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# 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:
* `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
chroot() environment. This is particularly relevant for systemctl, as it will
not alter its behaviour for chroot() environments if set. (Normally it
refrains from talking to PID 1 in such a case.)
* `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
will print latency information at runtime.
* `$SYSTEMD_PROC_CMDLINE` — if set, may contain a string that is used as kernel
command line instead of the actual one readable from /proc/cmdline. This is
useful for debugging, in order to test generators and other code against
specific kernel command lines.
systemctl:
* `$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.
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