doc/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 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
  41   completion. If no time unit is specified, assumes seconds. The usual other units
  42   are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set
  43   to 0, then the built-in default is used.
  44
  45 systemctl:
  46
  47 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
  48   listener, and instead always connect through the dbus-daemon D-bus broker.
  49
  50 * `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
  51   the client side, instead of asking PID 1 to do this.
  52
  53 * `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
  54
  55 systemd-nspawn:
  56
  57 * `$UNIFIED_CGROUP_HIERARCHY=1` — if set, force nspawn into unified cgroup
  58   hierarchy mode.
  59
  60 * `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
  61   friends writable in the container. If set to "network", leave only
  62   /proc/sys/net writable.
  63
  64 * `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
  65   uses to register with machined. If unset defaults to "nspawn", but with this
  66   variable may be set to any other value.
  67
  68 * `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
  69   it is available.
  70
  71 * `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
  72
  73 systemd-logind:
  74
  75 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
  76   hibernation is available even if the swap devices do not provide enough room
  77   for it.
  78
  79 installed systemd tests:
  80
  81 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
  82   a test executable is moved to an arbitrary location.
  83
  84 nss-systemd:
  85
  86 * `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
  87   user/group records for the `root` and `nobody` users if they are missing from
  88   `/etc/passwd`.
  89
  90 * `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
  91   user/group records for dynamically registered service users (i.e. users
  92   registered through `DynamicUser=1`).
  93
  94 * `$SYSTEMD_NSS_BYPASS_BUS=1` — if set, `nss-systemd` won't use D-Bus to do
  95   dynamic user lookups. This is primarily useful to make `nss-systemd` work
  96   safely from within `dbus-daemon`.
  97
  98 systemd-timedated:
  99
 100 * `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
 101   NTP client services. If set, `timedatectl set-ntp on` enables and starts the
 102   first existing unit listed in the environment variable, and
 103   `timedatectl set-ntp off` disables and stops all listed units.
 104
 105 systemd itself:
 106
 107 * `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
 108   are done by the service manager on behalf of a specific unit, in child
 109   processes that are later (after execve()) going to become unit
 110   processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM
 111   modules can use this information to determine in which context and on whose
 112   behalf they are being called, which may be useful to avoid deadlocks, for
 113   example to bypass IPC calls to the very service that is about to be
 114   started. Note that NSS and PAM modules should be careful to only rely on this
 115   data when invoked privileged, or possibly only when getppid() returns 1, as
 116   setting environment variables is of course possible in any even unprivileged
 117   contexts.
 118
 119 * `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$SYSTEMD_ACTIVATION_UNIT`,
 120   it is either set to `system` or `user` depending on whether the NSS/PAM
 121   module is called by systemd in `--system` or `--user` mode.