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_IN_CHROOT=0; 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_IN_CHROOT=0|1 — takes a boolean. If set, overrides chroot detection. This is particularly relevant for systemctl, as it will not alter its behaviour for chroot() environments if SYSTEMD_IN_CHROOT=0. 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.
$SYSTEMD_FIRST_BOOT=0|1 — if set, assume “first boot” condition to be false or true, instead of checking the flag file created by PID 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_OS_RELEASE — if set, use this path instead of /etc/os-release or /usr/lib/os-release. When operating under some root (e.g. systemctl --root=…), the path is prefixed with the root. Only useful for debugging.
$SYSTEMD_FSTAB — if set, use this path instead of /etc/fstab. Only useful for debugging.
$SYSTEMD_SYSROOT_FSTAB — if set, use this path instead of /sysroot/etc/fstab. Only useful for debugging systemd-fstab-generator.
$SYSTEMD_SYSFS_CHECK — takes a boolean. If set, overrides sysfs container detection that ignores /dev/ entries in fstab. Only useful for debugging systemd-fstab-generator.
$SYSTEMD_CRYPTTAB — if set, use this path instead of /etc/crypttab. Only useful for debugging. Currently only supported by systemd-cryptsetup-generator.
$SYSTEMD_INTEGRITYTAB — if set, use this path instead of /etc/integritytab. Only useful for debugging. Currently only supported by systemd-integritysetup-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 — takes a boolean. If set, 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_UTF8= — takes a boolean value, and overrides whether to generate non-ASCII special glyphs at various places (i.e. “→” instead of “->”). Usually this is determined automatically, based on $LC_CTYPE, but in scenarios where locale definitions are not installed it might make sense to override this check explicitly.
$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_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.
$SYSTEMD_ENABLE_LOG_CONTEXT — if set, extra fields will always be logged to the journal instead of only when logging in debug mode.
$SYSTEMD_NETLINK_DEFAULT_TIMEOUT — specifies the default timeout of waiting replies for netlink messages from the kernel. Defaults to 25 seconds.
$SYSTEMD_VERITY_SHARING=0 — if set, sharing dm-verity devices by using a stable <ROOTHASH>-verity device mapper name will be disabled.

systemctl:

$SYSTEMCTL_FORCE_BUS=1 — if set, do not connect to PID 1’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.
$SYSTEMCTL_SKIP_AUTO_KEXEC=1 — if set, do not automatically kexec instead of reboot when a new kernel has been loaded.
$SYSTEMCTL_SKIP_AUTO_SOFT_REBOOT=1 — if set, do not automatically soft-reboot instead of reboot when a new root file system has been loaded in /run/nextroot/.

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_NSPAWN_CHECK_OS_RELEASE=0 — if set, do not fail when trying to boot an OS tree without an os-release file (useful when trying to boot a container with empty /etc/ and bind-mounted /usr/)
$SYSTEMD_SUPPRESS_SYNC=1 — if set, all disk synchronization syscalls are blocked to the container payload (e.g. sync(), fsync(), syncfs(), …) and the O_SYNC/O_DSYNC flags are made unavailable to open() and friends. This is equivalent to passing --suppress-sync=yes on the systemd-nspawn command line.
$SYSTEMD_NSPAWN_NETWORK_MAC=... — if set, allows users to set a specific MAC address for a container, ensuring that it uses the provided value instead of generating a random one. It is effective when used with --network-veth. The expected format is six groups of two hexadecimal digits separated by colons, e.g. SYSTEMD_NSPAWN_NETWORK_MAC=12:34:56:78:90:AB

systemd-vmspawn:

$SYSTEMD_VMSPAWN_NETWORK_MAC=... — if set, allows users to set a specific MAC address for a VM, ensuring that it uses the provided value instead of generating a random one. It is effective when used with --network-tap. The expected format is six groups of two hexadecimal digits separated by colons, e.g. SYSTEMD_VMSPAWN_NETWORK_MAC=12:34:56:78:90:AB
$SYSTEMD_VMSPAWN_QEMU_EXTRA=… – may contain additional command line arguments to append the qemu command line.

