$ seq 1 1000 | systemd-cat

$ journalct -e
...
Oct 04 13:45:26 jessie-64 cat[1921]: 18
Oct 04 13:45:26 jessie-64 cat[1921]: 19
Oct 04 13:45:26 jessie-64 unknown[1921]: 20
Oct 04 13:45:26 jessie-64 unknown[1921]: 21
...

$ journalctl MESSAGE=1 _PID=1921 -o verbose
-- Logs begin at Mon 2015-09-28 10:32:57 VLAT, end at Sun 2015-10-04 13:47:01 VL
Sun 2015-10-04 13:45:26.009220 VLAT [s=602a2cf9ff774e51929098e5df2b9ac8;i=ce9a9;
    PRIORITY=6
    _UID=1000
    _GID=1000
    _CAP_EFFECTIVE=0
    _SYSTEMD_OWNER_UID=1000
    _SYSTEMD_SLICE=user-1000.slice
    _MACHINE_ID=a0bca4d8e31847608728ea41f8aedc17
    _TRANSPORT=stdout
    MESSAGE=1
    _COMM=cat
    _EXE=/bin/cat
    _CMDLINE=/bin/cat
    _AUDIT_LOGINUID=1000
    _BOOT_ID=1d5bc57f93c9418fbfdb363906321f6b
    _AUDIT_SESSION=3
    _SYSTEMD_CGROUP=/user.slice/user-1000.slice/session-3.scope
    _SYSTEMD_SESSION=3
    _SYSTEMD_UNIT=session-3.scope
    _HOSTNAME=jessie-64
    _PID=1921

$ journalctl MESSAGE=1000 _PID=1921 -o verbose
-- Logs begin at Mon 2015-09-28 10:32:57 VLAT, end at Sun 2015-10-04 13:47:01 VL
Sun 2015-10-04 13:45:26.079659 VLAT [s=602a2cf9ff774e51929098e5df2b9ac8;i=ced90;
    PRIORITY=6
    _UID=1000
    _GID=1000
    _MACHINE_ID=a0bca4d8e31847608728ea41f8aedc17
    _TRANSPORT=stdout
    _BOOT_ID=1d5bc57f93c9418fbfdb363906321f6b
    _HOSTNAME=jessie-64
    MESSAGE=1000
    _PID=1921