Java Reading Environment Variable - System.getEnv() Examples

1. Introduction 

In this tutorial, We will learn how to read System environment variables programmatically in java. We do set the PATH or JAVA_HOME values in system-level and now we want to retrieve them in java for further usage.

Java API has System class and has two methods to work with environment variables. The following are the methods and both are defined as static methods so which can be accessed directly with the class name.

getEnv(String keyName)
getEnv()

Example:

String value = System.getEnv("PATH");

2. Java Program to read PATH, JAVA_HOME and TEMP env variables

In the following program, we are going to retrieve PATH, JAVA_HOME, Temporary directory and SHELL from mac os env variables. Just need to call the getenv() method and pass the env variable name to it. All these env vars are OS-dependent.

package com.java.w3schools.blog.java.program.to.system.env;

/**
 * 
 * java Program to read system environment variables in Java
 * 
 * @author JavaProgramTo.com
 *
 */
public class ReadEnvVar {

 public static void main(String[] args) {

  String shellValue = System.getenv("SHELL");
  System.out.println("SHELL value : " + shellValue);

  String tempDir = System.getenv("TMPDIR");
  System.out.println("TMPDIR value : " + tempDir);

  String pathValue = System.getenv("PATH");
  System.out.println("PATH value : " + pathValue);

  String JavaHomeValue = System.getenv("JAVA_HOME");
  System.out.println("Java_home value : " + JavaHomeValue);

 }

}

Output:

SHELL value : /bin/bash
TMPDIR value : /var/folders/g0/0x5m1w1950z18h3m6z6h1_000000gn/T/
PATH value : /usr/bin:/bin:/usr/sbin:/sbin
Java_home value : /usr/bin/java

3. Reading all Env Variables in Java

Along with the getting value for a specific env var, System class has another method to fetch all keys and values in the OS. All are retrieved with System.getEnv() method. getEnv() method returns a Map and the iterating map will return key/value pairs.

package com.java.w3schools.blog.java.program.to.system.env;

import java.util.Map;

public class ReadAllEnv {

 public static void main(String[] args) {

  Map env = System.getenv();

  for (String key : env.keySet()) {

   String value = env.get(key);
   System.out.println(key + " : " + value);
  }

 }

}

Output:

PATH : /usr/bin:/bin:/usr/sbin:/sbin
SHELL : /bin/bash
SQLITE_EXEMPT_PATH_FROM_VNODE_GUARDS : /Users/javaprogramto/Library/WebKit/Databases
SECURITYSESSIONID : 186aa
USER : javaprogramto
APP_ICON_340 : ../Resources/Eclipse.icns
COMMAND_MODE : unix2003
TMPDIR : /var/folders/g0/0x5m1w1950z18h3m6z6h1_000000gn/T/
SSH_AUTH_SOCK : /private/tmp/com.apple.launchd.M0QVWzoZa9/Listeners
JAVA_MAIN_CLASS_24103 : com.java.w3schools.blog.java.program.to.system.env.ReadAllEnv
XPC_FLAGS : 0x0
__CF_USER_TEXT_ENCODING : 0x1F5:0x0:0x0
Apple_PubSub_Socket_Render : /private/tmp/com.apple.launchd.m7YJcjjT4h/Render
JAVA_STARTED_ON_FIRST_THREAD_340 : 1
LOGNAME : javaprogramto
XPC_SERVICE_NAME : com.apple.xpc.launchd.oneshot.0x10000003.eclipse
HOME : /Users/javaprogramto

5. Using Java 8 forEach

The above program can be iterated using Java 8 API forEach() method and will produce the same output as the above program.

import java.util.Map;

public class ReadAllEnvJava8 {

 public static void main(String[] args) {

  Map env = System.getenv();

  // using java 8 forEach
  env.forEach((key, value) -> System.out.println(key + " : " + value));
 }

}

5. Conclusion

In this short article, We have seen how to read environment variable value programmatically in java. System class has two utility methods getEnv(String key) which returns the corresponding value and getEnv() method returns all key, value pairs from OS.

Shown the example programs on these two methods.