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 and sd-device library:

$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_DEVICE_VERIFY_SYSFS — if set to “0”, disables verification that devices sysfs path are actually backed by sysfs. Relaxing this verification is useful for testing purposes.
$SYSTEMD_UDEV_EXTRA_TIMEOUT_SEC= — Specifies an extra timespan that the udev manager process waits for a worker process kills slow programs specified by IMPORT{program}=, PROGRAM=, or RUN=, and finalizes the processing event. If the worker process cannot finalize the event within the specified timespan, the worker process is killed by the manager process. Defaults to 10 seconds, maximum allowed is 5 hours.

udevadm and systemd-hwdb:

SYSTEMD_HWDB_UPDATE_BYPASS= — If set to “1”, execution of hwdb updates is skipped when udevadm hwdb --update or systemd-hwdb update are invoked. This can be useful if either of these tools are invoked unconditionally as a child process by another tool, such as package managers running either of these tools in a postinstall script.

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.
$KERNEL_INSTALL_CONF_ROOT=… — override the built in default configuration directory /etc/kernel/ to read files like entry-token and install.conf from.

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_SUPPORT_DEVICE, $SYSTEMD_SUPPORT_MOUNT, $SYSTEMD_SUPPORT_SWAP - can be set to 0 to mark respective unit type as unsupported. Generally, having less units saves system resources so these options might be useful for cases where we don’t need to track given unit type, e.g. --user manager often doesn’t need to deal with device or swap units because they are handled by the --system manager (PID 1). Note that setting certain unit type as unsupported might not prevent loading some units of that type if they are referenced by other units of another supported type.
$SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST — can be set to override the mount units burst rate limit for parsing /proc/self/mountinfo. On a system with few resources but many mounts the rate limit may be hit, which will cause the processing of mount units to stall. The burst limit may be adjusted when the default is not appropriate for a given system. Defaults to 5, accepts positive integers.
$SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_INTERVAL_SEC — can be set to override the mount units interval rate limit for parsing /proc/self/mountinfo. Similar to $SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST, the interval limit maybe adjusted when the default is not appropriate for a given system. The default value is 1 and the default application time unit is second, and the time unit can beoverriden as usual by specifying it explicitly, see the systemd.time(7) man page.

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, localectl, and systemd-localed:

$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_KEYMAP_DIRECTORIES= — takes a colon (:) separated list of keymap directories. The directories must be absolute and normalized. If unset, the default keymap directories (/usr/share/keymaps/, /usr/share/kbd/keymaps/, and /usr/lib/kbd/keymaps/) will be used.

systemd-resolved:

$SYSTEMD_RESOLVED_SYNTHESIZE_HOSTNAME — if set to “0”, systemd-resolved won’t synthesize A/AAAA/PTR RRs for the system hostname on either regular nor reverse lookups.

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. Similarly, $SYSTEMD_CONFEXT_HIERARCHIES works for confext images and supports the systemd-confext multi-call functionality of sysext.
$SYSTEMD_SYSEXT_MUTABLE_MODE — this variable may be used to override the default mutability mode for hierarchies managed by systemd-sysext. It takes the same values the --mutable= command line switch does. Note that the command line still overrides the effect of the environment variable. Similarly, $SYSTEMD_CONFEXT_MUTABLE_MODE works for confext images and supports the systemd-confext multi-call functionality of sysext.

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-sysusers:

$SOURCE_DATE_EPOCH — if unset, the field of the date of last password change in /etc/shadow will be the number of days from Jan 1, 1970 00:00 UTC until today. If $SOURCE_DATE_EPOCH is set to a valid UNIX epoch value in seconds, then the field will be the number of days until that time instead. This is to support creating bit-by-bit reproducible system images by choosing a reproducible value for the field of the date of last password change in /etc/shadow. See: https://reproducible-builds.org/specs/source-date-epoch/

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.
$TESTFUNCS – takes a colon separated list of test functions to invoke, causes all non-matching test functions to be skipped. Only applies to tests using our regular test boilerplate.

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

systemd-importd:

$SYSTEMD_IMPORT_BTRFS_SUBVOL — takes a boolean, which controls whether to prefer creating btrfs subvolumes over plain directories for machine images. Has no effect on non-btrfs file systems where subvolumes are not available anyway. If not set, defaults to true.
$SYSTEMD_IMPORT_BTRFS_QUOTA — takes a boolean, which controls whether to set up quota automatically for created btrfs subvolumes for machine images. If not set, defaults to true. Has no effect if machines are placed in regular directories, because btrfs subvolumes are not supported or disabled. If enabled, the quota group of the subvolume is automatically added to a combined quota group for all such machine subvolumes.
$SYSTEMD_IMPORT_SYNC — takes a boolean, which controls whether to synchronize images to disk after installing them, before completing the operation. If not set, defaults to true. If disabled installation of images will be quicker, but not as safe.

