[SOLVED] How can the same variable have different values at the same time???

Didier Spaier · 05-13-2022, 05:20 PM

I am trying these settings in ~/.profile

Code:

export XDG_CACHE_HOME=/run/user/$(id -u)
export XDG_RUNTIME_DIR=/run/user/$(id -u)

To test, I started 4 desktops at the same time: KDE (started with lightdm), MATE (started from tty2) XFCE (started from tty3) and LXQt (started from TTY4). I could check that /run/user/1000 was populated as expected and so far all DE seem to behave normally, but something puzzles me: typing "env|grep XDG" I see that XDG_CURRENT_DESKTOP is set respectively as KDE, MATE, XFCE and LXQT when I type the command from these desktops. Can it be that at each session opening the variable is stored at a new location in RAM, which is remembered only in this session? How does that work?

LuckyCyborg · 05-13-2022, 05:32 PM

Quote:

Originally Posted by Didier Spaier

I am trying these settings in ~/.profile

Code:

export XDG_CACHE_HOME=/run/user/$(id -u)
export XDG_RUNTIME_DIR=/run/user/$(id -u)

To test, I started 4 desktops at the same time: KDE (started with lightdm), MATE (started from tty2) XFCE (started from tty3) and LXQt (started from TTY4). I could check that /run/user/1000 was populated as expected and so far all DE seem to behave normally, but something puzzles me: typing "env|grep XDG" I see that XDG_CURRENT_DESKTOP is set respectively as KDE, MATE, XFCE and LXQT when I type the command from these desktops. Can it be that at each session opening the variable is stored at a new location in RAM, which is remembered only in this session? How does that work?

I'm not a programmer, but from what I know, I think that because on each console is spawned a specific Bash instance on login, and every instance has its own variables and data, because they are, well... executed instances of a particular program, each one being standalone, without data sharing.

Didier Spaier · 05-13-2022, 05:58 PM

Quote:

Originally Posted by LuckyCyborg

I'm not a programmer, but from what I know, I think that because on each console is spawned a specific Bash instance on login, and every instance has its own variables and data, because they are, well... executed instances of a particular program, each one being standalone, without data sharing.

Well, that makes sense. I made a simple test setting the same variable from two ttys and indeed there seem to be an isolation.

henca · 05-13-2022, 06:10 PM

Yes, that is really the explanation. Environment variables are not only unique for each shell, but can be unique for every process. This can easily be verified by doing:

md5sum /proc/*/environ

Different processes will have different environment variables set giving different md5sums.

regards Henrik

Pithium · 05-13-2022, 08:54 PM

If env variables were shared across multiple shell sessions that would make running multiple SlackBuild scripts at the same time very interesting.

Chuck56 · 05-13-2022, 09:59 PM

Bash is based on dynamic variable scoping. Most programming languages are based on static variable scoping.

Dynamic scoping is all about pushing the current environment down the stack of execution. Each stack can take on different values for variables with the same name.

ttk · 05-13-2022, 10:26 PM

Yes, what henca said.

When a new process is created, it inherits the env var values of its parent process, but keeps its own copy of them, and can change them.

If you run the command "ps auxe | less" you can see each process' environment. Be warned that it's ungainly and awkward to read.

Mark Pettit · 05-14-2022, 01:16 AM

It's called Quantum Theory.

Didier Spaier · 05-14-2022, 12:30 PM

I mark this thread as [SOLVED]

Still I will not use the settings I wrote in the initial post this thread, but rather put these lines in /etc/skel/.profile in Slint 15.0:

Code:

export XDG_RUNTIME_DIR=/run/user/$(id -u)
export XDG_CACHE_HOME=/dev/shm/$(whoami)
mkdir -p /dev/shm/$(whoami)
chmod 700 /dev/shm/$(whoami)

Rationale:

The default size of /run/user/$(id -u) is 10% of the physical RAM size, which is a bit small if you have few physical RAM to host the cached files of an user.
The default size of /dev/shm is 50% of the physical RAM.

As an aside if you have a swap in zram you can increase the latter with no issue. To test I have written this line in /etc/fstab:

Code:

tmpfs   /dev/shm   tmpfs   nosuid,nodev,noexec,size=110%  0 0

After a reboot I filled up /dev/shm copying big files in it. I was still able to run 3 DEs at the same time, with indeed several G of swap in zram as shown by df -th and zramctl but no swap on a drive needed, so systems still reasonably responsive.

dugan · 05-14-2022, 12:34 PM

Quote:

Originally Posted by Didier Spaier

typing "env|grep XDG" I see that XDG_CURRENT_DESKTOP is set respectively as KDE, MATE, XFCE and LXQT when I type the command from these desktops. How does that work?

They set it.

https://man7.org/linux/man-pages/man3/setenv.3.html

EDIT:

In the case of LxQt (I just checked), it's set in the "startlxqt" script.

Didier Spaier · 05-14-2022, 12:52 PM

Quote:

Originally Posted by dugan

They set it

Yeah, I have spent a lot of time checking that the XDG variables that matter most be correctly set in the three ways of starting a desktop session (startx, sddm, lightdm) for all provided sessions (X11 only), listed below:

Code:

didier[~]$ session-chooser
Usage: /usr/bin/session-chooser <desktop session>
Available desktop sessions:
fvwm
lxqt
mate
plasma
plasma-safe
wmaker
xfce
The session is currently set for didier to lxqt.
didier[~]$

PS: before someone asks: no, twm, mwm, blackbox and fluxbox are not proposed. But you can use them as window manager and very easily if running LXQt (by the way openbox will be the default in LXQt, but not proposed standalone either).

PPS: And that works, I just checked. LXQt is really awesome.