dump UTMP and WTMP files in raw format


Dump the /var/log/wtmp file to the standard output as plain text

$ utmpdump [/var/log/wtmp]

Load a previously dumped file into /var/log/wtmp

$ utmpdump -r [dumpfile] > [/var/log/wtmp]


utmpdump [options] [filename]


utmpdump is a simple program to dump UTMP and WTMP files in raw format, so they can be examined. utmpdump reads from stdin unless a filename is passed.


-f, --follow

Output appended data as the file grows.

-o, --output file

Write command output to file instead of standard output.

-r, --reverse

Undump, write back edited login information into the utmp or wtmp files.

-V, --version

Display version information and exit.

-h, --help

Display help text and exit.


utmpdump can be useful in cases of corrupted utmp or wtmp entries. It can dump out utmp/wtmp to an ASCII file, which can then be edited to remove bogus entries, and reintegrated using:

utmpdump -r < ascii_file > wtmp

But be warned, utmpdump was written for debugging purposes only.

File formats

Only the binary version of the utmp(5) is standardised. Textual dumps may become incompatible in future.

The version 2.28 was the last one that printed text output using ctime(3) timestamp format. Newer dumps use millisecond precision ISO-8601 timestamp format in UTC-0 timezone. Conversion from former timestamp format can be made to binary, although attempt to do so can lead the timestamps to drift amount of timezone offset.


You may not use the -r option, as the format for the utmp/wtmp files strongly depends on the input format. This tool was not written for normal use, but for debugging only.


Michael Krapp


The utmpdump command is part of the util-linux package and is available from Linux Kernel Archive.


last(1), w(1), who(1), utmp(5)

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