systemd.exec(5)
NAME
systemd.exec - Execution environment configuration
SYNOPSIS
service.service, socket.socket, mount.mount, swap.swap
DESCRIPTION
Unit configuration files for services, sockets, mount points, and swap devices share a subset of configuration options which define the execution environment of spawned processes. This man page lists the configuration options shared by these four unit types. See systemd.unit(5) for the common options of all unit configuration files, and systemd.service(5), systemd.socket(5), systemd.swap(5), and systemd.mount(5) for more information on the specific unit configuration files. The execution specific configuration options are configured in the [Service], [Socket], [Mount], or [Swap] sections, depending on the unit type. In addition, options which control resources through Linux Control Groups (cgroups) are listed in systemd.resource-control(5). Those options complement options listed here.
AUTOMATIC DEPENDENCIES
A few execution parameters result in additional, automatic dependencies to be added. Units with WorkingDirectory= or RootDirectory= set automatically gain dependencies of type Requires= and After= on all mount units required to access the specified paths. This is equivalent to having them listed explicitly in RequiresMountsFor=. Similar, units with PrivateTmp= enabled automatically get mount unit dependencies for all mounts required to access /tmp and /var/tmp. Units whose standard output or error output is connected to journal, syslog or kmsg (or their combinations with console output, see below) automatically acquire dependencies of type After= on systemd-journald.socket.
OPTIONS
WorkingDirectory=
Takes a directory path relative to the service's root directory
specified by RootDirectory=, or the special value "~". Sets the
working directory for executed processes. If set to "~", the home
directory of the user specified in User= is used. If not set,
defaults to the root directory when systemd is running as a system
instance and the respective user's home directory if run as user.
If the setting is prefixed with the "-" character, a missing
working directory is not considered fatal. If RootDirectory= is not
set, then WorkingDirectory= is relative to the root of the system
running the service manager. Note that setting this parameter might
result in additional dependencies to be added to the unit (see
above).
RootDirectory=
Takes a directory path relative to the host's root directory (i.e.
the root of the system running the service manager). Sets the root
directory for executed processes, with the chroot(2) system call.
If this is used, it must be ensured that the process binary and all
its auxiliary files are available in the chroot() jail. Note that
setting this parameter might result in additional dependencies to
be added to the unit (see above).
The PrivateUsers= setting is particularly useful in conjunction
with RootDirectory=. For details, see below.
User=, Group=
Set the UNIX user or group that the processes are executed as,
respectively. Takes a single user or group name, or numeric ID as
argument. For system services (services run by the system service
manager, i.e. managed by PID 1) and for user services of the root
user (services managed by root's instance of systemd --user), the
default is "root", but User= may be used to specify a different
user. For user services of any other user, switching user identity
is not permitted, hence the only valid setting is the same user the
user's service manager is running as. If no group is set, the
default group of the user is used. This setting does not affect
commands whose command line is prefixed with "+".
DynamicUser=
Takes a boolean parameter. If set, a UNIX user and group pair is
allocated dynamically when the unit is started, and released as
soon as it is stopped. The user and group will not be added to
/etc/passwd or /etc/group, but are managed transiently during
runtime. The nss-systemd(8) glibc NSS module provides integration
of these dynamic users/groups into the system's user and group
databases. The user and group name to use may be configured via
User= and Group= (see above). If these options are not used and
dynamic user/group allocation is enabled for a unit, the name of
the dynamic user/group is implicitly derived from the unit name. If
the unit name without the type suffix qualifies as valid user name
it is used directly, otherwise a name incorporating a hash of it is
used. If a statically allocated user or group of the configured
name already exists, it is used and no dynamic user/group is
allocated. Dynamic users/groups are allocated from the UID/GID
range 61184...65519. It is recommended to avoid this range for
regular system or login users. At any point in time each UID/GID
from this range is only assigned to zero or one dynamically
allocated users/groups in use. However, UID/GIDs are recycled after
a unit is terminated. Care should be taken that any processes
running as part of a unit for which dynamic users/groups are
enabled do not leave files or directories owned by these
users/groups around, as a different unit might get the same UID/GID
assigned later on, and thus gain access to these files or
directories. If DynamicUser= is enabled, RemoveIPC=, PrivateTmp=
are implied. This ensures that the lifetime of IPC objects and
temporary files created by the executed processes is bound to the
runtime of the service, and hence the lifetime of the dynamic
user/group. Since /tmp and /var/tmp are usually the only
world-writable directories on a system this ensures that a unit
making use of dynamic user/group allocation cannot leave files
around after unit termination. Moreover ProtectSystem=strict and
ProtectHome=read-only are implied, thus prohibiting the service to
write to arbitrary file system locations. In order to allow the
service to write to certain directories, they have to be
whitelisted using ReadWritePaths=, but care must be taken so that
UID/GID recycling doesn't create security issues involving files
created by the service. Use RuntimeDirectory= (see below) in order
to assign a writable runtime directory to a service, owned by the
dynamic user/group and removed automatically when the unit is
terminated. Defaults to off.
SupplementaryGroups=
Sets the supplementary Unix groups the processes are executed as.
This takes a space-separated list of group names or IDs. This
option may be specified more than once, in which case all listed
groups are set as supplementary groups. When the empty string is
assigned, the list of supplementary groups is reset, and all
assignments prior to this one will have no effect. In any way, this
option does not override, but extends the list of supplementary
groups configured in the system group database for the user. This
does not affect commands prefixed with "+".
RemoveIPC=
Takes a boolean parameter. If set, all System V and POSIX IPC
objects owned by the user and group the processes of this unit are
run as are removed when the unit is stopped. This setting only has
an effect if at least one of User=, Group= and DynamicUser= are
used. It has no effect on IPC objects owned by the root user.
Specifically, this removes System V semaphores, as well as System V
and POSIX shared memory segments and message queues. If multiple
units use the same user or group the IPC objects are removed when
the last of these units is stopped. This setting is implied if
DynamicUser= is set.
Nice=
Sets the default nice level (scheduling priority) for executed
processes. Takes an integer between -20 (highest priority) and 19
(lowest priority). See setpriority(2) for details.
OOMScoreAdjust=
Sets the adjustment level for the Out-Of-Memory killer for executed
processes. Takes an integer between -1000 (to disable OOM killing
for this process) and 1000 (to make killing of this process under
memory pressure very likely). See proc.txt[1] for details.
IOSchedulingClass=
Sets the I/O scheduling class for executed processes. Takes an
integer between 0 and 3 or one of the strings none, realtime,
best-effort or idle. See ioprio_set(2) for details.
