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NAME | DESCRIPTION | USER JOURNAL FIELDS | TRUSTED JOURNAL FIELDS | KERNEL JOURNAL FIELDS | FIELDS TO LOG ON BEHALF OF A DIFFERENT PROGRAM | ADDRESS FIELDS | SEE ALSO | NOTES | COLOPHON |
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SYSTEMD.JOURNAL-FIELDS(7) systemd.journal-fieldsSYSTEMD.JOURNAL-FIELDS(7)
systemd.journal-fields - Special journal fields
Entries in the journal (as written by systemd-journald.service(8))
resemble a UNIX process environment block in syntax but with field
values that may include binary data, and with non-unique field
names permitted. Primarily, field values are formatted UTF-8 text
strings — binary encoding is used only where formatting as UTF-8
text strings makes little sense. New fields may freely be defined
by applications, but a few fields have special meanings, which are
listed below. Typically, fields may only appear once per log
entry, however there are special exceptions: some fields may
appear more than once per entry, in which case this is explicitly
mentioned below. Even though the logging subsystem makes no
restrictions on which fields to accept non-unique values for, it
is strongly recommended to avoid relying on this for the fields
listed below (except where listed otherwise, as mentioned) in
order to avoid unnecessary incompatibilities with other
applications.
User fields are fields that are directly passed from clients and
stored in the journal.
MESSAGE=
The human-readable message string for this entry. This is
supposed to be the primary text shown to the user. It is
usually not translated (but might be in some cases), and is
not supposed to be parsed for metadata. In order to encode
multiple lines in a single log entry, separate them by newline
characters (ASCII code 10), but encode them as a single
MESSAGE= field. Do not add multiple values of this field type
to the same entry (also see above), as consuming applications
generally do not expect this and are unlikely to show all
values in that case.
MESSAGE_ID=
A 128-bit message identifier ID for recognizing certain
message types, if this is desirable. This should contain a
128-bit ID formatted as a lower-case hexadecimal string,
without any separating dashes or suchlike. This is recommended
to be a UUID-compatible ID, but this is not enforced, and
formatted differently. Developers can generate a new ID for
this purpose with systemd-id128 new.
PRIORITY=
A priority value between 0 ("emerg") and 7 ("debug") formatted
as a decimal string. This field is compatible with syslog's
priority concept.
CODE_FILE=, CODE_LINE=, CODE_FUNC=
The code location generating this message, if known. Contains
the source filename, the line number and the function name.
ERRNO=
The low-level Unix error number causing this entry, if any.
Contains the numeric value of errno(3) formatted as a decimal
string.
Added in version 188.
INVOCATION_ID=, USER_INVOCATION_ID=
A randomized, unique 128-bit ID identifying each runtime cycle
of the unit. This is different from _SYSTEMD_INVOCATION_ID in
that it is only used for messages coming from systemd code
(e.g. logs from the system/user manager or from forked
processes performing systemd-related setup).
Added in version 245.
SYSLOG_FACILITY=, SYSLOG_IDENTIFIER=, SYSLOG_PID=,
SYSLOG_TIMESTAMP=
Syslog compatibility fields containing the facility (formatted
as decimal string), the identifier string (i.e. "tag"), the
client PID, and the timestamp as specified in the original
datagram. (Note that the tag is usually derived from glibc's
program_invocation_short_name variable, see
program_invocation_short_name(3).)
Note that the journal service does not validate the values of
any structured journal fields whose name is not prefixed with
an underscore, and this includes any syslog related fields
such as these. Hence, applications that supply a facility,
PID, or log level are expected to do so properly formatted,
i.e. as integers formatted as decimals.
SYSLOG_RAW=
The original contents of the syslog line as received in the
syslog datagram. This field is only included if the MESSAGE=
field was modified compared to the original payload or the
timestamp could not be located properly and is not included in
SYSLOG_TIMESTAMP=. Message truncation occurs when the message
contains leading or trailing whitespace (trailing and leading
whitespace is stripped), or it contains an embedded NUL byte
(the NUL byte and anything after it is not included). Thus,
the original syslog line is either stored as SYSLOG_RAW= or it
can be recreated based on the stored priority and facility,
timestamp, identifier, and the message payload in MESSAGE=.
Added in version 240.
DOCUMENTATION=
A documentation URL with further information about the topic
of the log message. Tools such as journalctl will include a
hyperlink to a URL specified this way in their output. Should
be an "http://", "https://", "file:/", "man:" or "info:" URL.
Added in version 246.
TID=
The numeric thread ID (TID) the log message originates from.
Added in version 247.
UNIT=, USER_UNIT=
The name of a unit. Used by the system and user managers when
logging about specific units.
