+---
+title: Known Environment Variables
+---
+
# Known Environment Variables
A number of systemd components take additional runtime parameters via
hibernation is available even if the swap devices do not provide enough room
for it.
+systemd-udevd:
+
+* `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
+ "v238", "v239", "v240"…, or the special value "latest") as parameter. If
+ specified udev's net_id builtin will follow the specified naming scheme when
+ determining stable network interface names. This may be used to revert to
+ naming schemes of older udev versions, in order to provide more stable naming
+ across updates. This environment variable takes precedence over the kernel
+ command line option `net.naming-scheme=`, except if the value is prefixed
+ with `:` in which case the kernel command line option takes precedence, if it
+ is specified as well.
+
+* `$SYSTEMD_REBOOT_TO_FIRMWARE_SETUP` — if set overrides systemd-logind's
+ built-in EFI logic of requesting a reboot into the firmware. Takes a
+ boolean. If set to false the functionality is turned off entirely. If set to
+ true instead of requesting a reboot into the firmware setup UI through EFI a
+ file `/run/systemd/reboot-to-firmware-setup` is created whenever this is
+ requested. This file may be checked for by services run during system
+ shutdown in order to request the appropriate operation from the firmware in
+ an alternative fashion.
+
+* `$SYSTEMD_REBOOT_TO_BOOT_LOADER_MENU` — similar to the above, allows
+ overriding of systemd-logind's built-in EFI logic of requesting a reboot into
+ the boot loader menu. Takes a boolean. If set to false the functionality is
+ turned off entirely. If set to true instead of requesting a reboot into the
+ boot loader menu through EFI a file `/run/systemd/reboot-to-boot-loader-menu`
+ is created whenever this is requested. The file contains the requested boot
+ loader menu timeout in µs, formatted in ASCII decimals, or zero in case no
+ time-out is requested. This file may be checked for by services run during
+ system shutdown in order to request the appropriate operation from the boot
+ loader in an alternative fashion.
+
+* `$SYSTEMD_REBOOT_TO_BOOT_LOADER_ENTRY` — similar to the above, allows
+ overriding of systemd-logind's built-in EFI logic of requesting a reboot into
+ a specific boot loader entry. Takes a boolean. If set to false the
+ functionality is turned off entirely. If set to true instead of requesting a
+ reboot into a specific boot loader entry through EFI a file
+ `/run/systemd/reboot-to-boot-loader-entry` is created whenever this is
+ requested. The file contains the requested boot loader entry identifier. This
+ file may be checked for by services run during system shutdown in order to
+ request the appropriate operation from the boot loader in an alternative
+ fashion. Note that by default only boot loader entries which follow the [Boot
+ Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) and are
+ placed in the ESP or the Extended Boot Loader partition may be selected this
+ way. However, if a directory `/run/boot-loader-entries/` exists, the entries
+ are loaded from there instead. The directory should contain the usual
+ directory hierarchy mandated by the Boot Loader Specification, i.e. the entry
+ drop-ins should be placed in
+ `/run/boot-loader-entries/loader/entries/*.conf`, and the files referenced by
+ the drop-ins (including the kernels and initrds) somewhere else below
+ `/run/boot-loader-entries/`. Note that all these files may be (and are
+ supposed to be) symlinks. systemd-logind will load these files on-demand,
+ these files can hence be updated (ideally atomically) whenever the boot
+ loader configuration changes. A foreign boot loader installer script should
+ hence synthesize drop-in snippets and symlinks for all boot entries at boot
+ or whenever they change if it wants to integrate with systemd-logind's APIs.
+
installed systemd tests:
* `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
* `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$SYSTEMD_ACTIVATION_UNIT`,
it is either set to `system` or `user` depending on whether the NSS/PAM
module is called by systemd in `--system` or `--user` mode.
+
+systemd-remount-fs:
+
+* `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and no entry for the root directory
+ exists in /etc/fstab (this file always takes precedence), then the root
+ directory is remounted writable. This is primarily used by
+ systemd-gpt-auto-generator to ensure the root partition is mounted writable
+ in accordance to the GPT partition flags.
+
+systemd-firstboot and localectl:
+
+* `SYSTEMD_LIST_NON_UTF8_LOCALES=1` – if set non-UTF-8 locales are listed among
+ the installed ones. By default non-UTF-8 locales are suppressed from the
+ selection, since we are living in the 21st century.