IOSchedulingPriority=
Sets the I/O scheduling priority for executed processes. Takes an
integer between 0 (highest priority) and 7 (lowest priority). The
available priorities depend on the selected I/O scheduling class
(see above). See ioprio_set(2) for details.
CPUSchedulingPolicy=
Sets the CPU scheduling policy for executed processes. Takes one of
other, batch, idle, fifo or rr. See sched_setscheduler(2) for
details.
CPUSchedulingPriority=
Sets the CPU scheduling priority for executed processes. The
available priority range depends on the selected CPU scheduling
policy (see above). For real-time scheduling policies an integer
between 1 (lowest priority) and 99 (highest priority) can be used.
See sched_setscheduler(2) for details.
CPUSchedulingResetOnFork=
Takes a boolean argument. If true, elevated CPU scheduling
priorities and policies will be reset when the executed processes
fork, and can hence not leak into child processes. See
sched_setscheduler(2) for details. Defaults to false.
CPUAffinity=
Controls the CPU affinity of the executed processes. Takes a list
of CPU indices or ranges separated by either whitespace or commas.
CPU ranges are specified by the lower and upper CPU indices
separated by a dash. This option may be specified more than once,
in which case the specified CPU affinity masks are merged. If the
empty string is assigned, the mask is reset, all assignments prior
to this will have no effect. See sched_setaffinity(2) for details.
UMask=
Controls the file mode creation mask. Takes an access mode in octal
notation. See umask(2) for details. Defaults to 0022.
Environment=
Sets environment variables for executed processes. Takes a
space-separated list of variable assignments. This option may be
specified more than once, in which case all listed variables will
be set. If the same variable is set twice, the later setting will
override the earlier setting. If the empty string is assigned to
this option, the list of environment variables is reset, all prior
assignments have no effect. Variable expansion is not performed
inside the strings, however, specifier expansion is possible. The $
character has no special meaning. If you need to assign a value
containing spaces to a variable, use double quotes (") for the
assignment.
Example:
Environment="VAR1=word1 word2" VAR2=word3 "VAR3=$word 5 6"
gives three variables "VAR1", "VAR2", "VAR3" with the values "word1
word2", "word3", "$word 5 6".
See environ(7) for details about environment variables.
EnvironmentFile=
Similar to Environment= but reads the environment variables from a
text file. The text file should contain new-line-separated variable
assignments. Empty lines, lines without an "=" separator, or lines
starting with ; or # will be ignored, which may be used for
commenting. A line ending with a backslash will be concatenated
with the following one, allowing multiline variable definitions.
The parser strips leading and trailing whitespace from the values
of assignments, unless you use double quotes (").
The argument passed should be an absolute filename or wildcard
expression, optionally prefixed with "-", which indicates that if
the file does not exist, it will not be read and no error or
warning message is logged. This option may be specified more than
once in which case all specified files are read. If the empty
string is assigned to this option, the list of file to read is
reset, all prior assignments have no effect.
The files listed with this directive will be read shortly before
the process is executed (more specifically, after all processes
from a previous unit state terminated. This means you can generate
these files in one unit state, and read it with this option in the
next).
Settings from these files override settings made with Environment=.
If the same variable is set twice from these files, the files will
be read in the order they are specified and the later setting will
override the earlier setting.
PassEnvironment=
Pass environment variables from the systemd system manager to
executed processes. Takes a space-separated list of variable names.
This option may be specified more than once, in which case all
listed variables will be set. If the empty string is assigned to
this option, the list of environment variables is reset, all prior
assignments have no effect. Variables that are not set in the
system manager will not be passed and will be silently ignored.
Variables passed from this setting are overridden by those passed
from Environment= or EnvironmentFile=.
Example:
PassEnvironment=VAR1 VAR2 VAR3
passes three variables "VAR1", "VAR2", "VAR3" with the values set
for those variables in PID1.
See environ(7) for details about environment variables.
StandardInput=
Controls where file descriptor 0 (STDIN) of the executed processes
is connected to. Takes one of null, tty, tty-force, tty-fail,
socket or fd.
If null is selected, standard input will be connected to /dev/null,
i.e. all read attempts by the process will result in immediate EOF.
If tty is selected, standard input is connected to a TTY (as
configured by TTYPath=, see below) and the executed process becomes
the controlling process of the terminal. If the terminal is already
being controlled by another process, the executed process waits
until the current controlling process releases the terminal.
tty-force is similar to tty, but the executed process is forcefully
and immediately made the controlling process of the terminal,
potentially removing previous controlling processes from the
terminal.
tty-fail is similar to tty but if the terminal already has a
controlling process start-up of the executed process fails.
The socket option is only valid in socket-activated services, and
only when the socket configuration file (see systemd.socket(5) for
details) specifies a single socket only. If this option is set,
standard input will be connected to the socket the service was
activated from, which is primarily useful for compatibility with
daemons designed for use with the traditional inetd(8) daemon.
The fd option connects the input stream to a single file descriptor
provided by a socket unit. A custom named file descriptor can be
specified as part of this option, after a ":" (e.g. "fd:foobar").
If no name is specified, "stdin" is assumed (i.e. "fd" is
equivalent to "fd:stdin"). At least one socket unit defining such
name must be explicitly provided via the Sockets= option, and file
descriptor name may differ from the name of its containing socket
unit. If multiple matches are found, the first one will be used.
See FileDescriptorName= in systemd.socket(5) for more details about
named descriptors and ordering.
This setting defaults to null.
StandardOutput=
Controls where file descriptor 1 (STDOUT) of the executed processes
is connected to. Takes one of inherit, null, tty, journal, syslog,
kmsg, journal+console, syslog+console, kmsg+console, socket or fd.
inherit duplicates the file descriptor of standard input for
standard output.
null connects standard output to /dev/null, i.e. everything written
to it will be lost.
tty connects standard output to a tty (as configured via TTYPath=,
see below). If the TTY is used for output only, the executed
process will not become the controlling process of the terminal,
and will not fail or wait for other processes to release the
terminal.
journal connects standard output with the journal which is
accessible via journalctl(1). Note that everything that is written
to syslog or kmsg (see below) is implicitly stored in the journal
as well, the specific two options listed below are hence supersets
of this one.
syslog connects standard output to the syslog(3) system syslog
service, in addition to the journal. Note that the journal daemon
is usually configured to forward everything it receives to syslog
anyway, in which case this option is no different from journal.
kmsg connects standard output with the kernel log buffer which is
accessible via dmesg(1), in addition to the journal. The journal
daemon might be configured to send all logs to kmsg anyway, in
which case this option is no different from journal.
journal+console, syslog+console and kmsg+console work in a similar
way as the three options above but copy the output to the system
console as well.
socket connects standard output to a socket acquired via socket
activation. The semantics are similar to the same option of
StandardInput=.
