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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:
* `$SYSTEMD_OFFLINE=[0|1]` — if set to `1`, then `systemctl` will
refrain from talking to PID 1; this has the same effect as the historical
detection of `chroot()`. Setting this variable to `0` instead has a similar
effect as `SYSTEMD_IGNORE_CHROOT=1`; i.e. tools will try to
communicate with PID 1 even if a `chroot()` environment is detected.
You almost certainly want to set this to `1` if you maintain a package build system
or similar and are trying to use a modern container system and not plain
`chroot()`.
* `$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; turning most operations such
as `start` into no-ops. If that's what's explicitly desired, you might
consider setting `SYSTEMD_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.
* `$SYSTEMD_IN_INITRD` — takes a boolean. If set, overrides initrd detection.
This is useful for debugging and testing initrd-only programs in the main
system.
* `$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.
* `$SYSTEMD_MEMPOOL=0` — if set, the internal memory caching logic employed by
hash tables is turned off, and libc malloc() is used for all allocations.
* `$SYSTEMD_EMOJI=0` — if set, tools such as "systemd-analyze security" will
not output graphical smiley emojis, but ASCII alternatives instead. Note that
this only controls use of Unicode emoji glyphs, and has no effect on other
Unicode glyphs.
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.
* `$SYSTEMD_NSPAWN_TMPFS_TMP=0` — if set, do not overmount /tmp in the
container with a tmpfs, but leave the directory from the image in place.
systemd-logind:
* `$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.
systemd-udevd:
* `$NET_NAMING_SCHEME=` – if set, takes a network naming scheme (i.e. one of
"v238", "v239", "v240"…, or the special value "latest") as parameter. If
specified udev's net_id builtin will follow the specified naming scheme when
determining stable network interface names. This may be used to revert to
naming schemes of older udev versions, in order to provide more stable naming
across updates. This environment variable takes precedence over the kernel
command line option `net.naming-scheme=`, except if the value is prefixed
with `:` in which case the kernel command line option takes precedence, if it
is specified as well.
installed systemd tests:
* `$SYSTEMD_TEST_DATA` — override the location of test data. This is useful if
a test executable is moved to an arbitrary location.
nss-systemd:
* `$SYSTEMD_NSS_BYPASS_SYNTHETIC=1` — if set, `nss-systemd` won't synthesize
user/group records for the `root` and `nobody` users if they are missing from
`/etc/passwd`.
* `$SYSTEMD_NSS_DYNAMIC_BYPASS=1` — if set, `nss-systemd` won't return
user/group records for dynamically registered service users (i.e. users
registered through `DynamicUser=1`).
* `$SYSTEMD_NSS_BYPASS_BUS=1` — if set, `nss-systemd` won't use D-Bus to do
dynamic user lookups. This is primarily useful to make `nss-systemd` work
safely from within `dbus-daemon`.
systemd-timedated:
* `$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, and
`timedatectl set-ntp off` disables and stops all listed units.
systemd-sulogin-shell:
* `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
root password is not available (such as when the root account is locked).
See `sulogin(8)` for more details.
bootctl and other tools that access the EFI System Partition (ESP):
* `$SYSTEMD_RELAX_ESP_CHECKS=1` — if set, the ESP validation checks are
relaxed. Specifically, validation checks that ensure the specified ESP path
is a FAT file system are turned off, as are checks that the path is located
on a GPT partition with the correct type UUID.
* `$SYSTEMD_ESP_PATH=…` — override the path to the EFI System Partition. This
may be used to override ESP path auto detection, and redirect any accesses to
the ESP to the specified directory. Not that unlike with bootctl's --path=
switch only very superficial validation of the specified path is done when
this environment variable is used.
systemd itself:
* `$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 to `system` or `user` depending on whether the NSS/PAM
module is called by systemd in `--system` or `--user` mode.
systemd-remount-fs:
* `$SYSTEMD_REMOUNT_ROOT_RW=1` — if set and and no entry for the root directory
exists in /etc/fstab (this file always takes precedence), then the root
directory is remounted writable. This is primarily used by
systemd-gpt-auto-generator to ensure the root partition is mounted writable
in accordance to the GPT partition flags.
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