docs/ENVIRONMENT.md - systemd - Git at Google

 ---
 title: Known Environment Variables
 category: Interfaces
 layout: default
 ---

 # Known Environment Variables

 A number of systemd components take additional runtime parameters via
 environment variables. Many of these environment variables are not supported at
 the same level as command line switches and other interfaces are: we don't
 document them in the man pages and we make no stability guarantees for
 them. While they generally are unlikely to be dropped any time soon again, we
 do not want to guarantee that they stay around for good either.

 Below is an (incomprehensive) list of the environment variables understood by
 the various tools. Note that this list only covers environment variables not
 documented in the proper man pages.

 All tools:

 * `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will refrain from
   talking to PID 1; this has the same effect as the historical detection of
   `chroot()`. Setting this variable to `0` instead has a similar effect as
   `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to communicate with PID 1 even
   if a `chroot()` environment is detected. You almost certainly want to set
   this to `1` if you maintain a package build system or similar and are trying
   to use a modern container system and not plain `chroot()`.

 * `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
   `chroot()` environment. This is particularly relevant for systemctl, as it
   will not alter its behaviour for `chroot()` environments if set. Normally it
   refrains from talking to PID 1 in such a case; turning most operations such
   as `start` into no-ops.  If that's what's explicitly desired, you might
   consider setting `SYSTEMD_OFFLINE=1`.

 * `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
   will print latency information at runtime.

 * `$SYSTEMD_PROC_CMDLINE` — if set, the contents are used as the kernel command
   line instead of the actual one in `/proc/cmdline`. This is useful for
   debugging, in order to test generators and other code against specific kernel
   command lines.

 * `$SYSTEMD_FSTAB` — if set, use this path instead of `/etc/fstab`. Only useful
   for debugging.

 * `$SYSTEMD_CRYPTTAB` — if set, use this path instead of `/etc/crypttab`. Only
   useful for debugging. Currently only supported by
   `systemd-cryptsetup-generator`.

 * `$SYSTEMD_VERITYTAB` — if set, use this path instead of
   `/etc/veritytab`. Only useful for debugging. Currently only supported by
   `systemd-veritysetup-generator`.

 * `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
   `SystemdOptions` EFI variable. Analogous to `$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
           on `/`. If both conditions meet, then it's in initrd.
   `lenient`: Similar to `auto`, but the rootfs check is skipped.
   `0|1`: Simply overrides initrd detection. This is useful for debugging and
          testing initrd-only programs in the main system.

 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
   completion. If no time unit is specified, assumes seconds. The usual other units
   are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set
   to 0, then the built-in default is used.

 * `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
   hash tables is turned off, and libc `malloc()` is used for all allocations.

 * `$SYSTEMD_EMOJI=0` — if set, tools such as `systemd-analyze security` will
   not output graphical smiley emojis, but ASCII alternatives instead. Note that
   this only controls use of Unicode emoji glyphs, and has no effect on other
   Unicode glyphs.

 * `$RUNTIME_DIRECTORY` — various tools use this variable to locate the
   appropriate path under `/run/`. This variable is also set by the manager when
   `RuntimeDirectory=` is used, see systemd.exec(5).

 * `$SYSTEMD_CRYPT_PREFIX` — if set configures the hash method prefix to use for
   UNIX `crypt()` when generating passwords. By default the system's "preferred
   method" is used, but this can be overridden with this environment variable.
   Takes a prefix such as `$6$` or `$y$`. (Note that this is only honoured on
   systems built with libxcrypt and is ignored on systems using glibc's
   original, internal `crypt()` implementation.)

 * `$SYSTEMD_RDRAND=0` — if set, the RDRAND instruction will never be used,
   even if the CPU supports it.

 * `$SYSTEMD_SECCOMP=0` – if set, seccomp filters will not be enforced, even if
   support for it is compiled in and available in the kernel.

 * `$SYSTEMD_LOG_SECCOMP=1` — if set, system calls blocked by seccomp filtering,
   for example in `systemd-nspawn`, will be logged to the audit log, if the
   kernel supports this.

 `systemctl`:

 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
   listener, and instead always connect through the dbus-daemon D-bus broker.

 * `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
   the client side, instead of asking PID 1 to do this.

 * `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call SysV compatibility hooks.