The fd option connects the output stream to a single file
descriptor provided by a socket unit. A custom named file
descriptor can be specified as part of this option, after a ":"
(e.g. "fd:foobar"). If no name is specified, "stdout" is assumed
(i.e. "fd" is equivalent to "fd:stdout"). At least one socket unit
defining such name must be explicitly provided via the Sockets=
option, and file descriptor name may differ from the name of its
containing socket unit. If multiple matches are found, the first
one will be used. See FileDescriptorName= in systemd.socket(5) for
more details about named descriptors and ordering.
If the standard output (or error output, see below) of a unit is
connected to the journal, syslog or the kernel log buffer, the unit
will implicitly gain a dependency of type After= on
systemd-journald.socket (also see the automatic dependencies
section above).
This setting defaults to the value set with DefaultStandardOutput=
in systemd-system.conf(5), which defaults to journal. Note that
setting this parameter might result in additional dependencies to
be added to the unit (see above).
StandardError=
Controls where file descriptor 2 (STDERR) of the executed processes
is connected to. The available options are identical to those of
StandardOutput=, with some exceptions: if set to inherit the file
descriptor used for standard output is duplicated for standard
error, while fd operates on the error stream and will look by
default for a descriptor named "stderr".
This setting defaults to the value set with DefaultStandardError=
in systemd-system.conf(5), which defaults to inherit. Note that
setting this parameter might result in additional dependencies to
be added to the unit (see above).
TTYPath=
Sets the terminal device node to use if standard input, output, or
error are connected to a TTY (see above). Defaults to /dev/console.
TTYReset=
Reset the terminal device specified with TTYPath= before and after
execution. Defaults to "no".
TTYVHangup=
Disconnect all clients which have opened the terminal device
specified with TTYPath= before and after execution. Defaults to
"no".
TTYVTDisallocate=
If the terminal device specified with TTYPath= is a virtual console
terminal, try to deallocate the TTY before and after execution.
This ensures that the screen and scrollback buffer is cleared.
Defaults to "no".
SyslogIdentifier=
Sets the process name to prefix log lines sent to the logging
system or the kernel log buffer with. If not set, defaults to the
process name of the executed process. This option is only useful
when StandardOutput= or StandardError= are set to syslog, journal
or kmsg (or to the same settings in combination with +console).
SyslogFacility=
Sets the syslog facility to use when logging to syslog. One of
kern, user, mail, daemon, auth, syslog, lpr, news, uucp, cron,
authpriv, ftp, local0, local1, local2, local3, local4, local5,
local6 or local7. See syslog(3) for details. This option is only
useful when StandardOutput= or StandardError= are set to syslog.
Defaults to daemon.
SyslogLevel=
The default syslog level to use when logging to syslog or the
kernel log buffer. One of emerg, alert, crit, err, warning, notice,
info, debug. See syslog(3) for details. This option is only useful
when StandardOutput= or StandardError= are set to syslog or kmsg.
Note that individual lines output by the daemon might be prefixed
with a different log level which can be used to override the
default log level specified here. The interpretation of these
prefixes may be disabled with SyslogLevelPrefix=, see below. For
details, see sd-daemon(3). Defaults to info.
SyslogLevelPrefix=
Takes a boolean argument. If true and StandardOutput= or
StandardError= are set to syslog, kmsg or journal, log lines
written by the executed process that are prefixed with a log level
will be passed on to syslog with this log level set but the prefix
removed. If set to false, the interpretation of these prefixes is
disabled and the logged lines are passed on as-is. For details
about this prefixing see sd-daemon(3). Defaults to true.
TimerSlackNSec=
Sets the timer slack in nanoseconds for the executed processes. The
timer slack controls the accuracy of wake-ups triggered by timers.
See prctl(2) for more information. Note that in contrast to most
other time span definitions this parameter takes an integer value
in nano-seconds if no unit is specified. The usual time units are
understood too.
LimitCPU=, LimitFSIZE=, LimitDATA=, LimitSTACK=, LimitCORE=, LimitRSS=,
LimitNOFILE=, LimitAS=, LimitNPROC=, LimitMEMLOCK=, LimitLOCKS=,
LimitSIGPENDING=, LimitMSGQUEUE=, LimitNICE=, LimitRTPRIO=,
LimitRTTIME=
Set soft and hard limits on various resources for executed
processes. See setrlimit(2) for details on the resource limit
concept. Resource limits may be specified in two formats: either as
single value to set a specific soft and hard limit to the same
value, or as colon-separated pair soft:hard to set both limits
individually (e.g. "LimitAS=4G:16G"). Use the string infinity to
configure no limit on a specific resource. The multiplicative
suffixes K, M, G, T, P and E (to the base 1024) may be used for
resource limits measured in bytes (e.g. LimitAS=16G). For the
limits referring to time values, the usual time units ms, s, min, h
and so on may be used (see systemd.time(7) for details). Note that
if no time unit is specified for LimitCPU= the default unit of
seconds is implied, while for LimitRTTIME= the default unit of
microseconds is implied. Also, note that the effective granularity
of the limits might influence their enforcement. For example, time
limits specified for LimitCPU= will be rounded up implicitly to
multiples of 1s. For LimitNICE= the value may be specified in two
syntaxes: if prefixed with "+" or "-", the value is understood as
regular Linux nice value in the range -20..19. If not prefixed like
this the value is understood as raw resource limit parameter in the
range 0..40 (with 0 being equivalent to 1).
Note that most process resource limits configured with these
options are per-process, and processes may fork in order to acquire
a new set of resources that are accounted independently of the
original process, and may thus escape limits set. Also note that
LimitRSS= is not implemented on Linux, and setting it has no
effect. Often it is advisable to prefer the resource controls
listed in systemd.resource-control(5) over these per-process
limits, as they apply to services as a whole, may be altered
dynamically at runtime, and are generally more expressive. For
example, MemoryLimit= is a more powerful (and working) replacement
for LimitRSS=.
For system units these resource limits may be chosen freely. For
user units however (i.e. units run by a per-user instance of
systemd(1)), these limits are bound by (possibly more restrictive)
per-user limits enforced by the OS.
