How to reload /etc/environment without rebooting?

One thing you are mistaken about is that /etc/environment requires a reboot to reload. This is incorrect. The only time the file is read is on login, when the PAM stack is activated – specifically pam_env.so, which reads the file.

Logging out and back in would apply the changes – and in fact you must do this if you want all your processes to receive the new environment. All other "solutions"² will only apply the environment to the single shell process, but not to anything you launch through the GUI including new terminal windows.¹

If you're fine with that, though – the lack of export commands can be compensated for with set -a and set +a. However, it still remains a poor way, as the file doesn't use quoting either. But this should work fine:

while read -r env; do export "$env"; done

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¹ GNOME Session Manager provides a way to change its own environment, but only during the Initialization phase:

$ gdbus call -e -d org.gnome.SessionManager \
                -o /org/gnome/SessionManager \
                -m org.gnome.SessionManager.Setenv \
                "FOO" "bar"
Error: GDBus.Error:org.gnome.SessionManager.NotInInitialization: Setenv
    interface is only available during the Initialization phase

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² gdb is not a solution, but can be used sometimes. You have to attach it to the running processes of your session manager (e.g. gnome-session), your window manager (e.g. gnome-shell or openbox), your taskbar/panel if any (e.g. xfce4-panel), and generally anything else that possibly would run stuff. For each of those processes, you need to attach gdb to it by PID, invoke the putenv() function using p, then detach using q:

$ sudo gdb -p $(pidof gnome-session)
GNU gdb (GDB) 7.7.1
[...]
Attaching to process 718
[...]
0x00007fc2cefed81d in poll () from /usr/lib/libc.so.6

(gdb) p putenv("FOO=bar")
$1 = 0

(gdb) p putenv("BAZ=qux")
$2 = 0

(gdb) q
A debugging session is active.
Quit anyway? (y or n) y
Detaching from program: /usr/bin/gnome-session, process 718

Note that the debugger pauses the process, therefore you must attach to compositing window managers only from another tty (virtual console) or over SSH, otherwise the screen would freeze.

In addition to that, you should also update the environment used by dbus-daemon:

$ dbus-update-activation-environment --systemd FOO=bar BAZ=qux

For older systems:

$ gdbus call -e -d org.freedesktop.DBus \
                -o /org/freedesktop/DBus \
                -m org.freedesktop.DBus.UpdateActivationEnvironment \
                "{'FOO': 'bar', 'BAZ': 'qux'}"
()

This is not as comprehensive as the accepted answer, but if you have quoted variables in your /etc/environment files both previous methods (which use export $line somehow) will go wrong and you end up with quoted values.

• Option 1: with a temporary source-able file

sed 's/^/export /' /etc/environment > /tmp/env.sh && source /tmp/env.sh

• Option 2: with inline for (thanks @tim-jones):

for env in $( cat /etc/environment ); do export $(echo $env | sed -e 's/"//g'); done