 `systemd-nspawn`:

 * `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force `systemd-nspawn` into
   unified cgroup hierarchy mode.

 * `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make `/sys/`, `/proc/sys/`,
   and friends writable in the container. If set to "network", leave only
   `/proc/sys/net/` writable.

 * `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
   uses to register with machined. If unset defaults to "nspawn", but with this
   variable may be set to any other value.

 * `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
   it is available.

 * `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.

 * `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount `/tmp/` in the
   container with a tmpfs, but leave the directory from the image in place.

 `systemd-logind`:

 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
   hibernation is available even if the swap devices do not provide enough room
   for it.

 * `$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, the 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 timeout 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, the 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.

 `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.

 `nss-systemd`:

 * `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
   user/group records for the `root` and `nobody` users if they are missing from
   `/etc/passwd`.

 * `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
   user/group records for dynamically registered service users (i.e. users
   registered through `DynamicUser=1`).

 `systemd-timedated`:

 * `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
   NTP client services. If set, `timedatectl set-ntp on` enables and starts the
   first existing unit listed in the environment variable, and
   `timedatectl set-ntp off` disables and stops all listed units.

 `systemd-sulogin-shell`:

 * `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
   root password is not available (such as when the root account is locked).
   See `sulogin(8)` for more details.

 `bootctl` and other tools that access the EFI System Partition (ESP):

 * `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
   relaxed. Specifically, validation checks that ensure the specified ESP path
   is a FAT file system are turned off, as are checks that the path is located
   on a GPT partition with the correct type UUID.

 * `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
   may be used to override ESP path auto detection, and redirect any accesses to
   the ESP to the specified directory. Note that unlike with `bootctl`'s
   `--path=` switch only very superficial validation of the specified path is
   done when this environment variable is used.

 `systemd` itself:

 * `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
   are done by the service manager on behalf of a specific unit, in child
   processes that are later (after execve()) going to become unit
   processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM
   modules can use this information to determine in which context and on whose
   behalf they are being called, which may be useful to avoid deadlocks, for
   example to bypass IPC calls to the very service that is about to be
   started. Note that NSS and PAM modules should be careful to only rely on this
   data when invoked privileged, or possibly only when getppid() returns 1, as
   setting environment variables is of course possible in any even unprivileged
   contexts.

 * `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$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.

 `systemd-sysext`:

 * `SYSTEMD_SYSEXT_HIERARCHIES` – this variable may be used to override which
   hierarchies are managed by `systemd-sysext`. By default only `/usr/` and
   `/opt/` are managed, and directories may be added or removed to that list by
   setting this environment variable to a colon-separated list of absolute
   paths. Only "real" file systems and directories that only contain "real" file
   systems as submounts should be used. Do not specify API file systems such as
   `/proc/` or `/sys/` here, or hierarchies that have them as submounts. In
   particular, do not specify the root directory `/` here.

 `systemd-tmpfiles`:

 * `SYSTEMD_TMPFILES_FORCE_SUBVOL` — if unset, `v`/`q`/`Q` lines will create
   subvolumes only if the OS itself is installed into a subvolume. If set to `1`
   (or another value interpreted as true), these lines will always create
   subvolumes if the backing filesystem supports them. If set to `0`, these
   lines will always create directories.

 `systemd-sysv-generator`:

 * `$SYSTEMD_SYSVINIT_PATH` — Controls where `systemd-sysv-generator` looks for
   SysV init scripts.

 * `$SYSTEMD_SYSVRCND_PATH` — Controls where `systemd-sysv-generator` looks for
   SysV init script runlevel link farms.

 systemd tests:

 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
   a test executable is moved to an arbitrary location.

 * `$SYSTEMD_TEST_NSS_BUFSIZE` — size of scratch buffers for "reentrant"
   functions exported by the nss modules.

 fuzzers:

 * `$SYSTEMD_FUZZ_OUTPUT` — A boolean that specifies whether to write output to
   stdout. Setting to true is useful in manual invocations, since all output is
   suppressed by default.

 * `$SYSTEMD_FUZZ_RUNS` — The number of times execution should be repeated in
   manual invocations.

