# systemd.journal-fields

## 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 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 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. 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.
    


## 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 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 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.
    
               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 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).
    
               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.
    


## 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 assd_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.
    


## 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
    


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