When --unit=name or --user-unit=name are used with
journalctl(1), a match pattern that includes
"UNIT=name.service" or "USER_UNIT=name.service" will be
generated.
Added in version 251.
Fields prefixed with an underscore are trusted fields, i.e. fields
that are implicitly added by the journal and cannot be altered by
client code.
_PID=, _UID=, _GID=
The process number, user number, and group number of the
process the journal entry originates from formatted as
decimals. Note that entries obtained via "stdout" or "stderr"
of forked processes will contain credentials valid for a
parent process (that initiated the connection to
systemd-journald).
_COMM=, _EXE=, _CMDLINE=
The name, the executable path, and the command line of the
process the journal entry originates from.
_CAP_EFFECTIVE=
The effective capabilities(7) of the process the journal entry
originates from.
Added in version 206.
_AUDIT_SESSION=, _AUDIT_LOGINUID=
The session and login UID of the process the journal entry
originates from, as maintained by the kernel audit subsystem.
_SYSTEMD_CGROUP=, _SYSTEMD_SLICE=, _SYSTEMD_UNIT=,
_SYSTEMD_USER_UNIT=, _SYSTEMD_USER_SLICE=, _SYSTEMD_SESSION=,
_SYSTEMD_OWNER_UID=
The control group path in the systemd hierarchy, the systemd
slice unit name, the systemd unit name, the unit name in the
systemd user manager (if any), the systemd session ID (if
any), and the owner UID of the systemd user unit or systemd
session (if any) of the process the journal entry originates
from.
_SELINUX_CONTEXT=
The SELinux security context (label) of the process the
journal entry originates from.
_SOURCE_REALTIME_TIMESTAMP=
The earliest trusted timestamp of the message, if any is known
that is different from the reception time of the journal. The
timestamp is the CLOCK_REALTIME time in microseconds,
formatted as a decimal.
_SOURCE_BOOTTIME_TIMESTAMP=
The earliest trusted timestamp of the message in the
CLOCK_BOOTTIME time, in the same format as
_SOURCE_REALTIME_TIMESTAMP=.
Added in version 257.
_BOOT_ID=
The kernel boot ID for the boot the message was generated in,
formatted as a 128-bit hexadecimal string.
_MACHINE_ID=
The machine ID of the originating host, as described in
machine-id(5).
_SYSTEMD_INVOCATION_ID=
The invocation ID for the runtime cycle of the unit the
message was generated in, as available to processes of the
unit in $INVOCATION_ID (see systemd.exec(5)).
Added in version 233.
_HOSTNAME=
The hostname of the originating host.
_TRANSPORT=
How the entry was received by the journal service. Valid
transports are:
audit
for those read from the kernel audit subsystem
Added in version 227.
driver
for internally generated messages
Added in version 205.
syslog
for those received via the local syslog socket with the
syslog protocol
Added in version 205.
journal
for those received via the native journal protocol
Added in version 205.
stdout
for those read from a service's standard output or error
output
Added in version 205.
kernel
for those read from the kernel
Added in version 205.
_STREAM_ID=
Only applies to "_TRANSPORT=stdout" records: specifies a
randomized 128-bit ID assigned to the stream connection when
it was first created. This ID is useful to reconstruct
individual log streams from the log records: all log records
carrying the same stream ID originate from the same stream.
Added in version 235.
_LINE_BREAK=
Only applies to "_TRANSPORT=stdout" records: indicates that
the log message in the standard output/error stream was not
terminated with a normal newline character ("\n", i.e. ASCII
10). Specifically, when set this field is one of nul (in case
the line was terminated by a NUL byte), line-max (in case the
maximum log line length was reached, as configured with
LineMax= in journald.conf(5)), eof (if this was the last log
record of a stream and the stream ended without a final
newline character), or pid-change (if the process which
generated the log output changed in the middle of a line).
Note that this record is not generated when a normal newline
character was used for marking the log line end.
Added in version 235.
_NAMESPACE=
If this file was written by a systemd-journald instance
managing a journal namespace that is not the default, this
field contains the namespace identifier. See
systemd-journald.service(8) for details about journal
namespaces.
Added in version 245.
_RUNTIME_SCOPE=
A string field that specifies the runtime scope in which the
message was logged. If "initrd", the log message was processed
while the system was running inside the initrd. If "system",
the log message was generated after the system switched
execution to the host root filesystem.
Added in version 252.
Kernel fields are fields that are used by messages originating in
the kernel and stored in the journal.
_KERNEL_DEVICE=
The kernel device name. If the entry is associated to a block
device, contains the major and minor numbers of the device
node, separated by ":" and prefixed by "b". Similarly for
character devices, but prefixed by "c". For network devices,
this is the interface index prefixed by "n". For all other
devices, this is the subsystem name prefixed by "+", followed
by ":", followed by the kernel device name.
