docs/ENVIRONMENT.md

   1 ---
   2 title: Known Environment Variables
   3 ---
   4
   5 # Known Environment Variables
   6
   7 A number of systemd components take additional runtime parameters via
   8 environment variables. Many of these environment variables are not supported at
   9 the same level as command line switches and other interfaces are: we don't
  10 document them in the man pages and we make no stability guarantees for
  11 them. While they generally are unlikely to be dropped any time soon again, we
  12 do not want to guarantee that they stay around for good either.
  13
  14 Below is an (incomprehensive) list of the environment variables understood by
  15 the various tools. Note that this list only covers environment variables not
  16 documented in the proper man pages.
  17
  18 All tools:
  19
  20 * `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will
  21   refrain from talking to PID 1; this has the same effect as the historical
  22   detection of `chroot()`.  Setting this variable to `0` instead has a similar
  23   effect as `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to
  24   communicate with PID 1 even if a `chroot()` environment is detected.
  25   You almost certainly want to set this to `1` if you maintain a package build system
  26   or similar and are trying to use a modern container system and not plain
  27   `chroot()`.
  28
  29 * `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
  30   `chroot()` environment. This is particularly relevant for systemctl, as it
  31   will not alter its behaviour for `chroot()` environments if set.  Normally it
  32   refrains from talking to PID 1 in such a case; turning most operations such
  33   as `start` into no-ops.  If that's what's explicitly desired, you might
  34   consider setting `SYSTEMD_OFFLINE=1`.
  35
  36 * `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
  37   will print latency information at runtime.
  38
  39 * `$SYSTEMD_PROC_CMDLINE` — if set, the contents are used as the kernel command
  40   line instead of the actual one in /proc/cmdline. This is useful for
  41   debugging, in order to test generators and other code against specific kernel
  42   command lines.
  43
  44 * `$SYSTEMD_FSTAB` — if set, use this path instead of /etc/fstab. Only useful
  45   for debugging.
  46
  47 * `$SYSTEMD_CRYPTTAB` — if set, use this path instead of /etc/crypttab. Only
  48   useful for debugging. Currently only supported by systemd-cryptsetup-generator.
  49
  50 * `$SYSTEMD_EFI_OPTIONS` — if set, used instead of the string in SystemdOptions
  51   EFI variable. Analogous to `$SYSTEMD_PROC_CMDLINE`.
  52
  53 * `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
  54   This is useful for debugging and testing initrd-only programs in the main
  55   system.
  56
  57 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
  58   completion. If no time unit is specified, assumes seconds. The usual other units
  59   are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set
  60   to 0, then the built-in default is used.
  61
  62 * `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
  63   hash tables is turned off, and libc malloc() is used for all allocations.
  64
  65 * `$SYSTEMD_EMOJI=0` — if set, tools such as "systemd-analyze security" will
  66   not output graphical smiley emojis, but ASCII alternatives instead. Note that
  67   this only controls use of Unicode emoji glyphs, and has no effect on other
  68   Unicode glyphs.
  69
  70 * `$RUNTIME_DIRECTORY` — various tools use this variable to locate the
  71   appropriate path under /run. This variable is also set by the manager when
  72   RuntimeDirectory= is used, see systemd.exec(5).
  73
  74 systemctl:
  75
  76 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
  77   listener, and instead always connect through the dbus-daemon D-bus broker.
  78
  79 * `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
  80   the client side, instead of asking PID 1 to do this.
  81
  82 * `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
  83
  84 systemd-nspawn:
  85
  86 * `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force nspawn into unified
  87   cgroup hierarchy mode.
  88
  89 * `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
  90   friends writable in the container. If set to "network", leave only
  91   /proc/sys/net writable.
  92
  93 * `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
  94   uses to register with machined. If unset defaults to "nspawn", but with this
  95   variable may be set to any other value.
  96
  97 * `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
  98   it is available.
  99
 100 * `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
 101
 102 * `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount /tmp in the
 103   container with a tmpfs, but leave the directory from the image in place.
 104
 105 systemd-logind:
 106
 107 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
 108   hibernation is available even if the swap devices do not provide enough room
 109   for it.
 110
 111 systemd-udevd:
 112
 113 * `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
 114   "v238", "v239", "v240"…, or the special value "latest") as parameter. If
 115   specified udev's net_id builtin will follow the specified naming scheme when
 116   determining stable network interface names. This may be used to revert to
 117   naming schemes of older udev versions, in order to provide more stable naming
 118   across updates. This environment variable takes precedence over the kernel
 119   command line option `net.naming-scheme=`, except if the value is prefixed
 120   with `:` in which case the kernel command line option takes precedence, if it
 121   is specified as well.
 122
 123 * `$SYSTEMD_REBOOT_TO_FIRMWARE_SETUP` — if set overrides systemd-logind's
 124   built-in EFI logic of requesting a reboot into the firmware. Takes a
 125   boolean. If set to false the functionality is turned off entirely. If set to
 126   true instead of requesting a reboot into the firmware setup UI through EFI a
 127   file `/run/systemd/reboot-to-firmware-setup` is created whenever this is
 128   requested. This file may be checked for by services run during system
 129   shutdown in order to request the appropriate operation from the firmware in
 130   an alternative fashion.
