Environmental Variables

Environment variables are a fundamental concept in computer science and software development. They are essentially dynamic values that can affect the behavior of software and operating systems. Environment variables store configuration settings, system paths, and other essential information that programs and scripts can access.

Here are some key points about environment variables:

What They Are:

Environment variables are key-value pairs that store information at the system level. They are often used to configure software, define system-wide settings, or provide data that programs need to function correctly. These variables are accessible to all processes running on a system.

How to Use Them in Bash (Linux/macOS):

To view the value of an environment variable, use the echo command followed by the variable name preceded by a dollar sign (e.g., echo $HOME).

To set or modify an environment variable for the current session, use the export command (e.g., export MY_VARIABLE="some value").

To make an environment variable persist across sessions, you can add the export command to your shell profile file (e.g., .bashrc or .bash_profile).

How to Use Them in PowerShell (Windows):

To view the value of an environment variable, use the echo command with the variable enclosed in percentage signs (e.g., echo %USERPROFILE%).

To set or modify an environment variable for the current session, use the $env: prefix (e.g., $env:MY_VARIABLE = "some value").

To make an environment variable persist across sessions, you can use the “System Properties” settings in the Control Panel or modify the Windows Registry.

.env Files:

A .env file is a plain text configuration file used to store environment variables. These files are often used in web development projects to define variables like database connection strings, API keys, and other sensitive information.

Each line in a .env file typically consists of a variable name and its associated value (e.g., DB_USERNAME=johndoe).

Tools like dotenv in JavaScript and libraries in other languages can load variables from .env files into the environment.

Environment variables are versatile and widely used in various contexts, from system configuration to application development. They provide a way to separate configuration from code, making it easier to manage settings and secrets across different environments (e.g., development, testing, production) without hardcoding them into your scripts or applications.

Env Vars across languages

Language	Accessing Env Vars	Library for .env Files
Python	os.environ.get('VAR_NAME')	python-dotenv
JavaScript	process.env.VAR_NAME	dotenv
Ruby	ENV['VAR_NAME']	dotenv
PHP	getenv('VAR_NAME')	vlucas/phpdotenv
Go	os.Getenv("VAR_NAME")	github.com/joho/godotenv
Java	System.getenv("VAR_NAME")	N/A
C# / .NET	Environment.GetEnvironmentVariable("VAR_NAME")	N/A
Swift	ProcessInfo.processInfo.environment["VAR_NAME"]	N/A
Rust	std::env::var("VAR_NAME")	N/A
Perl	$ENV{'VAR_NAME'}	N/A
Bash	$VAR_NAME	N/A
PowerShell	$env:VAR_NAME	N/A
Lua	os.getenv("VAR_NAME")	N/A
Kotlin	System.getenv("VAR_NAME")	N/A
TypeScript	process.env.VAR_NAME	dotenv

Commonly Used Environmental Variables

Here is a list of commonly used variables and whether they are typically defined by the operating system or the developer.