Added in version 189.
_KERNEL_SUBSYSTEM=
The kernel subsystem name.
Added in version 189.
_UDEV_SYSNAME=
The kernel device name as it shows up in the device tree below
/sys/.
Added in version 189.
_UDEV_DEVNODE=
The device node path of this device in /dev/.
Added in version 189.
_UDEV_DEVLINK=
Additional symlink names pointing to the device node in /dev/.
This field is frequently set more than once per entry.
Added in version 189.
Fields in this section are used by programs to specify that they
are logging on behalf of another program or unit.
Fields used by the systemd-coredump coredump kernel helper:
COREDUMP_UNIT=, COREDUMP_USER_UNIT=
Used to annotate messages containing coredumps from system and
session units. See coredumpctl(1).
Added in version 198.
Privileged programs (currently UID 0) may attach OBJECT_PID= to a
message. This will instruct systemd-journald to attach additional
fields on behalf of the caller:
OBJECT_PID=PID
PID of the program that this message pertains to.
Added in version 205.
OBJECT_UID=, OBJECT_GID=, OBJECT_COMM=, OBJECT_EXE=,
OBJECT_CMDLINE=, OBJECT_AUDIT_SESSION=, OBJECT_AUDIT_LOGINUID=,
OBJECT_SYSTEMD_CGROUP=, OBJECT_SYSTEMD_SESSION=,
OBJECT_SYSTEMD_OWNER_UID=, OBJECT_SYSTEMD_UNIT=,
OBJECT_SYSTEMD_USER_UNIT=
These are additional fields added automatically by
systemd-journald. Their meaning is the same as _UID=, _GID=,
_COMM=, _EXE=, _CMDLINE=, _AUDIT_SESSION=, _AUDIT_LOGINUID=,
_SYSTEMD_CGROUP=, _SYSTEMD_SESSION=, _SYSTEMD_UNIT=,
_SYSTEMD_USER_UNIT=, and _SYSTEMD_OWNER_UID= as described
above, except that the process identified by PID is described,
instead of the process which logged the message.
Added in version 205.
OBJECT_SYSTEMD_INVOCATION_ID=
An additional field added automatically by systemd-journald.
The meaning is mostly the same as _SYSTEMD_INVOCATION_ID=,
with the difference described above.
Added in version 235.
During serialization into external formats, such as the Journal
Export Format[1] or the Journal JSON Format[2], the addresses of
journal entries are serialized into fields prefixed with double
underscores. Note that these are not proper fields when stored in
the journal but for addressing metadata of entries. They cannot be
written as part of structured log entries via calls such as
sd_journal_send(3). They may also not be used as matches for
sd_journal_add_match(3).
__CURSOR=
The cursor for the entry. A cursor is an opaque text string
that uniquely describes the position of an entry in the
journal and is portable across machines, platforms and journal
files.
__REALTIME_TIMESTAMP=
The wallclock time (CLOCK_REALTIME) at the point in time the
entry was received by the journal, in microseconds since the
epoch UTC, formatted as a decimal. This has different
properties from "_SOURCE_REALTIME_TIMESTAMP=", as it is
usually a bit later but more likely to be monotonic.
__MONOTONIC_TIMESTAMP=
The monotonic time (CLOCK_MONOTONIC) at the point in time the
entry was received by the journal in microseconds, formatted
as a decimal. To be useful as an address for the entry, this
should be combined with the boot ID in "_BOOT_ID=".
__SEQNUM=, __SEQNUM_ID=
The sequence number (and associated sequence number ID) of
this journal entry in the journal file it originates from. See
sd_journal_get_seqnum(3) for details.
Added in version 254.
systemd(1), systemd-journald.service(8), journalctl(1),
journald.conf(5), sd-journal(3), coredumpctl(1),
systemd.directives(7)
1. Journal Export Format
https://systemd.io/JOURNAL_EXPORT_FORMATS#journal-export-format
2. Journal JSON Format
https://systemd.io/JOURNAL_EXPORT_FORMATS#journal-json-format
This page is part of the systemd (systemd system and service
manager) project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a
bug report for this manual page, see
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
man-pages@man7.org
systemd 258~rc2 SYSTEMD.JOURNAL-FIELDS(7)
Pages that refer to this page: journalctl(1), logger(1), sd_bus_creds_get_pid(3), sd_journal_add_match(3), sd_journal_enumerate_fields(3), sd_journal_get_catalog(3), sd_journal_get_data(3), sd_journal_print(3), sd_journal_query_unique(3), sd_journal_stream_fd(3), journald.conf(5), systemd.exec(5), systemd.directives(7), systemd.index(7), systemd-coredump(8), systemd-journald.service(8), systemd-journal-gatewayd.service(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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