* `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
`SystemdOptions` EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
+* `$SYSTEMD_DEFAULT_HOSTNAME` — override the compiled-in fallback hostname
+ (relevant in particular for the system manager and `systemd-hostnamed`).
+ Must be a valid hostname (either a single label or a FQDN).
+
* `$SYSTEMD_IN_INITRD=[auto|lenient|0|1]` — if set, specifies initrd detection
method. Defaults to `auto`. Behavior is defined as follows:
`auto`: Checks if `/etc/initrd-release` exists, and a temporary fs is mounted