docs/ENVIRONMENT.md

   1 # Known Environment Variables
   2
   3 A number of systemd components take additional runtime parameters via
   4 environment variables. Many of these environment variables are not supported at
   5 the same level as command line switches and other interfaces are: we don't
   6 document them in the man pages and we make no stability guarantees for
   7 them. While they generally are unlikely to be dropped any time soon again, we
   8 do not want to guarantee that they stay around for good either.
   9
  10 Below is an (incomprehensive) list of the environment variables understood by
  11 the various tools. Note that this list only covers environment variables not
  12 documented in the proper man pages.
  13
  14 All tools:
  15
  16 * `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will
  17   refrain from talking to PID 1; this has the same effect as the historical
  18   detection of `chroot()`.  Setting this variable to `0` instead has a similar
  19   effect as `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to
  20   communicate with PID 1 even if a `chroot()` environment is detected.
  21   You almost certainly want to set this to `1` if you maintain a package build system
  22   or similar and are trying to use a modern container system and not plain
  23   `chroot()`.
  24
  25 * `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
  26   `chroot()` environment. This is particularly relevant for systemctl, as it
  27   will not alter its behaviour for `chroot()` environments if set.  Normally it
  28   refrains from talking to PID 1 in such a case; turning most operations such
  29   as `start` into no-ops.  If that's what's explicitly desired, you might
  30   consider setting `SYSTEMD_OFFLINE=1`.
  31
  32 * `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
  33   will print latency information at runtime.
  34
  35 * `$SYSTEMD_PROC_CMDLINE` — if set, may contain a string that is used as kernel
  36   command line instead of the actual one readable from /proc/cmdline. This is
  37   useful for debugging, in order to test generators and other code against
  38   specific kernel command lines.
  39
  40 * `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
  41   This is useful for debugging and testing initrd-only programs in the main
  42   system.
  43
  44 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
  45   completion. If no time unit is specified, assumes seconds. The usual other units
  46   are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set
  47   to 0, then the built-in default is used.
  48
  49 * `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
  50   hash tables is turned off, and libc malloc() is used for all allocations.
  51
  52 * `$SYSTEMD_EMOJI=0` — if set, tools such as "systemd-analyze security" will
  53   not output graphical smiley emojis, but ASCII alternatives instead. Note that
  54   this only controls use of Unicode emoji glyphs, and has no effect on other
  55   Unicode glyphs.
  56
  57 systemctl:
  58
  59 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
  60   listener, and instead always connect through the dbus-daemon D-bus broker.
  61
  62 * `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
  63   the client side, instead of asking PID 1 to do this.
  64
  65 * `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
  66
  67 systemd-nspawn:
  68
  69 * `$UNIFIED_CGROUP_HIERARCHY=1` — if set, force nspawn into unified cgroup
  70   hierarchy mode.
  71
  72 * `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
  73   friends writable in the container. If set to "network", leave only
  74   /proc/sys/net writable.
  75
  76 * `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
  77   uses to register with machined. If unset defaults to "nspawn", but with this
  78   variable may be set to any other value.
  79
  80 * `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
  81   it is available.
  82
  83 * `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
  84
  85 * `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount /tmp in the
  86   container with a tmpfs, but leave the directory from the image in place.
  87
  88 systemd-logind:
  89
  90 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
  91   hibernation is available even if the swap devices do not provide enough room
  92   for it.
  93
  94 * `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
  95   v238, v239, v240 …) as parameter. If specified udev's net_id builtin will
  96   follow the specified naming scheme when determining stable network interface
  97   names. This may be used to revert to naming schemes of older udev versions,
  98   in order to provide more stable naming across updates. This environment
  99   variable takes precedence over the kernel command line option
 100   `net.naming-scheme=`, except if the value is prefixed with `:` in which case
 101   the kernel command line option takes precedence, if it is specified as well.
 102
 103 installed systemd tests:
 104
 105 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
 106   a test executable is moved to an arbitrary location.
 107
 108 nss-systemd:
 109
 110 * `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
 111   user/group records for the `root` and `nobody` users if they are missing from
 112   `/etc/passwd`.
 113
 114 * `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
 115   user/group records for dynamically registered service users (i.e. users
 116   registered through `DynamicUser=1`).
 117
 118 * `$SYSTEMD_NSS_BYPASS_BUS=1` — if set, `nss-systemd` won't use D-Bus to do
 119   dynamic user lookups. This is primarily useful to make `nss-systemd` work
 120   safely from within `dbus-daemon`.
 121
 122 systemd-timedated:
 123
 124 * `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
 125   NTP client services. If set, `timedatectl set-ntp on` enables and starts the
 126   first existing unit listed in the environment variable, and
 127   `timedatectl set-ntp off` disables and stops all listed units.
 128
 129 systemd-sulogin-shell:
 130
 131 * `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
 132   root password is not available (such as when the root account is locked).
 133   See `sulogin(8)` for more details.
 134
 135 bootctl and other tools that access the EFI System Partition (ESP):
 136
 137 * `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
 138   relaxed. Specifically, validation checks that ensure the specified ESP path
 139   is a FAT file system are turned off, as are checks that the path is located
 140   on a GPT partition with the correct type UUID.
 141
 142 * `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
 143   may be used to override ESP path auto detection, and redirect any accesses to
 144   the ESP to the specified directory. Not that unlike with bootctl's --path=
 145   switch only very superficial validation of the specified path is done when
 146   this environment variable is used.
 147
 148 systemd itself:
 149
 150 * `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
 151   are done by the service manager on behalf of a specific unit, in child
 152   processes that are later (after execve()) going to become unit
 153   processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM
 154   modules can use this information to determine in which context and on whose
 155   behalf they are being called, which may be useful to avoid deadlocks, for
 156   example to bypass IPC calls to the very service that is about to be
 157   started. Note that NSS and PAM modules should be careful to only rely on this
 158   data when invoked privileged, or possibly only when getppid() returns 1, as
 159   setting environment variables is of course possible in any even unprivileged
 160   contexts.
 161
 162 * `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$SYSTEMD_ACTIVATION_UNIT`,
 163   it is either set to `system` or `user` depending on whether the NSS/PAM
 164   module is called by systemd in `--system` or `--user` mode.