Develop Reference

Maven set up required everytime i close the terminal

i'm very new to this world.. so forgive me for silly explanations.
I've installed maven and all the setting is done. Also i created a maven.sh
Content:
# Apache Maven Environment Variables
# MAVEN_HOME for Maven 1 - M2_HOME for Maven 2
export JAVA_HOME=/usr/lib/jvm/java-11
export M2_HOME=/opt/apache-maven
export MAVEN_HOME=/opt/apache-maven
export PATH=${M2_HOME}/bin:${PATH}
At this point everything is working as expected, but every time i close the terminal it seems to forget some settings..
I have to open the terminal and execute the following commands to make it work again:
~$ cd /etc/profile.d
~$ source maven.sh
I don't know if it is just the way to use maven or i'm missing something, but it's kind of annoying writting the same commands every single time i want to execute maven.. so i would appreciate if you can explain me a way to automate it.
Some extra information:
OS : Ubuntu 20.0.4
JDK : 11
Not first time creating JAVA_HOME variable
After executing those commands above, it shows the correct JAVA_HOME, but it's forgotten after closing the terminal and shows the wrong JAVA_HOME again
Let me tell you guys, this is my first question on stackoverflow, so i will understand if you want to correct me about my manners and my bad english (it's not my mother tongue, i have to get used to it)
Thank you in advance!

I can think of a couple of ways you could resolve this problem given your situation, which is not maven-specific but rather terminal-specific --
Add a reference to your maven.sh script to your profile so that it runs when you start a terminal session. This can typically be accomplished by creating (or adding to) the .bash_profile file in your home directory. You could either add the contents of your maven.sh script to that file, or add source /etc/profile.d/maven.sh to it. From that point forward when you start a terminal session, the script gets run automatically and your variables are set correctly. See https://joshstaiger.org/archives/2005/07/bash_profile_vs.html or the bash man page for some more details about the login shell
If you've already done the first step and it didn't work, consider checking to see if those environment variables are being set in another place (check ~/.bashrc, ~/.bash_profile, and anywhere your particular shell distribution / OS checks for startup scripts
Lastly, I recommend using SDKMan! to manage maven installs without these headaches -- but your mileage may vary. It handles keeping track of the environment variables, java version, &c and allows multiple Java/Maven versions to be installed and managed. https://sdkman.io
Sources:
https://joshstaiger.org/archives/2005/07/bash_profile_vs.html
https://linux.die.net/man/1/bash
https://sdkman.io

You can add these properties to individual user settings file. This file will be available in home directory
vi $HOME/.bash_profile
So everytime, you login to terminal, all these settings will be executed.

I solved the problem by doing a logout and login in Ubuntu

Setting my JAVA_HOME variable for Hadoop

I have tried setting my JAVA_HOME variable for Hadoop to no avail.
My Java runtime is in /usr/bin/java, but Hadoop keeps trying to access it in /usr/bin/java/bin/java.
I have set the JAVA_HOME in my .bash_profile and in bin/hadoop, but neither works. I am guessing that some other path setter is overriding this. Is this a known problem and what can be done about it?
A couple of other things I tried:
echo $JAVA_HOME from the hadoop script
add $JAVA_HOME to the conf/hadoop-env.sh

In your case:
JAVA_HOME = /usr
The code assumes that the executable is located at:
$JAVA_HOME/bin/java

It's unlikely your Java installation is in /usr/bin/java.
The executable may be there, or may just be a link to the executable in your installation.
IMO it's cleaner to use your actual Java JDK/JRE directory and use that instead: that's what JAVA_HOME is supposed to be.
Using Hadoop is quite a bit more complicated than setting an environment variable correctly; you may be in for a rough road. Consider something like Kiji's Bento Box to ease initial setup.

System.getenv doesn't get variables defined in ~/.bash_profile

Here is a line in file ~/.bash_profile
export MESSAGE="Hello World"
I want to access system variable MESSAGE in java.
System.getenv("MESSAGE"); doesn't work.

The .bash_profile file is only sourced for login shells. If your java process is spawned from a shell that is not a login shell (for example a script with a #!/bin/sh at the top) then it will not read it (though it may still inherit MESSAGE from the environment depending on how you run it).
Note also that .bash_profile is also not run for interactive shells that are not "login" shells, so you can't rely on it being run even when you have a shell prompt. People usually use .bashrc, which is sourced for all interactive shells, for that purpose.
If you want a variable to be set in all Bourne shell derivatives regardless of whether they are interactive or not, put it in both .profile and .bashrc.

I would recommend you to test if your environment variable is indeed "defined" by using echo $MESSAGE as suggested above.
In addition, changing the bash_profile file will not effect your current shell, you must use "source" in order for it to effect your current shell.
I would also recommend reading this article about differences between bashrc and bash_profile.
Maybe you should define the EXPORT at bashrc?

