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5.6 KiB
Known Environment Variables
A number of systemd components take additional runtime parameters via environment variables. Many of these environment variables are not supported at the same level as command line switches and other interfaces are: we don't document them in the man pages and we make no stability guarantees for them. While they generally are unlikely to be dropped any time soon again, we do not want to guarantee that they stay around for good either.
Below is an (incomprehensive) list of the environment variables understood by the various tools. Note that this list only covers environment variables not documented in the proper man pages.
All tools:
-
$SYSTEMD_OFFLINE=[0|1]
— if set to1
, thensystemctl
will refrain from talking to PID 1; this has the same effect as the historical detection ofchroot()
. Setting this variable to0
instead has a similar effect asSYSTEMD_IGNORE_CHROOT=1
; i.e. tools will try to communicate with PID 1 even if achroot()
environment is detected. You almost certainly want to set this to1
if you maintain a package build system or similar and are trying to use a modern container system and not plainchroot()
. -
$SYSTEMD_IGNORE_CHROOT=1
— if set, don't check whether being invoked in achroot()
environment. This is particularly relevant for systemctl, as it will not alter its behaviour forchroot()
environments if set. Normally it refrains from talking to PID 1 in such a case; turning most operations such asstart
into no-ops. If that's what's explicitly desired, you might consider settingSYSTEMD_OFFLINE=1
. -
$SD_EVENT_PROFILE_DELAYS=1
— if set, the sd-event event loop implementation will print latency information at runtime. -
$SYSTEMD_PROC_CMDLINE
— if set, may contain a string that is used as kernel command line instead of the actual one readable from /proc/cmdline. This is useful for debugging, in order to test generators and other code against specific kernel command lines. -
$SYSTEMD_BUS_TIMEOUT=SECS
— specifies the maximum time to wait for method call completion. If no time unit is specified, assumes seconds. The usual other units are understood, too (us, ms, s, min, h, d, w, month, y). If it is not set or set to 0, then the built-in default is used. -
$SYSTEMD_MEMPOOL=0
— if set the internal memory caching logic employed by hash tables is turned off, and libc malloc() is used for all allocations.
systemctl:
-
$SYSTEMCTL_FORCE_BUS=1
— if set, do not connect to PID1's private D-Bus listener, and instead always connect through the dbus-daemon D-bus broker. -
$SYSTEMCTL_INSTALL_CLIENT_SIDE=1
— if set, enable or disable unit files on the client side, instead of asking PID 1 to do this. -
$SYSTEMCTL_SKIP_SYSV=1
— if set, do not call out to SysV compatibility hooks.
systemd-nspawn:
-
$UNIFIED_CGROUP_HIERARCHY=1
— if set, force nspawn into unified cgroup hierarchy mode. -
$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1
— if set, make /sys and /proc/sys and friends writable in the container. If set to "network", leave only /proc/sys/net writable. -
$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…
— override the "service" name nspawn uses to register with machined. If unset defaults to "nspawn", but with this variable may be set to any other value. -
$SYSTEMD_NSPAWN_USE_CGNS=0
— if set, do not use cgroup namespacing, even if it is available. -
$SYSTEMD_NSPAWN_LOCK=0
— if set, do not lock container images when running.
systemd-logind:
$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1
— if set, report that hibernation is available even if the swap devices do not provide enough room for it.
installed systemd tests:
$SYSTEMD_TEST_DATA
— override the location of test data. This is useful if a test executable is moved to an arbitrary location.
nss-systemd:
-
$SYSTEMD_NSS_BYPASS_SYNTHETIC=1
— if set,nss-systemd
won't synthesize user/group records for theroot
andnobody
users if they are missing from/etc/passwd
. -
$SYSTEMD_NSS_DYNAMIC_BYPASS=1
— if set,nss-systemd
won't return user/group records for dynamically registered service users (i.e. users registered throughDynamicUser=1
). -
$SYSTEMD_NSS_BYPASS_BUS=1
— if set,nss-systemd
won't use D-Bus to do dynamic user lookups. This is primarily useful to makenss-systemd
work safely from withindbus-daemon
.
systemd-timedated:
$SYSTEMD_TIMEDATED_NTP_SERVICES=…
— colon-separated list of unit names of NTP client services. If set,timedatectl set-ntp on
enables and starts the first existing unit listed in the environment variable, andtimedatectl set-ntp off
disables and stops all listed units.
systemd itself:
-
$SYSTEMD_ACTIVATION_UNIT
— set for all NSS and PAM module invocations that are done by the service manager on behalf of a specific unit, in child processes that are later (after execve()) going to become unit processes. Contains the full unit name (e.g. "foobar.service"). NSS and PAM modules can use this information to determine in which context and on whose behalf they are being called, which may be useful to avoid deadlocks, for example to bypass IPC calls to the very service that is about to be started. Note that NSS and PAM modules should be careful to only rely on this data when invoked privileged, or possibly only when getppid() returns 1, as setting environment variables is of course possible in any even unprivileged contexts. -
$SYSTEMD_ACTIVATION_SCOPE
— closely related to$SYSTEMD_ACTIVATION_UNIT
, it is either set tosystem
oruser
depending on whether the NSS/PAM module is called by systemd in--system
or--user
mode.