systemd-dissect, systemd-nspawn and all other tools that may operate on disk images with --image= or similar:

$SYSTEMD_DISSECT_VERITY_SIDECAR — takes a boolean, which controls whether to load “sidecar” Verity metadata files. If enabled (which is the default), whenever a disk image is used, a set of files with the .roothash, .usrhash, .roothash.p7s, .usrhash.p7s, .verity suffixes are searched adjacent to disk image file, containing the Verity root hashes, their signatures or the Verity data itself. If disabled this automatic discovery of Verity metadata files is turned off.
$SYSTEMD_DISSECT_VERITY_EMBEDDED — takes a boolean, which controls whether to load the embedded Verity signature data. If enabled (which is the default), Verity root hash information and a suitable signature is automatically acquired from a signature partition, following the Discoverable Partitions Specification. If disabled any such partition is ignored. Note that this only disables discovery of the root hash and its signature, the Verity data partition itself is still searched in the GPT image.
$SYSTEMD_DISSECT_VERITY_SIGNATURE — takes a boolean, which controls whether to validate the signature of the Verity root hash if available. If enabled (which is the default), the signature of suitable disk images is validated against any of the certificates in /etc/verity.d/*.crt (and similar directories in /usr/lib/, /run, …) or passed to the kernel for validation against its built-in certificates.
$SYSTEMD_DISSECT_VERITY_TIMEOUT_SEC=sec — takes a timespan, which controls the timeout waiting for the image to be configured. Defaults to 100 msec.
$SYSTEMD_DISSECT_FILE_SYSTEMS= — takes a colon-separated list of file systems that may be mounted for automatically dissected disk images. If not specified defaults to something like: ext4:btrfs:xfs:vfat:erofs:squashfs
$SYSTEMD_LOOP_DIRECT_IO – takes a boolean, which controls whether to enable LO_FLAGS_DIRECT_IO (i.e. direct IO + asynchronous IO) on loopback block devices when opening them. Defaults to on, set this to “0” to disable this feature.
$SYSTEMD_ALLOW_USERSPACE_VERITY — takes a boolean, which controls whether to consider the userspace Verity public key store in /etc/verity.d/ (and related directories) to authenticate signatures on Verity hashes of disk images. Defaults to true, i.e. userspace signature validation is allowed. If false, authentication can be done only via the kernel’s internal keyring.

systemd-cryptsetup:

$SYSTEMD_CRYPTSETUP_USE_TOKEN_MODULE – takes a boolean, which controls whether to use the libcryptsetup “token” plugin module logic even when activating via FIDO2, PKCS#11, TPM2, i.e. mechanisms natively supported by systemd-cryptsetup. Defaults to enabled.
$SYSTEMD_CRYPTSETUP_TOKEN_PATH – takes a path to a directory in the file system. If specified overrides where libcryptsetup will look for token modules (.so). This is useful for debugging token modules: set this environment variable to the build directory and you are set. This variable is only supported when systemd is compiled in developer mode.

Various tools that read passwords from the TTY, such as systemd-cryptenroll and homectl:

$PASSWORD — takes a string: the literal password to use. If this environment variable is set it is used as password instead of prompting the user interactively. This exists primarily for debugging and testing purposes. Do not use this for production code paths, since environment variables are typically inherited down the process tree without restrictions and should thus not be used for secrets.
$NEWPASSWORD — similar to $PASSWORD above, but is used when both a current and a future password are required, for example if the password is to be changed. In that case $PASSWORD shall carry the current (i.e. old) password and $NEWPASSWORD the new.

systemd-homed:

$SYSTEMD_HOME_ROOT – defines an absolute path where to look for home directories/images. When unspecified defaults to /home/. This is useful for debugging purposes in order to run a secondary systemd-homed instance that operates on a different directory where home directories/images are placed.
$SYSTEMD_HOME_RECORD_DIR – defines an absolute path where to look for fixated home records kept on the host. When unspecified defaults to /var/lib/systemd/home/. Similar to $SYSTEMD_HOME_ROOT this is useful for debugging purposes, in order to run a secondary systemd-homed instance that operates on a record database entirely separate from the host’s.
$SYSTEMD_HOME_DEBUG_SUFFIX – takes a short string that is suffixed to systemd-homed’s D-Bus and Varlink service names/sockets. This is also understood by homectl. This too is useful for running an additional copy of systemd-homed that doesn’t interfere with the host’s main one.

