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Add a bit of documentation for the various undocumented environment variables we honour
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ENVIRONMENT.md
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ENVIRONMENT.md
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# Known Environment Variables
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A number of systemd components take additional runtime parameters via
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environment variables. Many of these environment variables are not supported at
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the same level as command line switches and other interfaces are: we don't
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document them in the man pages and we make no stability guarantees for
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them. While they generally are unlikely to be dropped any time soon again, we
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do not want to guarantee that they stay around for good either.
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Below is an (incomprehensive) list of the environment variables understood by
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the various tools. Note that this list only covers environment variables not
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documented in the proper man pages.
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All tools:
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* `$SYSTEMD_IGNORE_CHROOT=1` — if set, don't check whether being invoked in a
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chroot() environment. This is particularly relevant for systemctl, as it will
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not alter its behaviour for chroot() environments if set. (Normally it
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refrains from talking to PID 1 in such a case.)
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* `$SD_EVENT_PROFILE_DELAYS=1` — if set, the sd-event event loop implementation
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will print latency information at runtime.
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systemctl:
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* `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
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listener, and instead always connect through the dbus-daemon D-bus broker.
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* `$SYSTEMCTL_INSTALL_CLIENT_SIDE=1` — if set, enable or disable unit files on
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the client side, instead of asking PID 1 to do this.
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* `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
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systemd-nspawn:
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* `$UNIFIED_CGROUP_HIERARCHY=1` — if set, force nspawn into unified cgroup
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hierarchy mode.
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* `$SYSTEMD_NSPAWN_API_VFS_WRITABLE=1` — if set, make /sys and /proc/sys and
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friends writable in the container. If set to "network", leave only
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/proc/sys/net writable.
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* `$SYSTEMD_NSPAWN_CONTAINER_SERVICE=…` — override the "service" name nspawn
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uses to register with machined. If unset defaults to "nspawn", but with this
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variable may be set to any other value.
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* `$SYSTEMD_NSPAWN_USE_CGNS=0` — if set, do not use cgroup namespacing, even if
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it is available.
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* `$SYSTEMD_NSPAWN_LOCK=0` — if set, do not lock container images when running.
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systemd-logind:
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* `$SYSTEMD_BYPASS_HIBERNATION_MEMORY_CHECK=1` — if set, report that
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hibernation is available even if the swap devices do not provide enough room
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for it.
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