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SYSTEMD.JOURNAL-FIELDS(7) systemd.journal-fields SYSTEMD.JOURNAL-FIELDS(7)
NAME
systemd.journal-fields - Special journal fields
DESCRIPTION
Entries in the journal (as written by systemd-journald.service(8)) resemble a UNIX process
environment block in syntax but with fields that may include binary data. Primarily,
fields are formatted UTF-8 text strings, and 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. All fields with special meanings are
optional. In some cases, fields may appear more than once per entry.
USER JOURNAL FIELDS
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.
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.
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).
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 numeric integers formatted as
decimal strings.
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 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=.
DOCUMENTATION=
A documentation URL with further information about the topic of the log message. Tools
such as journalctl will include a hyperlink to an URL specified this way in their
output. Should be an "http://", "https://", "file:/", "man:" or "info:" URL.
TID=
The numeric thread ID (TID) the log message originates from.
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.
TRUSTED JOURNAL FIELDS
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, user, and group ID of the process the journal entry originates from
formatted as a decimal string. 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.
_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. This is the time in microseconds since the epoch
UTC, formatted as a decimal string.
_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 available 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)).
_HOSTNAME=
The name 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
driver
for internally generated messages
syslog
for those received via the local syslog socket with the syslog protocol
journal
for those received via the native journal protocol
stdout
for those read from a service's standard output or error output
kernel
for those read from the kernel
_STREAM_ID=
Only applies to "_TRANSPORT=stdout" records: specifies a randomized 128bit 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.
_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.
_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.
KERNEL JOURNAL FIELDS
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.
_KERNEL_SUBSYSTEM=
The kernel subsystem name.
_UDEV_SYSNAME=
The kernel device name as it shows up in the device tree below /sys/.
_UDEV_DEVNODE=
The device node path of this device in /dev/.
_UDEV_DEVLINK=
Additional symlink names pointing to the device node in /dev/. This field is
frequently set more than once per entry.
FIELDS TO LOG ON BEHALF OF A DIFFERENT PROGRAM
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).
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.
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.
ADDRESS FIELDS
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 string. 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 string. To be useful as an address
for the entry, this should be combined with the boot ID in "_BOOT_ID=".
SEE ALSO
systemd(1), systemd-journald.service(8), journalctl(1), journald.conf(5), sd-journal(3),
coredumpctl(1), systemd.directives(7)
NOTES
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
systemd 249 SYSTEMD.JOURNAL-FIELDS(7)
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