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, may contain a string that is used as kernel
  40   command line instead of the actual one readable from /proc/cmdline. This is
  41   useful for debugging, in order to test generators and other code against
  42   specific kernel command lines.
  43
  44 * `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
  45   This is useful for debugging and testing initrd-only programs in the main
  46   system.
  47
  48 * `$SYSTEMD_BUS_TIMEOUT=SECS` — specifies the maximum time to wait for method call
  49   completion. If no time unit is specified, assumes seconds. The usual other units
  50   are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set
  51   to 0, then the built-in default is used.
  52
  53 * `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
  54   hash tables is turned off, and libc malloc() is used for all allocations.
  55
  56 * `$SYSTEMD_EMOJI=0` — if set, tools such as "systemd-analyze security" will
  57   not output graphical smiley emojis, but ASCII alternatives instead. Note that
  58   this only controls use of Unicode emoji glyphs, and has no effect on other
  59   Unicode glyphs.
  60
  61 systemctl:
  62
  63 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
  64   listener, and instead always connect through the dbus-daemon D-bus broker.
  65
  66 * `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
  67   the client side, instead of asking PID 1 to do this.
  68
  69 * `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
  70
  71 systemd-nspawn:
  72
  73 * `$UNIFIED_CGROUP_HIERARCHY=1` — if set, force nspawn into unified cgroup
  74   hierarchy mode.
  75
  76 * `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
  77   friends writable in the container. If set to "network", leave only
  78   /proc/sys/net writable.
  79
  80 * `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
  81   uses to register with machined. If unset defaults to "nspawn", but with this
  82   variable may be set to any other value.
  83
  84 * `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
  85   it is available.
  86
  87 * `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
  88
  89 * `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount /tmp in the
  90   container with a tmpfs, but leave the directory from the image in place.
  91
  92 systemd-logind:
  93
  94 * `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
  95   hibernation is available even if the swap devices do not provide enough room
  96   for it.
  97
  98 systemd-udevd:
  99
 100 * `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
 101   "v238", "v239", "v240"…, or the special value "latest") as parameter. If
 102   specified udev's net_id builtin will follow the specified naming scheme when
 103   determining stable network interface names. This may be used to revert to
 104   naming schemes of older udev versions, in order to provide more stable naming
 105   across updates. This environment variable takes precedence over the kernel
 106   command line option `net.naming-scheme=`, except if the value is prefixed
 107   with `:` in which case the kernel command line option takes precedence, if it
 108   is specified as well.
 109
 110 installed systemd tests:
 111
 112 * `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
 113   a test executable is moved to an arbitrary location.
 114
 115 nss-systemd:
 116
 117 * `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
 118   user/group records for the `root` and `nobody` users if they are missing from
 119   `/etc/passwd`.
 120
 121 * `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
 122   user/group records for dynamically registered service users (i.e. users
 123   registered through `DynamicUser=1`).
 124
 125 * `$SYSTEMD_NSS_BYPASS_BUS=1` — if set, `nss-systemd` won't use D-Bus to do
 126   dynamic user lookups. This is primarily useful to make `nss-systemd` work
 127   safely from within `dbus-daemon`.
 128
 129 systemd-timedated:
 130
 131 * `$SYSTEMD_TIMEDATED_NTP_SERVICES=…` — colon-separated list of unit names of
 132   NTP client services. If set, `timedatectl set-ntp on` enables and starts the
 133   first existing unit listed in the environment variable, and
 134   `timedatectl set-ntp off` disables and stops all listed units.
 135
 136 systemd-sulogin-shell:
 137
 138 * `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
 139   root password is not available (such as when the root account is locked).
 140   See `sulogin(8)` for more details.
 141
 142 bootctl and other tools that access the EFI System Partition (ESP):
 143
 144 * `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
 145   relaxed. Specifically, validation checks that ensure the specified ESP path
 146   is a FAT file system are turned off, as are checks that the path is located
 147   on a GPT partition with the correct type UUID.
 148
 149 * `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
 150   may be used to override ESP path auto detection, and redirect any accesses to
 151   the ESP to the specified directory. Not that unlike with bootctl's --path=
 152   switch only very superficial validation of the specified path is done when
 153   this environment variable is used.
 154
 155 systemd itself:
 156
 157 * `$SYSTEMD_ACTIVATION_UNIT` — set for all NSS and PAM module invocations that
 158   are done by the service manager on behalf of a specific unit, in child
 159   processes that are later (after execve()) going to become unit
 160   processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM
 161   modules can use this information to determine in which context and on whose
 162   behalf they are being called, which may be useful to avoid deadlocks, for
 163   example to bypass IPC calls to the very service that is about to be
 164   started. Note that NSS and PAM modules should be careful to only rely on this
 165   data when invoked privileged, or possibly only when getppid() returns 1, as
 166   setting environment variables is of course possible in any even unprivileged
 167   contexts.
 168
 169 * `$SYSTEMD_ACTIVATION_SCOPE` — closely related to `$SYSTEMD_ACTIVATION_UNIT`,
 170   it is either set to `system` or `user` depending on whether the NSS/PAM
 171   module is called by systemd in `--system` or `--user` mode.
 172
 173 systemd-remount-fs:
 174
 175 * `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and and no entry for the root directory
 176   exists in /etc/fstab (this file always takes precedence), then the root
 177   directory is remounted writable. This is primarily used by
 178   systemd-gpt-auto-generator to ensure the root partition is mounted writable
 179   in accordance to the GPT partition flags.