Resource limits not configured explicitly for a unit default to the
value configured in the various DefaultLimitCPU=,
DefaultLimitFSIZE=, ... options available in systemd-
system.conf(5), and -- if not configured there -- the kernel or
per-user defaults, as defined by the OS (the latter only for user
services, see above).
Table 1. Resource limit directives, their equivalent ulimit shell
commands and the unit used
Directive ulimit equivalent Unit
LimitCPU= ulimit -t Seconds
LimitFSIZE= ulimit -f Bytes
LimitDATA= ulimit -d Bytes
LimitSTACK= ulimit -s Bytes
LimitCORE= ulimit -c Bytes
LimitRSS= ulimit -m Bytes
LimitNOFILE= ulimit -n Number of File
Descriptors
LimitAS= ulimit -v Bytes
LimitNPROC= ulimit -u Number of Processes
LimitMEMLOCK= ulimit -l Bytes
LimitLOCKS= ulimit -x Number of Locks
LimitSIGPENDING= ulimit -i Number of Queued
Signals
LimitMSGQUEUE= ulimit -q Bytes
LimitNICE= ulimit -e Nice Level
LimitRTPRIO= ulimit -r Realtime Priority
LimitRTTIME= No equivalent Microseconds
PAMName=
Sets the PAM service name to set up a session as. If set, the
executed process will be registered as a PAM session under the
specified service name. This is only useful in conjunction with the
User= setting. If not set, no PAM session will be opened for the
executed processes. See pam(8) for details.
CapabilityBoundingSet=
Controls which capabilities to include in the capability bounding
set for the executed process. See capabilities(7) for details.
Takes a whitespace-separated list of capability names, e.g.
CAP_SYS_ADMIN, CAP_DAC_OVERRIDE, CAP_SYS_PTRACE. Capabilities
listed will be included in the bounding set, all others are
removed. If the list of capabilities is prefixed with "~", all but
the listed capabilities will be included, the effect of the
assignment inverted. Note that this option also affects the
respective capabilities in the effective, permitted and inheritable
capability sets. If this option is not used, the capability
bounding set is not modified on process execution, hence no limits
on the capabilities of the process are enforced. This option may
appear more than once, in which case the bounding sets are merged.
If the empty string is assigned to this option, the bounding set is
reset to the empty capability set, and all prior settings have no
effect. If set to "~" (without any further argument), the bounding
set is reset to the full set of available capabilities, also
undoing any previous settings. This does not affect commands
prefixed with "+".
AmbientCapabilities=
Controls which capabilities to include in the ambient capability
set for the executed process. Takes a whitespace-separated list of
capability names, e.g. CAP_SYS_ADMIN, CAP_DAC_OVERRIDE,
CAP_SYS_PTRACE. This option may appear more than once in which case
the ambient capability sets are merged. If the list of capabilities
is prefixed with "~", all but the listed capabilities will be
included, the effect of the assignment inverted. If the empty
string is assigned to this option, the ambient capability set is
reset to the empty capability set, and all prior settings have no
effect. If set to "~" (without any further argument), the ambient
capability set is reset to the full set of available capabilities,
also undoing any previous settings. Note that adding capabilities
to ambient capability set adds them to the process's inherited
capability set.
Ambient capability sets are useful if you want to execute a process
as a non-privileged user but still want to give it some
capabilities. Note that in this case option keep-caps is
automatically added to SecureBits= to retain the capabilities over
the user change. AmbientCapabilities= does not affect commands
prefixed with "+".
SecureBits=
Controls the secure bits set for the executed process. Takes a
space-separated combination of options from the following list:
keep-caps, keep-caps-locked, no-setuid-fixup,
no-setuid-fixup-locked, noroot, and noroot-locked. This option may
appear more than once, in which case the secure bits are ORed. If
the empty string is assigned to this option, the bits are reset to
0. This does not affect commands prefixed with "+". See
capabilities(7) for details.
ReadWritePaths=, ReadOnlyPaths=, InaccessiblePaths=
Sets up a new file system namespace for executed processes. These
options may be used to limit access a process might have to the
file system hierarchy. Each setting takes a space-separated list of
paths relative to the host's root directory (i.e. the system
running the service manager). Note that if paths contain symlinks,
they are resolved relative to the root directory set with
RootDirectory=.
Paths listed in ReadWritePaths= are accessible from within the
namespace with the same access modes as from outside of it. Paths
listed in ReadOnlyPaths= are accessible for reading only, writing
will be refused even if the usual file access controls would permit
this. Nest ReadWritePaths= inside of ReadOnlyPaths= in order to
provide writable subdirectories within read-only directories. Use
ReadWritePaths= in order to whitelist specific paths for write
access if ProtectSystem=strict is used. Paths listed in
InaccessiblePaths= will be made inaccessible for processes inside
the namespace (along with everything below them in the file system
hierarchy).
Note that restricting access with these options does not extend to
submounts of a directory that are created later on. Non-directory
paths may be specified as well. These options may be specified more
than once, in which case all paths listed will have limited access
from within the namespace. If the empty string is assigned to this
option, the specific list is reset, and all prior assignments have
no effect.
Paths in ReadWritePaths=, ReadOnlyPaths= and InaccessiblePaths= may
be prefixed with "-", in which case they will be ignored when they
do not exist. Note that using this setting will disconnect
propagation of mounts from the service to the host (propagation in
the opposite direction continues to work). This means that this
setting may not be used for services which shall be able to install
mount points in the main mount namespace. Note that the effect of
these settings may be undone by privileged processes. In order to
set up an effective sandboxed environment for a unit it is thus
recommended to combine these settings with either
CapabilityBoundingSet=~CAP_SYS_ADMIN or SystemCallFilter=~@mount.
PrivateTmp=
Takes a boolean argument. If true, sets up a new file system
namespace for the executed processes and mounts private /tmp and
/var/tmp directories inside it that is not shared by processes
outside of the namespace. This is useful to secure access to
temporary files of the process, but makes sharing between processes
via /tmp or /var/tmp impossible. If this is enabled, all temporary
files created by a service in these directories will be removed
after the service is stopped. Defaults to false. It is possible to
run two or more units within the same private /tmp and /var/tmp
namespace by using the JoinsNamespaceOf= directive, see
systemd.unit(5) for details. This setting is implied if
DynamicUser= is set. For this setting the same restrictions
regarding mount propagation and privileges apply as for
ReadOnlyPaths= and related calls, see above.