 131
 132 * `$SYSTEMD_REBOOT_TO_BOOT_LOADER_MENU` — similar to the above, allows
 133   overriding of systemd-logind's built-in EFI logic of requesting a reboot into
 134   the boot loader menu. Takes a boolean. If set to false the functionality is
 135   turned off entirely. If set to true instead of requesting a reboot into the
 136   boot loader menu through EFI a file `/run/systemd/reboot-to-boot-loader-menu`
 137   is created whenever this is requested. The file contains the requested boot
 138   loader menu timeout in µs, formatted in ASCII decimals, or zero in case no
 139   time-out is requested. This file may be checked for by services run during
 140   system shutdown in order to request the appropriate operation from the boot
 141   loader in an alternative fashion.
 142
 143 * `$SYSTEMD_REBOOT_TO_BOOT_LOADER_ENTRY` — similar to the above, allows
 144   overriding of systemd-logind's built-in EFI logic of requesting a reboot into
 145   a specific boot loader entry. Takes a boolean. If set to false the
 146   functionality is turned off entirely. If set to true instead of requesting a
 147   reboot into a specific boot loader entry through EFI a file
 148   `/run/systemd/reboot-to-boot-loader-entry` is created whenever this is
 149   requested. The file contains the requested boot loader entry identifier. This
 150   file may be checked for by services run during system shutdown in order to
 151   request the appropriate operation from the boot loader in an alternative
 152   fashion. Note that by default only boot loader entries which follow the [Boot
 153   Loader Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) and are
 154   placed in the ESP or the Extended Boot Loader partition may be selected this
 155   way. However, if a directory `/run/boot-loader-entries/` exists, the entries
 156   are loaded from there instead. The directory should contain the usual
 157   directory hierarchy mandated by the Boot Loader Specification, i.e. the entry
 158   drop-ins should be placed in
 159   `/run/boot-loader-entries/loader/entries/*.conf`, and the files referenced by
 160   the drop-ins (including the kernels and initrds) somewhere else below
 161   `/run/boot-loader-entries/`. Note that all these files may be (and are
 162   supposed to be) symlinks. systemd-logind will load these files on-demand,
 163   these files can hence be updated (ideally atomically) whenever the boot
 164   loader configuration changes. A foreign boot loader installer script should
 165   hence synthesize drop-in snippets and symlinks for all boot entries at boot
 166   or whenever they change if it wants to integrate with systemd-logind's APIs.
 167
 168 installed systemd tests:
 169
 170 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
 171   a test executable is moved to an arbitrary location.
 172
 173 nss-systemd:
 174
 175 * `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
 176   user/group records for the `root` and `nobody` users if they are missing from
 177   `/etc/passwd`.
 178
 179 * `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
 180   user/group records for dynamically registered service users (i.e. users
 181   registered through `DynamicUser=1`).
 182
 183 * `$SYSTEMD_NSS_BYPASS_BUS=1` — if set, `nss-systemd` won't use D-Bus to do
 184   dynamic user lookups. This is primarily useful to make `nss-systemd` work
 185   safely from within `dbus-daemon`.
 186
 187 systemd-timedated:
 188
 189 * `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
 190   NTP client services. If set, `timedatectl set-ntp on` enables and starts the
 191   first existing unit listed in the environment variable, and
 192   `timedatectl set-ntp off` disables and stops all listed units.
 193
 194 systemd-sulogin-shell:
 195
 196 * `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
 197   root password is not available (such as when the root account is locked).
 198   See `sulogin(8)` for more details.
 199
 200 bootctl and other tools that access the EFI System Partition (ESP):
 201
 202 * `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
 203   relaxed. Specifically, validation checks that ensure the specified ESP path
 204   is a FAT file system are turned off, as are checks that the path is located
 205   on a GPT partition with the correct type UUID.
 206
 207 * `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
 208   may be used to override ESP path auto detection, and redirect any accesses to
 209   the ESP to the specified directory. Not that unlike with bootctl's --path=
 210   switch only very superficial validation of the specified path is done when
 211   this environment variable is used.
 212
 213 systemd itself:
 214
 215 * `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
 216   are done by the service manager on behalf of a specific unit, in child
 217   processes that are later (after execve()) going to become unit
 218   processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM
 219   modules can use this information to determine in which context and on whose
 220   behalf they are being called, which may be useful to avoid deadlocks, for
 221   example to bypass IPC calls to the very service that is about to be
 222   started. Note that NSS and PAM modules should be careful to only rely on this
 223   data when invoked privileged, or possibly only when getppid() returns 1, as
 224   setting environment variables is of course possible in any even unprivileged
 225   contexts.
 226
 227 * `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$SYSTEMD_ACTIVATION_UNIT`,
 228   it is either set to `system` or `user` depending on whether the NSS/PAM
 229   module is called by systemd in `--system` or `--user` mode.
 230
 231 systemd-remount-fs:
 232
 233 * `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and no entry for the root directory
 234   exists in /etc/fstab (this file always takes precedence), then the root
 235   directory is remounted writable. This is primarily used by
 236   systemd-gpt-auto-generator to ensure the root partition is mounted writable
 237   in accordance to the GPT partition flags.
 238
 239 systemd-firstboot and localectl:
 240
 241 * `SYSTEMD_LIST_NON_UTF8_LOCALES=1` – if set non-UTF-8 locales are listed among
 242   the installed ones. By default non-UTF-8 locales are suppressed from the
 243   selection, since we are living in the 21st century.