
• PowerShell: Get-ChildItem Env: lists them all, and Get-ChildItem Env:COURSE shows one. Remove one with Remove-Item Env:COURSE. Microsoft's page also says that from PowerShell 7.5 on, assigning $null ($env:COURSE = $null) removes it.

Output can be long. Pipe it into a filter from Lesson 4: printenv | grep COURSE, or Get-ChildItem Env: | Select-String COURSE.

How long does it last?

Only until you close that terminal, or more exactly until that shell session ends. Microsoft's documentation states that changes you make in PowerShell affect only the current session. In both shells, a variable set this way is invisible in a different terminal window, and gone when you reopen.

The reason is that each shell has its own copy. A new terminal gets its variables fresh from the system and your configuration files, not from the terminal you closed.

Making a variable permanent

Do not do it yet, but know where it lives:

• Bash and zsh: a line such as export COURSE=java in a startup file such as ~/.bashrc or ~/.zshrc. Microsoft's page mentions those two files for non-login shells. Which file applies depends on the shell and how it was started, so check your system's documentation.
• Windows: the Environment Variables dialog (reached through the system settings), which stores a value for your user or for the whole machine. PowerShell can also write a user-level value with [Environment]::SetEnvironmentVariable('COURSE','java','User'). Microsoft's page documents that method and the scopes. After a permanent change, open a new terminal to see it, exactly as in Module 5.

Adding a folder to PATH for one session

PATH is a list of folders separated by : in Bash and on macOS and Linux, and by ; on Windows (Microsoft's page confirms both). To add a folder without losing the existing ones, put the old value inside the new one:

$ export PATH="$HOME/tools:$PATH"
PS> $env:PATH += ";C:\Tools"

The Bash line puts ~/tools first and keeps everything else. The PowerShell line appends C:\Tools to the end, as in Microsoft's own example; on macOS or Linux use : instead of ;. The classic error is replacing PATH (export PATH=$HOME/tools), which removes everything else. Almost every command then stops being found, until you close the terminal.

Quoting and expansion in Bash

• echo "$COURSE" and echo $COURSE print the value. Double quotes allow expansion.
• echo '$COURSE' prints the literal text $COURSE. Single quotes block expansion.

In PowerShell, double-quoted strings also expand variables ("Course: $env:COURSE"), and single-quoted strings do not.