PrivateDevices=
Takes a boolean argument. If true, sets up a new /dev namespace for
the executed processes and only adds API pseudo devices such as
/dev/null, /dev/zero or /dev/random (as well as the pseudo TTY
subsystem) to it, but no physical devices such as /dev/sda, system
memory /dev/mem, system ports /dev/port and others. This is useful
to securely turn off physical device access by the executed
process. Defaults to false. Enabling this option will install a
system call filter to block low-level I/O system calls that are
grouped in the @raw-io set, will also remove CAP_MKNOD and
CAP_SYS_RAWIO from the capability bounding set for the unit (see
above), and set DevicePolicy=closed (see systemd.resource-
control(5) for details). Note that using this setting will
disconnect propagation of mounts from the service to the host
(propagation in the opposite direction continues to work). This
means that this setting may not be used for services which shall be
able to install mount points in the main mount namespace. The /dev
namespace will be mounted read-only and 'noexec'. The latter may
break old programs which try to set up executable memory by using
mmap(2) of /dev/zero instead of using MAP_ANON. This setting is
implied if DynamicUser= is set. For this setting the same
restrictions regarding mount propagation and privileges apply as
for ReadOnlyPaths= and related calls, see above.
PrivateNetwork=
Takes a boolean argument. If true, sets up a new network namespace
for the executed processes and configures only the loopback network
device "lo" inside it. No other network devices will be available
to the executed process. This is useful to securely turn off
network access by the executed process. Defaults to false. It is
possible to run two or more units within the same private network
namespace by using the JoinsNamespaceOf= directive, see
systemd.unit(5) for details. Note that this option will disconnect
all socket families from the host, this includes AF_NETLINK and
AF_UNIX. The latter has the effect that AF_UNIX sockets in the
abstract socket namespace will become unavailable to the processes
(however, those located in the file system will continue to be
accessible).
PrivateUsers=
Takes a boolean argument. If true, sets up a new user namespace for
the executed processes and configures a minimal user and group
mapping, that maps the "root" user and group as well as the unit's
own user and group to themselves and everything else to the
"nobody" user and group. This is useful to securely detach the user
and group databases used by the unit from the rest of the system,
and thus to create an effective sandbox environment. All files,
directories, processes, IPC objects and other resources owned by
users/groups not equaling "root" or the unit's own will stay
visible from within the unit but appear owned by the "nobody" user
and group. If this mode is enabled, all unit processes are run
without privileges in the host user namespace (regardless if the
unit's own user/group is "root" or not). Specifically this means
that the process will have zero process capabilities on the host's
user namespace, but full capabilities within the service's user
namespace. Settings such as CapabilityBoundingSet= will affect only
the latter, and there's no way to acquire additional capabilities
in the host's user namespace. Defaults to off.
This setting is particularly useful in conjunction with
RootDirectory=, as the need to synchronize the user and group
databases in the root directory and on the host is reduced, as the
only users and groups who need to be matched are "root", "nobody"
and the unit's own user and group.
ProtectSystem=
Takes a boolean argument or the special values "full" or "strict".
If true, mounts the /usr and /boot directories read-only for
processes invoked by this unit. If set to "full", the /etc
directory is mounted read-only, too. If set to "strict" the entire
file system hierarchy is mounted read-only, except for the API file
system subtrees /dev, /proc and /sys (protect these directories
using PrivateDevices=, ProtectKernelTunables=,
ProtectControlGroups=). This setting ensures that any modification
of the vendor-supplied operating system (and optionally its
configuration, and local mounts) is prohibited for the service. It
is recommended to enable this setting for all long-running
services, unless they are involved with system updates or need to
modify the operating system in other ways. If this option is used,
ReadWritePaths= may be used to exclude specific directories from
being made read-only. This setting is implied if DynamicUser= is
set. For this setting the same restrictions regarding mount
propagation and privileges apply as for ReadOnlyPaths= and related
calls, see above. Defaults to off.
ProtectHome=
Takes a boolean argument or "read-only". If true, the directories
/home, /root and /run/user are made inaccessible and empty for
processes invoked by this unit. If set to "read-only", the three
directories are made read-only instead. It is recommended to enable
this setting for all long-running services (in particular
network-facing ones), to ensure they cannot get access to private
user data, unless the services actually require access to the
user's private data. This setting is implied if DynamicUser= is
set. For this setting the same restrictions regarding mount
propagation and privileges apply as for ReadOnlyPaths= and related
calls, see above.
ProtectKernelTunables=
Takes a boolean argument. If true, kernel variables accessible
through /proc/sys, /sys, /proc/sysrq-trigger, /proc/latency_stats,
/proc/acpi, /proc/timer_stats, /proc/fs and /proc/irq will be made
read-only to all processes of the unit. Usually, tunable kernel
variables should only be written at boot-time, with the sysctl.d(5)
mechanism. Almost no services need to write to these at runtime; it
is hence recommended to turn this on for most services. For this
setting the same restrictions regarding mount propagation and
privileges apply as for ReadOnlyPaths= and related calls, see
above. Defaults to off. Note that this option does not prevent
kernel tuning through IPC interfaces and external programs. However
InaccessiblePaths= can be used to make some IPC file system objects
inaccessible.
ProtectControlGroups=
Takes a boolean argument. If true, the Linux Control Groups
(cgroups(7)) hierarchies accessible through /sys/fs/cgroup will be
made read-only to all processes of the unit. Except for container
managers no services should require write access to the control
groups hierarchies; it is hence recommended to turn this on for
most services. For this setting the same restrictions regarding
mount propagation and privileges apply as for ReadOnlyPaths= and
related calls, see above. Defaults to off.
MountFlags=
Takes a mount propagation flag: shared, slave or private, which
control whether mounts in the file system namespace set up for this
unit's processes will receive or propagate mounts or unmounts. See
mount(2) for details. Defaults to shared. Use shared to ensure that
mounts and unmounts are propagated from the host to the container
and vice versa. Use slave to run processes so that none of their
mounts and unmounts will propagate to the host. Use private to also
ensure that no mounts and unmounts from the host will propagate
into the unit processes' namespace. Note that slave means that file
systems mounted on the host might stay mounted continuously in the
unit's namespace, and thus keep the device busy. Note that the file
system namespace related options (PrivateTmp=, PrivateDevices=,
ProtectSystem=, ProtectHome=, ProtectKernelTunables=,
ProtectControlGroups=, ReadOnlyPaths=, InaccessiblePaths=,
ReadWritePaths=) require that mount and unmount propagation from
the unit's file system namespace is disabled, and hence downgrade
shared to slave.