$SYSTEMD_HOMEWORK_PATH – configures the path to the systemd-homework binary to invoke. If not specified defaults to /usr/lib/systemd/systemd-homework.

Combining these four environment variables is pretty useful when debugging/developing systemd-homed:

SYSTEMD_HOME_DEBUG_SUFFIX=foo \
    SYSTEMD_HOMEWORK_PATH=/home/lennart/projects/systemd/build/systemd-homework \
    SYSTEMD_HOME_ROOT=/home.foo/ \
    SYSTEMD_HOME_RECORD_DIR=/var/lib/systemd/home.foo/ \
    /home/lennart/projects/systemd/build/systemd-homed

$SYSTEMD_HOME_MOUNT_OPTIONS_BTRFS, $SYSTEMD_HOME_MOUNT_OPTIONS_EXT4, $SYSTEMD_HOME_MOUNT_OPTIONS_XFS – configure the default mount options to use for LUKS home directories, overriding the built-in default mount options. There’s one variable for each of the supported file systems for the LUKS home directory backend.
$SYSTEMD_HOME_MKFS_OPTIONS_BTRFS, $SYSTEMD_HOME_MKFS_OPTIONS_EXT4, $SYSTEMD_HOME_MKFS_OPTIONS_XFS – configure additional arguments to use for mkfs when formatting LUKS home directories. There’s one variable for each of the supported file systems for the LUKS home directory backend.
$SYSTEMD_HOME_LOCK_FREEZE_SESSION - Takes a boolean. When false, the user’s session will not be frozen when the home directory is locked. Note that the kernel may still freeze any task that tries to access data from the user’s locked home directory. This can lead to data loss, security leaks, or other undesired behavior caused by parts of the session becoming unresponsive due to disk I/O while other parts of the session continue running. Thus, we highly recommend that this variable isn’t used unless necessary. Defaults to true.

kernel-install:

$KERNEL_INSTALL_BYPASS – If set to “1”, execution of kernel-install is skipped when kernel-install is invoked. This can be useful if kernel-install is invoked unconditionally as a child process by another tool, such as package managers running kernel-install in a postinstall script.

systemd-journald, journalctl:

$SYSTEMD_JOURNAL_COMPACT – Takes a boolean. If enabled, journal files are written in a more compact format that reduces the amount of disk space required by the journal. Note that journal files in compact mode are limited to 4G to allow use of 32-bit offsets. Enabled by default.
$SYSTEMD_JOURNAL_COMPRESS – Takes a boolean, or one of the compression algorithms “XZ”, “LZ4”, and “ZSTD”. If enabled, the default compression algorithm set at compile time will be used when opening a new journal file. If disabled, the journal file compression will be disabled. Note that the compression mode of existing journal files are not changed. To make the specified algorithm takes an effect immediately, you need to explicitly run journalctl --rotate.
$SYSTEMD_CATALOG – path to the compiled catalog database file to use for journalctl -x, journalctl --update-catalog, journalctl --list-catalog and related calls.
$SYSTEMD_CATALOG_SOURCES – path to the catalog database input source directory to use for journalctl --update-catalog.

systemd-pcrextend, systemd-cryptsetup:

$SYSTEMD_FORCE_MEASURE=1 — If set, force measuring of resources (which are marked for measurement) even if not booted on a kernel equipped with systemd-stub. Normally, requested measurement of resources is conditionalized on kernels that have booted with systemd-stub. With this environment variable the test for that my be bypassed, for testing purposes.

systemd-repart:

$SYSTEMD_REPART_MKFS_OPTIONS_<FSTYPE> – configure additional arguments to use for mkfs when formatting partition file systems. There’s one variable for each of the supported file systems.
$SYSTEMD_REPART_OVERRIDE_FSTYPE – if set the value will override the file system type specified in Format= lines in partition definition files. Additionally, the filesystem for all partitions with a specific designator can be overridden via a correspondingly named environment variable. For example, to override the filesystem type for all partitions with Type=root, you can set SYSTEMD_REPART_OVERRIDE_FSTYPE_ROOT=ext4.

systemd-nspawn, systemd-networkd:

$SYSTEMD_FIREWALL_BACKEND – takes a string, either iptables or nftables. Selects the firewall backend to use. If not specified tries to use nftables and falls back to iptables if that’s not available.

systemd-networkd:

