Set environment variables for a systemd service correctly (Environment, EnvironmentFile, quoting gotchas)

Documented steps

1. Inline variables go in [Service]: `Environment=PORT=8080` or several at once: `Environment="VAR1=a b" "VAR2=c"` — quote the whole VAR=value token when the value contains spaces.
2. File-based: `EnvironmentFile=/etc/myapp/env` with lines of VAR=value. Prefix the path with '-' (`EnvironmentFile=-/etc/myapp/env`) to make a missing file non-fatal instead of a start failure.
3. Multiple Environment=/EnvironmentFile= lines accumulate; later assignments of the same variable win. An empty `Environment=` clears everything set so far in the unit.
4. systemd does NOT run a shell: no $HOME expansion, no command substitution, no `export`. `$VAR`/`${VAR}` inside ExecStart expand from the unit's own environment only; use `${VAR}` for exact single-argument substitution and `$VAR` for word-split substitution.
5. To compute values dynamically, wrap in a shell explicitly: `ExecStart=/bin/sh -c 'exec /usr/local/bin/myapp --host=$(hostname -f)'` — remember `exec` so signals reach the app, and note ExecStartPre cannot export variables to ExecStart.
6. Inspect the result: `systemctl show myapp -p Environment,EnvironmentFiles` and, live, `cat /proc/$(systemctl show -p MainPID --value myapp)/environ | tr '\0' '\n'`.
7. Manager-level defaults for all units: `systemctl set-environment FOO=bar` / DefaultEnvironment= in /etc/systemd/system.conf (affects new starts only).