UtmpIdentifier=
Takes a four character identifier string for an utmp(5) and wtmp
entry for this service. This should only be set for services such
as getty implementations (such as agetty(8)) where utmp/wtmp
entries must be created and cleared before and after execution, or
for services that shall be executed as if they were run by a getty
process (see below). If the configured string is longer than four
characters, it is truncated and the terminal four characters are
used. This setting interprets %I style string replacements. This
setting is unset by default, i.e. no utmp/wtmp entries are created
or cleaned up for this service.
UtmpMode=
Takes one of "init", "login" or "user". If UtmpIdentifier= is set,
controls which type of utmp(5)/wtmp entries for this service are
generated. This setting has no effect unless UtmpIdentifier= is set
too. If "init" is set, only an INIT_PROCESS entry is generated and
the invoked process must implement a getty-compatible utmp/wtmp
logic. If "login" is set, first an INIT_PROCESS entry, followed by
a LOGIN_PROCESS entry is generated. In this case, the invoked
process must implement a login(1)-compatible utmp/wtmp logic. If
"user" is set, first an INIT_PROCESS entry, then a LOGIN_PROCESS
entry and finally a USER_PROCESS entry is generated. In this case,
the invoked process may be any process that is suitable to be run
as session leader. Defaults to "init".
SELinuxContext=
Set the SELinux security context of the executed process. If set,
this will override the automated domain transition. However, the
policy still needs to authorize the transition. This directive is
ignored if SELinux is disabled. If prefixed by "-", all errors will
be ignored. This does not affect commands prefixed with "+". See
setexeccon(3) for details.
AppArmorProfile=
Takes a profile name as argument. The process executed by the unit
will switch to this profile when started. Profiles must already be
loaded in the kernel, or the unit will fail. This result in a non
operation if AppArmor is not enabled. If prefixed by "-", all
errors will be ignored. This does not affect commands prefixed with
"+".
SmackProcessLabel=
Takes a SMACK64 security label as argument. The process executed by
the unit will be started under this label and SMACK will decide
whether the process is allowed to run or not, based on it. The
process will continue to run under the label specified here unless
the executable has its own SMACK64EXEC label, in which case the
process will transition to run under that label. When not
specified, the label that systemd is running under is used. This
directive is ignored if SMACK is disabled.
The value may be prefixed by "-", in which case all errors will be
ignored. An empty value may be specified to unset previous
assignments. This does not affect commands prefixed with "+".
IgnoreSIGPIPE=
Takes a boolean argument. If true, causes SIGPIPE to be ignored in
the executed process. Defaults to true because SIGPIPE generally is
useful only in shell pipelines.
NoNewPrivileges=
Takes a boolean argument. If true, ensures that the service process
and all its children can never gain new privileges. This option is
more powerful than the respective secure bits flags (see above), as
it also prohibits UID changes of any kind. This is the simplest and
most effective way to ensure that a process and its children can
never elevate privileges again. Defaults to false, but in the user
manager instance certain settings force NoNewPrivileges=yes,
ignoring the value of this setting. Those is the case when
SystemCallFilter=, SystemCallArchitectures=,
RestrictAddressFamilies=, PrivateDevices=, ProtectKernelTunables=,
ProtectKernelModules=, MemoryDenyWriteExecute=, or
RestrictRealtime= are specified.
SystemCallFilter=
Takes a space-separated list of system call names. If this setting
is used, all system calls executed by the unit processes except for
the listed ones will result in immediate process termination with
the SIGSYS signal (whitelisting). If the first character of the
list is "~", the effect is inverted: only the listed system calls
will result in immediate process termination (blacklisting). If
running in user mode, or in system mode, but without the
CAP_SYS_ADMIN capability (e.g. setting User=nobody),
NoNewPrivileges=yes is implied. This feature makes use of the
Secure Computing Mode 2 interfaces of the kernel ('seccomp
filtering') and is useful for enforcing a minimal sandboxing
environment. Note that the execve, exit, exit_group, getrlimit,
rt_sigreturn, sigreturn system calls and the system calls for
querying time and sleeping are implicitly whitelisted and do not
need to be listed explicitly. This option may be specified more
than once, in which case the filter masks are merged. If the empty
string is assigned, the filter is reset, all prior assignments will
have no effect. This does not affect commands prefixed with "+".
Note that strict system call filters may impact execution and error
handling code paths of the service invocation. Specifically, access
to the execve system call is required for the execution of the
service binary --- if it is blocked service invocation will
necessarily fail. Also, if execution of the service binary fails
for some reason (for example: missing service executable), the
error handling logic might require access to an additional set of
system calls in order to process and log this failure correctly. It
might be necessary to temporarily disable system call filters in
order to simplify debugging of such failures.
If you specify both types of this option (i.e. whitelisting and
blacklisting), the first encountered will take precedence and will
dictate the default action (termination or approval of a system
call). Then the next occurrences of this option will add or delete
the listed system calls from the set of the filtered system calls,
depending of its type and the default action. (For example, if you
have started with a whitelisting of read and write, and right after
it add a blacklisting of write, then write will be removed from the
set.)
As the number of possible system calls is large, predefined sets of
system calls are provided. A set starts with "@" character,
followed by name of the set.
Table 2. Currently predefined system call sets
Set Description
@basic-io System calls for basic
I/O: reading, writing,
seeking, file descriptor
duplication and closing
(read(2), write(2), and
related calls)
@clock System calls for changing
the system clock
(adjtimex(2),
settimeofday(2), and
related calls)
@cpu-emulation System calls for CPU
emulation functionality
(vm86(2) and related
calls)
@debug Debugging, performance
monitoring and tracing
functionality (ptrace(2),
perf_event_open(2) and
related calls)
@io-event Event loop system calls
(poll(2), select(2),
epoll(7), eventfd(2) and
related calls)
@ipc Pipes, SysV IPC, POSIX
Message Queues and other
IPC (mq_overview(7),
svipc(7))
@keyring Kernel keyring access
(keyctl(2) and related
calls)
@module Kernel module control
(init_module(2),
delete_module(2) and
related calls)
@mount File system mounting and
unmounting (mount(2),
chroot(2), and related
calls)
@network-io Socket I/O (including
local AF_UNIX): socket(7),
unix(7)
@obsolete Unusual, obsolete or
unimplemented
(create_module(2),
gtty(2), ...)
@privileged All system calls which
need super-user
capabilities
(capabilities(7))
@process Process control,
execution, namespaces
(clone(2), kill(2),
namespaces(7), ...