$SYSTEMD_NETWORK_PERSISTENT_STORAGE_READY – takes a boolean. If true, systemd-networkd tries to open the persistent storage on start. To make this work, ProtectSystem=strict in systemd-networkd.service needs to be downgraded or disabled.

systemd-storagetm:

$SYSTEMD_NVME_MODEL, $SYSTEMD_NVME_FIRMWARE, $SYSTEMD_NVME_SERIAL, $SYSTEMD_NVME_UUID – these take a model string, firmware version string, serial number string, and UUID formatted as string. If specified these override the defaults exposed on the NVME subsystem and namespace, which are derived from the underlying block device and system identity. Do not set the latter two via the environment variable unless systemd-storagetm is invoked to expose a single device only, since those identifiers better should be kept unique.

systemd-pcrlock, systemd-pcrextend:

$SYSTEMD_MEASURE_LOG_USERSPACE – the path to the tpm2-measure.log file (containing userspace measurement data) to read. This allows overriding the default of /run/log/systemd/tpm2-measure.log.
$SYSTEMD_MEASURE_LOG_FIRMWARE – the path to the binary_bios_measurements file (containing firmware measurement data) to read. This allows overriding the default of /sys/kernel/security/tpm0/binary_bios_measurements.

systemd-sleep:

$SYSTEMD_SLEEP_FREEZE_USER_SESSIONS - Takes a boolean. When true (the default), user.slice will be frozen during sleep. When false it will not be. We recommend against using this variable, because it can lead to undesired behavior, especially for systems that use home directory encryption and for systemd-suspend-then-hibernate.service.

Tools using the Varlink protocol (such as varlinkctl) or sd-bus (such as busctl):

$SYSTEMD_SSH – the ssh binary to invoke when the ssh: transport is used. May be a filename (which is searched for in $PATH) or absolute path.
$SYSTEMD_VARLINK_LISTEN – interpreted by some tools that provide a Varlink service. Takes a file system path: if specified the tool will listen on an AF_UNIX stream socket on the specified path in addition to whatever else it would listen on. If set to “-“ the tool will turn stdin/stdout into a Varlink connection.

systemd-mountfsd:

$SYSTEMD_MOUNTFSD_TRUSTED_DIRECTORIES – takes a boolean argument. If true disk images from the usual disk image directories (/var/lib/machines/, /var/lib/confexts/, …) will be considered “trusted”, i.e. are validated with a more relaxed image policy (typically not requiring Verity signature checking) than those from other directories (where Verity signature checks are mandatory). If false all images are treated the same, regardless if placed in the usual disk image directories or elsewhere. If not set defaults to a compile time setting.
$SYSTEMD_MOUNTFSD_IMAGE_POLICY_TRUSTED, $SYSTEMD_MOUNTFSD_IMAGE_POLICY_UNTRUSTED – the default image policy to apply to trusted and untrusted disk images. An image is considered trusted if placed in a trusted disk image directory (see above), or if suitable polkit authentication was acquired. See systemd.image-policy(7) for the valid syntax for image policy strings.

systemd-run, run0, systemd-nspawn, systemd-vmspawn:

$SYSTEMD_TINT_BACKGROUND – Takes a boolean. When false the automatic tinting of the background for containers, VMs, and interactive systemd-run and run0 invocations is turned off. Note that this environment variable has no effect if the background color is explicitly selected via the relevant --background= switch of the tool.
$SYSTEMD_ADJUST_TERMINAL_TITLE – Takes a boolean. When false the terminal window title will not be updated for interactive invocation of the mentioned tools.

systemd-hostnamed, systemd-importd, systemd-localed, systemd-machined, systemd-portabled, systemd-timedated:

SYSTEMD_EXIT_ON_IDLE – Takes a boolean. When false, the exit-on-idle logic of these services is disabled, making it easier to debug them.

systemd-ask-password:

$SYSTEMD_ASK_PASSWORD_KEYRING_TIMEOUT_SEC - takes a timespan, which controls the expiration time of keys stored in the kernel keyring by systemd-ask-password. If unset, the default expiration of 150 seconds is used. If set to 0, keys are not cached in the kernel keyring. If set to infinity, keys are cached without an expiration time in the kernel keyring.
SYSTEMD_ASK_PASSWORD_KEYRING_TYPE - takes a keyring ID or one of thread, process, session, user, user-session, or group. Controls the kernel keyring in which systemd-ask-password caches the queried password. Defaults to user.