 Note that is may be also useful to set `$SYSTEMD_LOG_LEVEL`, since all logging
 is suppressed by default.
	---
	title: Known Environment Variables
	category: Interfaces
	layout: default
	---

	# Known Environment Variables

	A number of systemd components take additional runtime parameters via
	environment variables. Many of these environment variables are not supported at
	the same level as command line switches and other interfaces are: we don't
	document them in the man pages and we make no stability guarantees for
	them. While they generally are unlikely to be dropped any time soon again, we
	do not want to guarantee that they stay around for good either.

	Below is an (incomprehensive) list of the environment variables understood by
	the various tools. Note that this list only covers environment variables not
	documented in the proper man pages.

	All tools:

	* `$SYSTEMD_OFFLINE=[0\|1]` — if set to `1`, then `systemctl` will refrain from
	talking to PID 1; this has the same effect as the historical detection of
	`chroot()`. Setting this variable to `0` instead has a similar effect as
	`SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to communicate with PID 1 even
	if a `chroot()` environment is detected. You almost certainly want to set
	this to `1` if you maintain a package build system or similar and are trying
	to use a modern container system and not plain `chroot()`.

	* `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
	`chroot()` environment. This is particularly relevant for systemctl, as it
	will not alter its behaviour for `chroot()` environments if set. Normally it
	refrains from talking to PID 1 in such a case; turning most operations such
	as `start` into no-ops. If that's what's explicitly desired, you might
	consider setting `SYSTEMD_OFFLINE=1`.

	* `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
	will print latency information at runtime.

	* `$SYSTEMD_PROC_CMDLINE` — if set, the contents are used as the kernel command
	line instead of the actual one in `/proc/cmdline`. This is useful for
	debugging, in order to test generators and other code against specific kernel
	command lines.

	* `$SYSTEMD_FSTAB` — if set, use this path instead of `/etc/fstab`. Only useful
	for debugging.

	* `$SYSTEMD_CRYPTTAB` — if set, use this path instead of `/etc/crypttab`. Only
	useful for debugging. Currently only supported by
	`systemd-cryptsetup-generator`.

	* `$SYSTEMD_VERITYTAB` — if set, use this path instead of
	`/etc/veritytab`. Only useful for debugging. Currently only supported by
	`systemd-veritysetup-generator`.

	* `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in the
	`SystemdOptions` EFI variable. Analogous to `$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
	on `/`. If both conditions meet, then it's in initrd.
	`lenient`: Similar to `auto`, but the rootfs check is skipped.
	`0\|1`: Simply overrides initrd detection. This is useful for debugging and
	testing initrd-only programs in the main system.

	* `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
	completion. If no time unit is specified, assumes seconds. The usual other units
	are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set
	to 0, then the built-in default is used.

	* `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
	hash tables is turned off, and libc `malloc()` is used for all allocations.

	* `$SYSTEMD_EMOJI=0` — if set, tools such as `systemd-analyze security` will
	not output graphical smiley emojis, but ASCII alternatives instead. Note that
	this only controls use of Unicode emoji glyphs, and has no effect on other
	Unicode glyphs.

	* `$RUNTIME_DIRECTORY` — various tools use this variable to locate the
	appropriate path under `/run/`. This variable is also set by the manager when
	`RuntimeDirectory=` is used, see systemd.exec(5).

	* `$SYSTEMD_CRYPT_PREFIX` — if set configures the hash method prefix to use for
	UNIX `crypt()` when generating passwords. By default the system's "preferred
	method" is used, but this can be overridden with this environment variable.
	Takes a prefix such as `$6$` or `$y$`. (Note that this is only honoured on
	systems built with libxcrypt and is ignored on systems using glibc's
	original, internal `crypt()` implementation.)

	* `$SYSTEMD_RDRAND=0` — if set, the RDRAND instruction will never be used,
	even if the CPU supports it.

	* `$SYSTEMD_SECCOMP=0` – if set, seccomp filters will not be enforced, even if
	support for it is compiled in and available in the kernel.

	* `$SYSTEMD_LOG_SECCOMP=1` — if set, system calls blocked by seccomp filtering,
	for example in `systemd-nspawn`, will be logged to the audit log, if the
	kernel supports this.

	`systemctl`:

	* `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
	listener, and instead always connect through the dbus-daemon D-bus broker.

	* `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
	the client side, instead of asking PID 1 to do this.

	* `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call SysV compatibility hooks.