@raw-io Raw I/O port access
(ioperm(2), iopl(2),
pciconfig_read(), ...)
@resources System calls for changing
resource limits, memory
and scheduling parameters
(setrlimit(2),
setpriority(2), ...)
Note that as new system calls are added to the kernel, additional
system calls might be added to the groups above, so the contents of
the sets may change between systemd versions.
It is recommended to combine the file system namespacing related
options with SystemCallFilter=~@mount, in order to prohibit the
unit's processes to undo the mappings. Specifically these are the
options PrivateTmp=, PrivateDevices=, ProtectSystem=, ProtectHome=,
ProtectKernelTunables=, ProtectControlGroups=, ReadOnlyPaths=,
InaccessiblePaths= and ReadWritePaths=.
SystemCallErrorNumber=
Takes an "errno" error number name to return when the system call
filter configured with SystemCallFilter= is triggered, instead of
terminating the process immediately. Takes an error name such as
EPERM, EACCES or EUCLEAN. When this setting is not used, or when
the empty string is assigned, the process will be terminated
immediately when the filter is triggered.
SystemCallArchitectures=
Takes a space-separated list of architecture identifiers to include
in the system call filter. The known architecture identifiers are
the same as for ConditionArchitecture= described in
systemd.unit(5), as well as x32, mips64-n32, mips64-le-n32, and the
special identifier native. Only system calls of the specified
architectures will be permitted to processes of this unit. This is
an effective way to disable compatibility with non-native
architectures for processes, for example to prohibit execution of
32-bit x86 binaries on 64-bit x86-64 systems. The special native
identifier implicitly maps to the native architecture of the system
(or more strictly: to the architecture the system manager is
compiled for). If running in user mode, or in system mode, but
without the CAP_SYS_ADMIN capability (e.g. setting User=nobody),
NoNewPrivileges=yes is implied. Note that setting this option to a
non-empty list implies that native is included too. By default,
this option is set to the empty list, i.e. no architecture system
call filtering is applied.
RestrictAddressFamilies=
Restricts the set of socket address families accessible to the
processes of this unit. Takes a space-separated list of address
family names to whitelist, such as AF_UNIX, AF_INET or AF_INET6.
When prefixed with ~ the listed address families will be applied as
blacklist, otherwise as whitelist. Note that this restricts access
to the socket(2) system call only. Sockets passed into the process
by other means (for example, by using socket activation with socket
units, see systemd.socket(5)) are unaffected. Also, sockets created
with socketpair() (which creates connected AF_UNIX sockets only)
are unaffected. Note that this option has no effect on 32-bit x86
and is ignored (but works correctly on x86-64). If running in user
mode, or in system mode, but without the CAP_SYS_ADMIN capability
(e.g. setting User=nobody), NoNewPrivileges=yes is implied. By
default, no restriction applies, all address families are
accessible to processes. If assigned the empty string, any previous
list changes are undone.
Use this option to limit exposure of processes to remote systems,
in particular via exotic network protocols. Note that in most
cases, the local AF_UNIX address family should be included in the
configured whitelist as it is frequently used for local
communication, including for syslog(2) logging. This does not
affect commands prefixed with "+".
ProtectKernelModules=
Takes a boolean argument. If true, explicit module loading will be
denied. This allows to turn off module load and unload operations
on modular kernels. It is recommended to turn this on for most
services that do not need special file systems or extra kernel
modules to work. Default to off. Enabling this option removes
CAP_SYS_MODULE from the capability bounding set for the unit, and
installs a system call filter to block module system calls, also
/usr/lib/modules is made inaccessible. For this setting the same
restrictions regarding mount propagation and privileges apply as
for ReadOnlyPaths= and related calls, see above. Note that limited
automatic module loading due to user configuration or kernel
mapping tables might still happen as side effect of requested user
operations, both privileged and unprivileged. To disable module
auto-load feature please see sysctl.d(5) kernel.modules_disabled
mechanism and /proc/sys/kernel/modules_disabled documentation.
Personality=
Controls which kernel architecture uname(2) shall report, when
invoked by unit processes. Takes one of the architecture
identifiers x86, x86-64, ppc, ppc-le, ppc64, ppc64-le, s390 or
s390x. Which personality architectures are supported depends on the
system architecture. Usually the 64bit versions of the various
system architectures support their immediate 32bit personality
architecture counterpart, but no others. For example, x86-64
systems support the x86-64 and x86 personalities but no others. The
personality feature is useful when running 32-bit services on a
64-bit host system. If not specified, the personality is left
unmodified and thus reflects the personality of the host system's
kernel.
RuntimeDirectory=, RuntimeDirectoryMode=
Takes a list of directory names. If set, one or more directories by
the specified names will be created below /run (for system
services) or below $XDG_RUNTIME_DIR (for user services) when the
unit is started, and removed when the unit is stopped. The
directories will have the access mode specified in
RuntimeDirectoryMode=, and will be owned by the user and group
specified in User= and Group=. Use this to manage one or more
runtime directories of the unit and bind their lifetime to the
daemon runtime. The specified directory names must be relative, and
may not include a "/", i.e. must refer to simple directories to
create or remove. This is particularly useful for unprivileged
daemons that cannot create runtime directories in /run due to lack
of privileges, and to make sure the runtime directory is cleaned up
automatically after use. For runtime directories that require more
complex or different configuration or lifetime guarantees, please
consider using tmpfiles.d(5).
MemoryDenyWriteExecute=
Takes a boolean argument. If set, attempts to create memory
mappings that are writable and executable at the same time, or to
change existing memory mappings to become executable, or mapping
shared memory segments as executable are prohibited. Specifically,
a system call filter is added that rejects mmap(2) system calls
with both PROT_EXEC and PROT_WRITE set, mprotect(2) system calls
with PROT_EXEC set and shmat(2) system calls with SHM_EXEC set.
Note that this option is incompatible with programs that generate
program code dynamically at runtime, such as JIT execution engines,
or programs compiled making use of the code "trampoline" feature of
various C compilers. This option improves service security, as it
makes harder for software exploits to change running code
dynamically.
RestrictRealtime=
Takes a boolean argument. If set, any attempts to enable realtime
scheduling in a process of the unit are refused. This restricts
access to realtime task scheduling policies such as SCHED_FIFO,
SCHED_RR or SCHED_DEADLINE. See sched(7) for details about these
scheduling policies. Realtime scheduling policies may be used to
monopolize CPU time for longer periods of time, and may hence be
used to lock up or otherwise trigger Denial-of-Service situations
on the system. It is hence recommended to restrict access to
realtime scheduling to the few programs that actually require them.
