systemd.journal-fields(7) — Linux manual page


SYSTEMD....NAL-FIELDS(7) systemd.journal-fields SYSTEMD....NAL-FIELDS(7)

NAME         top

       systemd.journal-fields - Special journal fields

DESCRIPTION         top

       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.

           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.

           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.

           A priority value between 0 ("emerg") and 7 ("debug")
           formatted as a decimal string. This field is compatible with
           syslog's priority concept.

           The code location generating this message, if known. Contains
           the source filename, the line number and the function name.

           The low-level Unix error number causing this entry, if any.
           Contains the numeric value of errno(3) formatted as a decimal

           Added in version 188.

           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

           Added in version 245.

           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

           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.

           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.

           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.

           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

           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, 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.

           The effective capabilities(7) of the process the journal
           entry originates from.

           Added in version 206.

           The session and login UID of the process the journal entry
           originates from, as maintained by the kernel audit subsystem.

           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

           The SELinux security context (label) of the process the
           journal entry originates from.

           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.

           The kernel boot ID for the boot the message was generated in,
           formatted as a 128-bit hexadecimal string.

           The machine ID of the originating host, as available in

           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.

           The name of the originating host.

           How the entry was received by the journal service. Valid
           transports are:

               for those read from the kernel audit subsystem

               Added in version 227.

               for internally generated messages

               Added in version 205.

               for those received via the local syslog socket with the
               syslog protocol

               Added in version 205.

               for those received via the native journal protocol

               Added in version 205.

               for those read from a service's standard output or error

               Added in version 205.

               for those read from the kernel

               Added in version 205.

           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.

           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.

           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

           Added in version 245.

           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.

           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.

           The kernel subsystem name.

           Added in version 189.

           The kernel device name as it shows up in the device tree
           below /sys/.

           Added in version 189.

           The device node path of this device in /dev/.

           Added in version 189.

           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:

           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:

           PID of the program that this message pertains to.

           Added in version 205.

           These are additional fields added automatically by
           systemd-journald. Their meaning is the same as _UID=, _GID=,
           _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.

ADDRESS FIELDS         top

       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

           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.

           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.

           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

       __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.

SEE ALSO         top

       systemd(1), systemd-journald.service(8), journalctl(1),
       journald.conf(5), sd-journal(3), coredumpctl(1),

NOTES         top

        1. Journal Export Format

        2. Journal JSON Format

COLOPHON         top

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       manager) project.  Information about the project can be found at
       ⟨⟩.  If you have
       a bug report for this manual page, see
       This page was obtained from the project's upstream Git repository
       ⟨⟩ on 2023-12-22.  (At that
       time, the date of the most recent commit that was found in the
       repository was 2023-12-22.)  If you discover any rendering
       problems in this HTML version of the page, or you believe there
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       corrections or improvements to the information in this COLOPHON
       (which is not part of the original manual page), send a mail to

systemd 255                                     SYSTEMD....NAL-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)