* Slim notes

- The reason why using slim triggers /etc/profile to run is because the "login_cmd" defined in /etc/slim.conf explicitly runs bash as a login shell. Bash then runs /etc/profile.

login_cmd           exec /bin/bash -login /etc/X11/Xsession %session

- /etc/slim.conf does have a "default_path" setting. But if you append a path to this setting, it doesn't appear to work. That's because the bash login runs /etc/profile, which overwrites the current PATH anyway.

You can confirm that the "default_path" does work by adding a dummy path to "default_path":

default_path        /usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games:/dbg/slimconf

then printing the PATH variable out to a file at the very top of /etc/profile:

echo "Top of /etc/profile, PATH is $PATH" >> /tmp/dbglog.txt

You'll see in the log file that "/dbg/slimconf" is in the PATH, confirming that slim really is using "default_path".

* Lightdm notes

- I downloaded the lightdm source code, then compiled and debugged it. The source code hardcodes the root and regular users' PATH variable:

- session-child.c, session_child_run():

    /* Set POSIX variables */
    if (user_get_uid (user) == 0)
      pam_putenv (pam_handle, "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin");
    else
      pam_putenv (pam_handle, "PATH=/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games");
    pam_putenv (pam_handle, g_strdup_printf ("USER=%s", username));
    pam_putenv (pam_handle, g_strdup_printf ("LOGNAME=%s", username));
    pam_putenv (pam_handle, g_strdup_printf ("HOME=%s", user_get_home_directory (user)));
    pam_putenv (pam_handle, g_strdup_printf ("SHELL=%s", user_get_shell (user)));

I added a dummy path and confirmed that the PATH variable really comes from this code, which answers my questions. And it also means that unlike slim, there's currently no external way to change these paths, other than recompiling the code.

So using ~/.xsessionrc to modify the PATH seems to be the best option so far.

- The reason why lightdm doesn't run /etc/profile is because it doesn't run bash at all. Instead, it directly runs another app "/sbin/lightdm-gtk-greeter". I couldn't figure out where lightdm gets the greeter path from.

* Solution for lightdm

- Create a .sh file in /etc/profile.d, append /usr/sbin to the PATH variable. This covers the tty/telnet/ssh cases.
- Create a local file ~/.xsessionrc, append to the PATH variable. This covers the MATE Terminal and double-click-on-script cases.

A quick test of all of my test cases running "ifconfig" without sudo confirms this solution works for me. I'll marked this as solved.

Evidently, TTY login gets path from /etc/profile, then ~/.profile - both of which seem to be ignored by lightdm

I know it's not answering your questions (I don't know the answers), but for now you could use a combination of /etc/profile (for tty) and ~/.xsessionrc (for X)

(edit)
From this page https://wiki.archlinux.org/title/LightDM

I was not able to get xprofile to work, either.  I'm wondering if something is missing or misconfigured by default, or if this is a bug.

In slim + Xfce, I can create a .sh file in /etc/profile.d, and append to the PATH:

  PATH=$PATH:/example/path

But this doesn't work with lightdm. My testing shows that lightdm doesn't run /etc/profile, which means none of the scripts in /etc/profile.d are run. And I've confirmed that the problem is lightdm, because when I switched from slim + Xfce to lightdm + Xfce, /etc/profile is NOT run anymore.

* Solutions that don't work for me
 
- Use ~/.xsessionrc. This doesn't work because TTY (CTRL+ALT+F1), ssh, or telnet does NOT run ~/.xsessionrc.

- Use ~/.bashrc. This doesn't work if I double click on a script file to launch it. ~/.bashrc is NOT run.
 
- Use /etc/environment. This also doesn't work in TTY, ssh, or telnet. It also requires me to hardcode the rest of the system paths, because /etc/environment does not support script syntax, so I can't append to an existing PATH value.

- Maybe I can find where PATH is first initialized globally, and append my path there. I've tried:
  - The initial PATH value is "/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games", and I've confirmed that even though /etc/profile sets the PATH to this string, this string is ALREADY set to PATH at the start of /etc/profile. Where is it coming from?
  - I searched /etc for "/usr/games", and the only file that could make sense is /etc/login.defs. But when I manually appended a dummy path "/dbg/logindefs", and rebooted, the dummy path is NOT in PATH.
  - I extracted the ramdisk /boot/initrd.img-6.12.63+deb13-amd64, and did not find "/usr/games". Now I'm out of ideas.

* Questions

? Where can I append to the PATH globally?

? Also, where exactly is the PATH first initialized?