Revert "docs: use collections to structure the data"

This reverts commit 5e8ff010a1436d33bbf3c108335af6e0b4ff7a2a.

This broke all the URLs, we can't have that. (And actually, we probably don't
_want_ to make the change either. It's nicer to have all the pages in one
directory, so one doesn't have to figure out to which collection the page
belongs.)

1 files changed, 644 insertions, 0 deletions

diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md
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+---
+title: Known Environment Variables
+category: Interfaces
+layout: default
+SPDX-License-Identifier: LGPL-2.1-or-later
+---
+
+# 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`.
+
+* `$SYSTEMD_FIRST_BOOT=0|1` — if set, assume "first boot" condition to be false
+  or true, instead of checking the flag file created by PID 1.
+
+* `$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.
+
+* `$SYSTEMD_VERITY_SHARING=0` — if set, sharing dm-verity devices by
+  using a stable `<ROOTHASH>-verity` device mapper name will be disabled.
+
+* `$SYSTEMD_OPENSSL_KEY_LOADER`— when using OpenSSL to load a key via an engine
+  or a provider, can be used to force the usage of one or the other interface.
+  Set to 'engine' to force the usage of the old engine API, and to 'provider'
+  force the usage of the new provider API. If unset, the provider will be tried
+  first and the engine as a fallback if that fails. Providers are the new OpenSSL
+  3 API, but there are very few if any in a production-ready state, so engines
+  are still needed.
+
+`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-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://uapi-group.org/specifications/specs/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 may not prevent loading some units of that type if they
+  are referenced by other units of another supported type.
+
+* `$SYSTEMD_DEFAULT_MOUNT_RATE_LIMIT_BURST` — can be set to override the mount
+  units burst rate limit for parsing `/proc/self/mountinfo`. On a system with
+  few resources but many mounts the rate limit may be hit, which will cause the
+  processing of mount units to stall. The burst limit may be adjusted when the
+  default is not appropriate for a given system. Defaults to `5`, accepts
+  positive integers.
+
+`systemd-remount-fs`:
+
+* `$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`:
+
+* `$SYSTEMD_LIST_NON_UTF8_LOCALES=1` — if set, non-UTF-8 locales are listed among
+  the installed ones. By default non-UTF-8 locales are suppressed from the
+  selection, since we are living in the 21st century.
+
+`systemd-resolved`:
+
+* `$SYSTEMD_RESOLVED_SYNTHESIZE_HOSTNAME` — if set to "0", `systemd-resolved`
+  won't synthesize system hostname on both regular and 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-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](https://uapi-group.org/specifications/specs/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-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`:
+```sh
+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.
+
+`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.
+
+`systemd-nspawn`, `systemd-networkd`:
+
+* `$SYSTEMD_FIREWALL_BACKEND` – takes a string, either `iptables` or
+  `nftables`. Selects the firewall backend to use. If not specified tries to
+  use `nftables` and falls back to `iptables` if that's not available.
+
+`systemd-storagetm`:
+
+* `$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`.
+
+* `$SYSTEMD_MEASURE_LOG_FIRMWARE` – the path to the `binary_bios_measurements`
+  file (containing firmware measurement data) to read. This allows overriding
+  the default of `/sys/kernel/security/tpm0/binary_bios_measurements`.
+
+Tools using the Varlink protocol (such as `varlinkctl`) or sd-bus (such as
+`busctl`):
+
+* `$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.