	`systemd-nspawn`:

	* `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force `systemd-nspawn` into
	unified cgroup hierarchy mode.

	* `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make `/sys/`, `/proc/sys/`,
	and friends writable in the container. If set to "network", leave only
	`/proc/sys/net/` writable.

	* `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
	uses to register with machined. If unset defaults to "nspawn", but with this
	variable may be set to any other value.

	* `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
	it is available.

	* `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.

	* `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount `/tmp/` in the
	container with a tmpfs, but leave the directory from the image in place.

	`systemd-logind`:

	* `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
	hibernation is available even if the swap devices do not provide enough room
	for it.

	* `$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, the 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 timeout 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, the 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.

	`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.

	`nss-systemd`:

	* `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
	user/group records for the `root` and `nobody` users if they are missing from
	`/etc/passwd`.

	* `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
	user/group records for dynamically registered service users (i.e. users
	registered through `DynamicUser=1`).

	`systemd-timedated`:

	* `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
	NTP client services. If set, `timedatectl set-ntp on` enables and starts the
	first existing unit listed in the environment variable, and
	`timedatectl set-ntp off` disables and stops all listed units.

	`systemd-sulogin-shell`:

	* `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
	root password is not available (such as when the root account is locked).
	See `sulogin(8)` for more details.

	`bootctl` and other tools that access the EFI System Partition (ESP):

	* `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
	relaxed. Specifically, validation checks that ensure the specified ESP path
	is a FAT file system are turned off, as are checks that the path is located
	on a GPT partition with the correct type UUID.

	* `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
	may be used to override ESP path auto detection, and redirect any accesses to
	the ESP to the specified directory. Note that unlike with `bootctl`'s
	`--path=` switch only very superficial validation of the specified path is
	done when this environment variable is used.

	`systemd` itself:

	* `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
	are done by the service manager on behalf of a specific unit, in child
	processes that are later (after execve()) going to become unit
	processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM
	modules can use this information to determine in which context and on whose
	behalf they are being called, which may be useful to avoid deadlocks, for
	example to bypass IPC calls to the very service that is about to be
	started. Note that NSS and PAM modules should be careful to only rely on this
	data when invoked privileged, or possibly only when getppid() returns 1, as
	setting environment variables is of course possible in any even unprivileged
	contexts.

	* `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$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.

	`systemd-sysext`:

	* `SYSTEMD_SYSEXT_HIERARCHIES` – this variable may be used to override which
	hierarchies are managed by `systemd-sysext`. By default only `/usr/` and
	`/opt/` are managed, and directories may be added or removed to that list by
	setting this environment variable to a colon-separated list of absolute
	paths. Only "real" file systems and directories that only contain "real" file
	systems as submounts should be used. Do not specify API file systems such as
	`/proc/` or `/sys/` here, or hierarchies that have them as submounts. In
	particular, do not specify the root directory `/` here.

	`systemd-tmpfiles`:

	* `SYSTEMD_TMPFILES_FORCE_SUBVOL` — if unset, `v`/`q`/`Q` lines will create
	subvolumes only if the OS itself is installed into a subvolume. If set to `1`
	(or another value interpreted as true), these lines will always create
	subvolumes if the backing filesystem supports them. If set to `0`, these
	lines will always create directories.

	`systemd-sysv-generator`:

	* `$SYSTEMD_SYSVINIT_PATH` — Controls where `systemd-sysv-generator` looks for
	SysV init scripts.

	* `$SYSTEMD_SYSVRCND_PATH` — Controls where `systemd-sysv-generator` looks for
	SysV init script runlevel link farms.

	systemd tests:

	* `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
	a test executable is moved to an arbitrary location.

	* `$SYSTEMD_TEST_NSS_BUFSIZE` — size of scratch buffers for "reentrant"
	functions exported by the nss modules.

	fuzzers:

	* `$SYSTEMD_FUZZ_OUTPUT` — A boolean that specifies whether to write output to
	stdout. Setting to true is useful in manual invocations, since all output is
	suppressed by default.

	* `$SYSTEMD_FUZZ_RUNS` — The number of times execution should be repeated in
	manual invocations.

	Note that is may be also useful to set `$SYSTEMD_LOG_LEVEL`, since all logging
	is suppressed by default.