This actually gets even more interesting for users who have a .profile (for the old Bourne shell), that gets read in by .bash_profile automatically (providing compatability). In either case, the environment variables are read in once only at login shell startup, and all subshells inherit those variables for free. The .bashrc is for tty-dependent things and unheritables like functions (the old sh used $ENV, I think, if it were set, for something similar).
Your use of ~/.bash_profile looks fine (although single quotes are more reliable than double quotes, which allow some substitutions). Make sure you've logged out and back in between editing that file and trying your test, or use ". ~/.bash_profile" (no quotes and note the leading dot and space, since the dot is the command here).
The article at http://www.joshstaiger.org/archives/2005/07/bash_profile_vs.html covers some good things, like using ". ~/.bashrc" at then end of your ~/.bash_profile (excepth that you should use -r, not -f). The comment about using export in your .bashrc is wrong, you should not do this, for two reasons (1) an pretty insignificant performance penalty, (2) a pretty high chance that some command you execute won't get the environment variable - particularly things spawned from your window manager menus and other places where an actual command prompt didn't appear in any of their parents.
Lastly, make sure the $MESSAGE actually exists in your environment - look at the output of the "env" command - if it isn't there, the Java process won't see it unless it's internally creating it and storing it in its own internal copy of the environment variables.
Another note, if you set env variables in the .profile, use this syntax:
VAR=VALUE ; export VAR
...since the old sh shell doesn't support "export VAR=VALUE". Using set -e before a bunch of these and set +e after can remove the need to use "export" at all, if I remember correctly.

Another place to look at: /etc/environment (this may overrule/replace your .bashrc or .bash_profile in shells opened via your IDE)

Variables in your bash profile only get used in the terminal session you are in, so IntelliJ won't pick them up.
Instead, go to Edit Configurations in your Run Configurations box, and add your environment variables in a semi-colon separated list in the environment variables box!

Setting environment variable permanently from java

Using Java,
How can I add environment variable permanently to the existing env variables.
so that when I do a restart operation for windows or Linux, this environment variable is still there.

You might want to take a look at this.
In Windows you can set a Path Variable from command line so it should do the trick.
I realize this is only applicable to Windows.

Not in any cross platform sort of way. In Linux, these are typically controlled via shell init scripts. You would have to edit one of those (which one depends on the user, system, and shell type). In Windows, this is controlled via system configuration (i'd imagine there are some windows specific APIs to modify those).

coppy the path of jdk upto C:\Program Files\Java\jdk1.6.0\bin from program and past in user variables and put ;.; at the end and give name .
and in system variables click on new and enter the name and past the path....and save ...

go to command prompt
..
to check current paths >echo %path%
to set path >set path="C:\Program Files\Java\jdk1.6.0\bin" enter ok now check and run java program

Environment variables are platform specific. Windows stores them in Registry.
*In the registry the User environment variables are stored at
HKEY_CURRENT_USER\Environment
and the System environment variables are stored at
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment
(from http://demins.blogspot.co.il/2007/10/where-does-windows-xp-store-evrironment.html)*
There are a lot of ways to access windows registry from java. You can for example execute command line using utility named reg that has a reach command line. You can also use one of interoparability APIs like JaWin, Jinterop, Jintegra. You can also refer to my solution explained here.
On linux you can use command line like export MYVAR=myvalue. I mean execute this command line from java using Runtime.exec() or ProcessBuilder. The problem is that this variable will not become really persistent. It will be visiable for all users until the computers is restarted. To make it really persistent you have to modify user login script (e.g. bashrc file for most linux systems if users's default shell is bash).

Java JDK Installation

OK, what am I doing wrong, this is driving me nuts.
I am trying to install the latest JDK (1.6.0_23). So, I downloaded it from Oracle's awful site and then ran the installation. I installed it to C:\Java\jdk1.6.0_23
Then, I created a JAVA_HOME User Variable that pointed to C:\Java\jdk1.6.0_23. I then added a piece to the end of my Path environment variable that says %JAVA_HOME%\bin.
However, when I try to simply open a command prompt and run simple java commands, I am told it is not a recognizable command. I have to manually cd into the that bin directory to do anything.
Do I also need a Classpath variable that points to the JRE? I noticed there was a Classpath variable there previously that pointed to jre/lib/QTJava.zip, but I deleted it.
If you have the JDK installed and a JAVA_HOME variable setup, do I still need the JRE in the classpath? I am running Windows 7 and do all of my development in Eclipse.

Maybe the problem is because you set JAVA_HOME as a user variable, but trying to reference it from the PATH which is a system variable (or is it?). You cannot do this, because system variables are evaluated before user variables.
There are two possible solutions:
1. Set JAVA_HOME as a system variable instead
2. Create a new user variable PATH and set %JAVA_HOME%\bin there. The user PATH and the system PATH variables will be concatenated at runtime automatically.

From http://social.answers.microsoft.com/Forums/en-US/vistainstall/thread/48b23109-9fbc-47c5-a5d1-465773f94704
(at the end)
1) Enable 'delayed variable expansion'
in the registry (see
http://batcheero.blogspot.com/2007/06/how-to-enabledelayedexpansion.html)
2) Change the '%' signs around var2 to
'!', e.g. "%var2%" becomes "!var2!"
I've done some limited testing on
Windows 7 and this appears to fix the
problem.
Maybe try that, see if it fixes it (I don't have windows here to try)

Do I also need a CLASSPATH variable that points to the JRE?
Strictly speaking, no. The CLASSPATH variable may be used if you try to run a java class and you don't use the -cp or -jar options.
The CLASSPATH variable doesn't need to point to the JRE. The java.exe command etc all know where to find the JRE's runtime classes. (And they don't look on the CLASSPATH for them anyway.)
For the PATH problem, try running:
C:\Java\jdk1.6.0_23\bin\java.exe -version
If that doesn't work then there's a problem with your actual installation. If it does work, try looking at what JAVA_HOME and PATH are set to in the environment variables of the command shell.