Environmental and shell variables

Important!

To understand the difference between environmental and shell variables know that:

  • You can spawn child shells from the parent shell that is initiated when you first open the terminal. To do this just run bash or zsh .
  • This is a self-contained new instance of the shell. This means:
    • It will have access to environmental variables (since they belong to the parent / are global)
    • It will not have access to any shell variables that are defined in the parent.
  • How do you get back to the ‘upper’ parent shell? Type exit .
  • Note that:
    • Custom (user-created) shell variables do not pass down to spawned shell instances, nor do they pass up to the parent
    • Custom (user-created) environment variables do pass down to spawned shell instances but do not pass up to the parent. They are lost on exit .

Q. What methods are there for keeping track of, preserving, and jumping between spawned instances? Is this even possible or do they die on exit .

Questions, research

  1. If you create a variable manually I guess it won’t make it to any config file. How would you create a persistent var that is added to the .bashrc and thus which would be initialised on every session? Is this where the path comes in?
  2. What methods are there for keeping track of, preserving, and jumping between spawned instances? Is this even possible or do they die on exit ?

What is the shell environment and what are environment variables?

  • Every time that you interact with the shell you do so within an environment. This is the context within which you are working and it determines your access to resources and the behaviour that is permitted.

  • The environment is an area that the shell builds every time that it starts a session. It contains variables that define system properties.

  • Every time a Shell-sessions-e6dd743dec1d4fe3b1ee672c8f9731f6 spawns, a process takes place to gather and compile information that should be available to the shell process and its child processes. It obtains the data for these settings from a variety of different files and settings on the system.

  • The environment is represented by strings comprising key-value pairs. For example:

    KEY=value1:value2
KEY="value with spaces":"another value with spaces"

    As the above shows, a key can have multiple related values. Each one is demarcated with a : . If the value is longer than a single word, quotation marks are used.

  • The keys are variables. They come in two types: environmental variables and shell variables:

    • Environmental variables are much more permanent and pertain to things like the user and his path (the overall session)
    • Shell variables are more changeable for instance the current working directory (the current program instance)

Variables can be created via config files that run on the initialisation of the session or manually created via the command line in the current session

What are shell variables useful for?

Some deployment mechanisms rely on environmental variables to configure authentication information. This is useful because it does not require keeping these in files that may be seen by outside parties.

More generally they are used for when you will need to read or alter the environment of your system.

Viewing shell and environmental variables

To view the settings of your current environment you can execute the env command which returns a list of the key-value pairs introduced above. Here are some of the more intelligible variables that are returned when I run this command:

SHELL=/usr/bin/zsh
DESKTOP_SESSION=plasma
HOME=/home/thomas
USER=thomas
PWD=/home/thomas/repos/bash-scripting
PATH=/home/thomas/.nvm/versions/node/v16.8.0/bin:/home/thomas/.local/bin:/usr/local/sbin:/usr/local/bin:/usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/var/lib/snapd/snap/bin

However if you want to target a specific viable you need to invoke printenv with the relevant key, for example:

printenv SHELL
# SHELL=/usr/bin/zsh

Note that env and printenv do not show all the shell variables, only a selection. To view all the shell variables along with the environmental variables use set .

Creating, exporting and deleting variable shell and environment variables

  • You set shell variables using the same syntax you would within a script file:

    TEST_SHELL_VAR="This is a test"
set | grep TEST_SH
TEST_SHELL_VAR='This is a test'

# We can also print it with an echo, again exactly as we would with a shell script
echo S{TEST_SHELL_VAR}

  • We can verify that it is not an environmental variable based on the fact that following does not return anything:

    printenv | grep TEST-SH

  • We can verify that this is a shell variable by spawning a new shell and calling it. Nothing will be returned from the child shell.

  • You can upgrade a shell variable to an environment variable with export :

    export TEST_SHELL_VAR
# And confirm:
printenv | grep TEST_SH
TEST_SHELL_VAR='This is a test'

  • We can use the same syntax to create new environment variables from scratch:

    export NEW_ENV_VAR="A new var"

Using config files to create variables

You can also add variables to config files that run on login such as your user .bashrc / .zshrc . This is obviously best for when you want the variables to persist and be accessible within every Shell-sessions-e6dd743dec1d4fe3b1ee672c8f9731f6.

Important environmental and shell variables

PATH

A list of directories that the system will check when looking for commands. When a user types in a command, the system will check directories in this order for the executable.

echo ${PATH}
# /home/thomas/.nvm/versions/node/v16.8.0/bin:/home/thomas/.local/bin:/usr/local/sbin:/usr/local/bin:
# /usr/bin:/usr/bin/site_perl:/usr/bin/vendor_perl:/usr/bin/core_perl:/var/lib/snapd/snap/bin

For example, if you wish to use npm commands globally (in any directory) you will need to have the requisite Node executable in your path, which you can see above.

SHELL

This describes the shell that will be interpreting any commands you type in. In most cases, this will be bash by default, but other values can be set if you prefer other options.

echo ${SHELL}
# /usr/bin/zsh

USER

The current logged in user.

echo ${USER}
# thomas

PWD

The current working directory.

echo ${PWD}
# /home/thomas
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