Variable Name	Description	Provided By
PATH	Specifies the directories where executable programs are located	OS
HOME	Specifies the user’s home directory	OS
USER	Specifies the username of the current user	OS
LOGNAME	Specifies the login name of the current user	OS
TEMP or TMPDIR	Specifies a temporary directory for storing temporary files	OS
PWD	Specifies the current working directory	OS
LANG or LANGUAGE	Specifies the current language and locale settings	OS
LD_LIBRARY_PATH	Specifies the directories where shared libraries are located	OS/Programmer
JAVA_HOME	Specifies the location of the installed Java SDK	Programmer
PYTHONPATH	Specifies additional directories to search for Python modules	Programmer
CLASSPATH	Specifies the location of Java class files	Programmer
DB_URL	Specifies the URL of a database server	Programmer
API_KEY	Specifies an API key for accessing external services	Programmer
SHELL	Specifies the path to the user’s preferred shell	OS
EDITOR	Specifies the user’s preferred text editor	OS/Programmer
MAIL	Specifies the location of the user’s mailbox	OS
DISPLAY	Specifies the X Window System display server	OS
TZ	Specifies the timezone of the system	OS
TERM	Specifies the type of terminal	OS
HTTP_PROXY	Specifies the HTTP proxy server	Programmer
HTTPS_PROXY	Specifies the HTTPS proxy server	Programmer
NO_PROXY	Specifies the domains that should bypass the proxy server	Programmer
HOSTNAME	Specifies the hostname of the system	OS
PORT	Specifies the port number for a network service	Programmer
DEBUG	Specifies whether to run the application in debug mode	Programmer
CONFIG_PATH	Specifies the path to a configuration file	Programmer
NODE_ENV	Specifies the environment in which a Node.js app is running	Programmer
DATABASE_URL	Specifies the URL of a database server	Programmer
SECRET_KEY	Specifies a secret key for encrypting data	Programmer
AWS_ACCESS_KEY_ID	Specifies the AWS access key ID	Programmer
AWS_SECRET_ACCESS_KEY	Specifies the AWS secret access key	Programmer
REDIS_URL	Specifies the URL of a Redis server	Programmer
RACK_ENV	Specifies the environment in which a Ruby app is running	Programmer
MONGO_URL	Specifies the URL of a MongoDB server	Programmer
GOPATH	Specifies the workspace directory for Go projects	Programmer
ANDROID_HOME	Specifies the location of the Android SDK	Programmer
SDKROOT	Specifies the root directory of the SDK being used	Programmer
PGDATA	Specifies the location of the PostgreSQL data directory	Programmer
NPM_CONFIG_PREFIX	Prefix configuration where npm will find/keep packages	Programmer
NVM_DIR	NVM (Node Version Manager) directory path	Programmer
RBENV_ROOT	Directory path for rbenv (Ruby Version Manager)	Programmer
RUBYOPT	Specifies additional options to Ruby interpreter	Programmer
BUNDLE_PATH	Path where Bundler installs your gems	Programmer
RAILS_ENV	Specifies the environment in which Rails app is running	Programmer
GEM_HOME	The directory where gems are installed	Programmer
GEM_PATH	Path where RubyGems looks for gems	Programmer
RUSTUP_HOME	Directory path for rustup (Rust toolchain installer)	Programmer
CARGO_HOME	Directory path for Cargo, Rust’s package manager	Programmer
LOCALAPPDATA	Path to the local app data directory on Windows	OS
APPDATA	Path to the roaming app data directory on Windows	OS
CommonProgramFiles	Path to the common program files directory on Windows	OS
ProgramFiles	Path to the program files directory on Windows	OS
SystemRoot	Path to the Windows system root directory	OS
GOPROXY	URL of a Go module proxy	Programmer
GRADLE_HOME	Specifies the location of the installed Gradle	Programmer
KUBECONFIG	Path to a kubeconfig file for access to Kubernetes	Programmer
DOCKER_HOST	Specifies the hostname or IP address of the Docker daemon	Programmer
DOCKER_CERT_PATH	Path to directory with Docker TLS certificates	Programmer
DOCKER_TLS_VERIFY	Enable or Disable TLS verification for Docker	Programmer
COMPOSE_PROJECT_NAME	Specifies a project name for Docker Compose	Programmer
VIRTUAL_ENV	Specifies the path to a Python virtual environment	Programmer
FLASK_ENV	Specifies the environment in which Flask app is running	Programmer
FLASK_APP	Specifies the name of the Flask application	Programmer
COMPOSER_HOME	Directory path for Composer, the PHP dependency manager	Programmer
COMPOSER_VENDOR_DIR	Directory path for Composer’s vendor directory	Programmer
ANT_HOME	Specifies the location of the installed Apache Ant	Programmer
MAVEN_HOME	Specifies the location of the installed Apache Maven	Programmer
TOMCAT_HOME	Specifies the location of the installed Apache Tomcat	Programmer
SPRING_PROFILES_ACTIVE	Specifies active profiles for a Spring application	Programmer
KAFKA_HOME	Specifies the location of the installed Apache Kafka	Programmer
ZOOKEEPER_HOME	Specifies the location of the installed Apache ZooKeeper	Programmer
HADOOP_HOME	Specifies the location of the installed Apache Hadoop	Programmer
HADOOP_CONF_DIR	Specifies the directory of Hadoop configuration files	Programmer
SPARK_HOME	Specifies the location of the installed Apache Spark	Programmer
PYSPARK_PYTHON	Specifies the Python binary executable for PySpark	Programmer
PYSPARK_DRIVER_PYTHON	Specifies the Python binary executable for the PySpark driver	Programmer
CASSANDRA_HOME	Specifies the location of the installed Apache Cassandra	Programmer
MYSQL_HOME	Specifies the location of the installed MySQL server	Programmer
PGHOST	Specifies the host name of the PostgreSQL server	Programmer
PGPORT	Specifies the port number of the PostgreSQL server	Programmer
PGUSER	Specifies the username for the PostgreSQL server	Programmer
PGPASSWORD	Specifies the password for the PostgreSQL server	Programmer
ANDROID_SDK_ROOT	Specifies the location of the Android SDK	Programmer
GOBIN	Specifies the directory where Go binaries are installed	Programmer
JRE_HOME	Specifies the location of the installed Java Runtime Environment	Programmer
JDK_HOME	Specifies the location of the installed Java Development Kit	Programmer
SCALA_HOME	Specifies the location of the installed Scala	Programmer
PERL5LIB	Specifies additional directories to search for Perl libraries	Programmer
R_LIBS	Specifies additional directories to search for R libraries	Programmer