*NIX – does exporting a variable only last while machine is on

bash

I know that I can set a variable, 'export' the variables and put an alias statement in my .bash_rc file.

So:

When I just set a variable it just lasts for that session and is not visible to other sessions, correct?
Then when I export it it it is visible to other shell sessions (is that only to other 'new' sessions though?
Can existing sessions reload or soemthing to get it?)
Most critically does that export 'stay' through a reboot?
Or is that when the .bashrc file comes in for 'really' permanent setting of variables going forward?
Finally any known differences between Ubuntu and Mac on this?

Best Answer

You are correct about .bashrc. On the initial boot, the OS knows nothing of environment variables except what it reads in .profilerc, .bashrc, .inputrc, etc. And anything exported only lasts for the current session unless placed in one of those files.

If values are added to one of these files and you want it to take effect immediately, run:

source .bashrc

or:

. /etc/bashrc

etc...

Edit (adding from @ThomasAndrews comments):

However, this only changes the env variables for the current session (terminal window) in which you execute it. The key to understanding is to realize that environment variables are inherited from parent processes to child process at the time the child process starts.

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Tmux Commands – Execute Command in Session on Startup

Thanks to JohnKiller's suggestion, I realized that .bashrc executes both when a new tty is opened, and when TMUX is opened in a terminal.

For future readers: The $TMUX variable is typically referenced to see if TMUX is running at all, but you could also use "pidof tmux". The $TMUX variable will be populated with something like: "/tmp/tmux-0/default,27389,0" if TMUX is running.

In my particular case, I'm running CentOS 6, and have it set to auto-login with root since it's just a test image. I was able to do that by editing /etc/init/tty.conf:

exec /bin/mingetty --autologin root $TTY

Now that it's set to auto-login, I added the following to /root/.bashrc:

if [[ `tty` == "/dev/tty1" ]] && [[ -z "$TMUX" ]];then
        tmux
fi

For newbies reading this, this says "If my terminal is terminal 1, and the $TMUX variable is zero-length, run tmux".

It's followed by:

if [[ -n "$TMUX" ]] && [[ ! -e "/root/.automatic_start_occurred" ]];then
        touch /root/.automatic_start_occurred
        /usr/bin/hello_world
fi

Again for newbies, this says "If $TMUX is non-zero in length, and the file ".automatic_start_occurred" does not exist (the "!" in the if statement), make the file "/root/.automatic_start_occurred" and then execute "hello_world" in /usr/bin.

This is exactly what I was looking for my system to do - After booting, TTY1 will pop up with TMUX, and the other TTYs will be left alone. When TMUX pops up for the first time, it will execute some arbitrary commands, and never do them again unless the file ".automatic_startup_occurred" is removed.

Best Answer

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