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 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.