Environment Variables

Simply put, environment variables are a collection of key value strings held by the current shell or process. Each process has access to an array of environment variables held in its user-space memory.

The environment variables are available to all applications, and allow these programs or scripts use them within the shell environment. They can allow custom modification to defaults that permeate throughout the user-space rather than be homed within each applications configuration.

Each process that is created via fork(), will inherit a copy of its parent’s environment. This ensures the communication and transfer of variables from the parent is a once-only one-way transaction. This is important to prevent updated environment variables from one child polluting the global user-space should it go wrong.

PATH

Each environment will have access to several default or standard variables such as:

• PATH
• HOME
• LOGNAME
• SHELL
• EDITOR
• MAIL

By typing echo $HOME the users home location will be printed to the terminal. Environmental variables are case sensitive, and by convention the key is written in capitals.

echoing $PATH on a unix platform will output a : separated list of file paths such as:

/usr/local/bin:/usr/local/sbin:/usr/bin

The PATH is set by system and user start-up scripts - the exact process can change between implementations. Its function is to search in the PATH locations for the name of any program being called for execution. In linux everything is file, and when a file is set with the execution bit, that file can be called with the exec() method. What the PATH does is search recursively through each location, from left to right as its printed, looking for that file if its not given with an absolute path.

Example

$ which echo
/bin/echo
$ ls -l /bin/echo
-rwxr-xr-x 1 root root 30k Dec 29 2017 /bin/echo
$ echo $PATH
/home/userName/bin:/usr/local/bin:/usr/bin:/bin  # /bin in PATH

Above we see that calling which echo returns the location of echo (in my distro it returns echo: shell built-in command but /bin/echo will call the file nonetheless). Now when we call echo which is not an absolute path, the PATH variable is consulted. Moving left to right the system checks for an executable file named echo, which is finds inside /bin. Once found it executes and ceases the search. If we called /bin/echo the PATH would be searched.

Example Two

In a real world case, I use Go’s /bin location as an exemplar of why you may need to add to the PATH variable.

Dave Cheney has a pretty sweet Go knock off of an awesome package by the same name httpstat. The TL;DR is that it gives you statistic on a site by simply calling httpstat https://reorx.com. Astute readers will see that this requires httpstat to be on the PATH or it will generate an error.

Fixing this was simple as we see below.

Creation & Deletion

Environment variables are key value pairs, case sensitive and by convention have the key written in caps.

Creation

Adding to your current shell environments list of variables is simple.

# method 1
$ VAR=foo       # create key:value
$ export VAR    # export is called on VAR
$ echo $VAR     # echo VAR to terminal
foo

# method 2
$ export VAR=foo # combine export and variable assignment
$ echo $VAR
foo

Bourne shells accept the second method, and its probably the most common and convenient method. export is unix command to set the attribute for a variable. See man export for more information.

Deletion

$ unset VAR
$ echo $VAR
# will echo empty string

Continuing with the httpstat example we will append the location of Go’s package binaries to our PATH.

$ export PATH=~/go/bin:$PATH # now added to PATH

Once the variable is exported, it will only live within the current shell. If we close it down, that updated PATH is now lost.

To make the variables persistent we add them to our shell’s .profile or .bashrc if using bash. In my case, I use Zsh so its added to .zshrc. Calling source .zshrc will reload .zshrc and make the new variables available if not already. And, because each process inherits form the parent, removing it form the file will prevent it being added to the environment list if needed in the future.

Security tidbits

Of note linux implements a security measure within the Superusers PATH. The current working directory is normally omitted from the PATH meaning files must be called using ./ as the prefix or as an absolute path. This prevents malicious users from placing an executable file with the same name as a system-wide executable such as ls being run as root.

Conclusion

• Environment variables allows the system to access a list of values for use in applications
• Users can set and unset variables
• PATH is important when you need to access an executable without calling its absolute path