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Journal Export Format

  • Note that this document describes the binary serialization format of journals only, as used for transfer across the network. For interfacing with web technologies there's the Journal JSON Format. The binary format on disk is documented as Journal File Format. Before reading on, please make sure you are aware of the basic properties of journal entries, in particular realize that they may include binary non-text data (though usually don't), and the same field might have multiple values assigned within the same entry (though usually hasn't).

When exporting journal data for other uses or transferring it via the network/local IPC the journal export format is used. It's a simple serialization of journal entries, that is easy to read without any special tools, but still binary safe where necessary. The format is like this:

  • Two journal entries that follow each other are separated by a double newline.
  • Journal fields consisting only of characters >= 32 (space) and < 127 (i.e. printable, non-control ASCII) are serialized as they are (i.e. field name, followed by '=', followed by field data), followed by a newline as separator to the next field. Note that fields containing tabs or newlines cannot be formatted like this.
  • Other journal fields are serialized in a special binary safe way: field name, followed by newline, followed by a binary 64bit little endian size value, followed by the binary field data, followed by a newline as separator to the next field.
  • Entry metadata that is not actually a field is serialized like it was a field, but beginning with two underscores. More specifically, __CURSOR=, __REALTIME_TIMESTAMP=, __MONOTONIC_TIMESTAMP= are introduced this way. Note that these meta-fields are only generated when actual journal files are serialized. They are omitted for entries that do not originate from a journal file (for example because they are transferred for the first time to be stored in one). Or in other words: if you are generating this format you shouldn't care about these special double-underscore fields. But you might find them usable when you deserialize the format generated by us. Additional fields prefixed with two underscores might be added later on, your parser should skip over the fields it does not know.
  • The order in which fields appear in an entry is undefined and might be different for each entry that is serialized. And that's already it.

This format can be generated via "journalctl -o export".

Here's an example for two serialized entries which consist only of text data:

__CURSOR=s=739ad463348b4ceca5a9e69c95a3c93f;i=4ece7;b=6c7c6013a26343b29e964691ff25d04c;m=4fc72436e;t=4c508a72423d9;x=d3e5610681098c10;p=system.journal
__REALTIME_TIMESTAMP=1342540861416409
__MONOTONIC_TIMESTAMP=21415215982
_BOOT_ID=6c7c6013a26343b29e964691ff25d04c
_TRANSPORT=syslog
PRIORITY=4
SYSLOG_FACILITY=3
SYSLOG_IDENTIFIER=gdm-password]
SYSLOG_PID=587
MESSAGE=AccountsService-DEBUG(+): ActUserManager: ignoring unspecified session '8' since it's not graphical: Success
_PID=587
_UID=0
_GID=500
_COMM=gdm-session-wor
_EXE=/usr/libexec/gdm-session-worker
_CMDLINE=gdm-session-worker [pam/gdm-password]
_AUDIT_SESSION=2
_AUDIT_LOGINUID=500
_SYSTEMD_CGROUP=/user/lennart/2
_SYSTEMD_SESSION=2
_SELINUX_CONTEXT=system_u:system_r:xdm_t:s0-s0:c0.c1023
_SOURCE_REALTIME_TIMESTAMP=1342540861413961
_MACHINE_ID=a91663387a90b89f185d4e860000001a
_HOSTNAME=epsilon

__CURSOR=s=739ad463348b4ceca5a9e69c95a3c93f;i=4ece8;b=6c7c6013a26343b29e964691ff25d04c;m=4fc72572f;t=4c508a7243799;x=68597058a89b7246;p=system.journal
__REALTIME_TIMESTAMP=1342540861421465
__MONOTONIC_TIMESTAMP=21415221039
_BOOT_ID=6c7c6013a26343b29e964691ff25d04c
_TRANSPORT=syslog
PRIORITY=6
SYSLOG_FACILITY=9
SYSLOG_IDENTIFIER=/USR/SBIN/CROND
SYSLOG_PID=8278
MESSAGE=(root) CMD (run-parts /etc/cron.hourly)
_PID=8278
_UID=0
_GID=0
_COMM=run-parts
_EXE=/usr/bin/bash
_CMDLINE=/bin/bash /bin/run-parts /etc/cron.hourly
_AUDIT_SESSION=8
_AUDIT_LOGINUID=0
_SYSTEMD_CGROUP=/user/root/8
_SYSTEMD_SESSION=8
_SELINUX_CONTEXT=system_u:system_r:crond_t:s0-s0:c0.c1023
_SOURCE_REALTIME_TIMESTAMP=1342540861416351
_MACHINE_ID=a91663387a90b89f185d4e860000001a
_HOSTNAME=epsilon

Yupp, no example with binary data here: the wiki can't show this nicely. Sorry.