Defaults to off.
ENVIRONMENT VARIABLES IN SPAWNED PROCESSES
Processes started by the system are executed in a clean environment in
which select variables listed below are set. System processes started
by systemd do not inherit variables from PID 1, but processes started
by user systemd instances inherit all environment variables from the
user systemd instance.
$PATH
Colon-separated list of directories to use when launching
executables. Systemd uses a fixed value of
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin.
$LANG
Locale. Can be set in locale.conf(5) or on the kernel command line
(see systemd(1) and kernel-command-line(7)).
$USER, $LOGNAME, $HOME, $SHELL
User name (twice), home directory, and the login shell. The
variables are set for the units that have User= set, which includes
user systemd instances. See passwd(5).
$INVOCATION_ID
Contains a randomized, unique 128bit ID identifying each runtime
cycle of the unit, formatted as 32 character hexadecimal string. A
new ID is assigned each time the unit changes from an inactive
state into an activating or active state, and may be used to
identify this specific runtime cycle, in particular in data stored
offline, such as the journal. The same ID is passed to all
processes run as part of the unit.
$XDG_RUNTIME_DIR
The directory for volatile state. Set for the user systemd
instance, and also in user sessions. See pam_systemd(8).
$XDG_SESSION_ID, $XDG_SEAT, $XDG_VTNR
The identifier of the session, the seat name, and virtual terminal
of the session. Set by pam_systemd(8) for login sessions.
$XDG_SEAT and $XDG_VTNR will only be set when attached to a seat
and a tty.
$MAINPID
The PID of the unit's main process if it is known. This is only set
for control processes as invoked by ExecReload= and similar.
$MANAGERPID
The PID of the user systemd instance, set for processes spawned by
it.
$LISTEN_FDS, $LISTEN_PID, $LISTEN_FDNAMES
Information about file descriptors passed to a service for socket
activation. See sd_listen_fds(3).
$NOTIFY_SOCKET
The socket sd_notify() talks to. See sd_notify(3).
$WATCHDOG_PID, $WATCHDOG_USEC
Information about watchdog keep-alive notifications. See
sd_watchdog_enabled(3).
$TERM
Terminal type, set only for units connected to a terminal
(StandardInput=tty, StandardOutput=tty, or StandardError=tty). See
termcap(5).
$JOURNAL_STREAM
If the standard output or standard error output of the executed
processes are connected to the journal (for example, by setting
StandardError=journal) $JOURNAL_STREAM contains the device and
inode numbers of the connection file descriptor, formatted in
decimal, separated by a colon (":"). This permits invoked processes
to safely detect whether their standard output or standard error
output are connected to the journal. The device and inode numbers
of the file descriptors should be compared with the values set in
the environment variable to determine whether the process output is
still connected to the journal. Note that it is generally not
sufficient to only check whether $JOURNAL_STREAM is set at all as
services might invoke external processes replacing their standard
output or standard error output, without unsetting the environment
variable.
This environment variable is primarily useful to allow services to
optionally upgrade their used log protocol to the native journal
protocol (using sd_journal_print(3) and other functions) if their
standard output or standard error output is connected to the
journal anyway, thus enabling delivery of structured metadata along
with logged messages.
$SERVICE_RESULT
Only defined for the service unit type, this environment variable
is passed to all ExecStop= and ExecStopPost= processes, and encodes
the service "result". Currently, the following values are defined:
"timeout" (in case of an operation timeout), "exit-code" (if a
service process exited with a non-zero exit code; see $EXIT_CODE
below for the actual exit code returned), "signal" (if a service
process was terminated abnormally by a signal; see $EXIT_CODE below
for the actual signal used for the termination), "core-dump" (if a
service process terminated abnormally and dumped core), "watchdog"
(if the watchdog keep-alive ping was enabled for the service but it
missed the deadline), or "resources" (a catch-all condition in case
a system operation failed).
This environment variable is useful to monitor failure or
successful termination of a service. Even though this variable is
available in both ExecStop= and ExecStopPost=, it is usually a
better choice to place monitoring tools in the latter, as the
former is only invoked for services that managed to start up
correctly, and the latter covers both services that failed during
their start-up and those which failed during their runtime.
$EXIT_CODE, $EXIT_STATUS
Only defined for the service unit type, these environment variables
are passed to all ExecStop=, ExecStopPost= processes and contain
exit status/code information of the main process of the service.
For the precise definition of the exit code and status, see
wait(2). $EXIT_CODE is one of "exited", "killed", "dumped".
$EXIT_STATUS contains the numeric exit code formatted as string if
$EXIT_CODE is "exited", and the signal name in all other cases.
Note that these environment variables are only set if the service
manager succeeded to start and identify the main process of the
service.
Table 3. Summary of possible service result variable values
$SERVICE_RESULT $EXIT_STATUS $EXIT_CODE
"timeout" "killed" "TERM", "KILL"
"exited" "0", "1", "2", "3",
..., "255"
"exit-code" "exited" "0", "1", "2", "3",
..., "255"
"signal" "killed" "HUP", "INT",
"KILL", ...
"core-dump" "dumped" "ABRT", "SEGV",
"QUIT", ...
"watchdog" "dumped" "ABRT"
"killed" "TERM", "KILL"
"exited" "0", "1", "2", "3",
..., "255"
"resources" any of the above any of the above
Note: the process may be also terminated by a signal not
sent by systemd. In particular the process may send an
arbitrary signal to itself in a handler for any of the
non-maskable signals. Nevertheless, in the "timeout" and
"watchdog" rows above only the signals that systemd sends
have been included.
Additional variables may be configured by the following means: for
processes spawned in specific units, use the Environment=,
EnvironmentFile= and PassEnvironment= options above; to specify
variables globally, use DefaultEnvironment= (see systemd-
system.conf(5)) or the kernel option systemd.setenv= (see systemd(1)).
Additional variables may also be set through PAM, cf. pam_env(8).
SEE ALSO
systemd(1), systemctl(1), journalctl(8), systemd.unit(5), systemd.service(5), systemd.socket(5), systemd.swap(5), systemd.mount(5), systemd.kill(5), systemd.resource-control(5), systemd.time(7), systemd.directives(7), tmpfiles.d(5), exec(3)
NOTES
1. proc.txt
https://www.kernel.org/doc/Documentation/filesystems/proc.txt
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Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
