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