Question

I’m hacking around in some scripts trying to parse some data written by Javas DataOutputStream#writeLong(...). Since java always seems to write big endian, I have a problem feeding the bytes to od. This is due to the fact that od always assumes that the endianess matches the endianess of the arch that you are currently on, and I’m on a little endian machine.

I’m looking for an easy one-liner to reverse the byte order. Let’s say that you know that the last 8 bytes of a file is a long written by the aforementioned writeLong(...) method. My current best attempt to print this long is

tail -c 8 file | tac | od -t d8

, but tac only seems to work on text (fair enough). I’ve found some references to dd conv=swab, but this only swaps bytes in pairs, and cannot reverse these eight bytes.

Does anyone know a good one-liner for this?

Answer 1

Resorted to Perl in the end. Used a one-liner which I found at PERL One Liners:

tail -c 8 file | perl -0777e 'print scalar reverse <>' | od -t d8

The 0777 separator char was a bit puzzling to me, but this page at debian admin seems to suggest that it is a placeholder for ‘no record separator’, triggering a complete reverse byte-per byte.

Other suggestions are welcome.

EDIT: Found another command in a comment to tac.c, which I downloaded from GNU coreutils:

Copy each FILE, or the standard input if none are given or when a
FILE name of “-” is encountered, to the standard output with the
order of the records reversed. The records are separated by
instances of a string, or a newline if none is given. By default, the
separator string is attached to the end of the record that it
follows in the file.

Options:
-b, –before The separator is attached to the beginning
of the record that it precedes in the file.
-r, –regex The separator is a regular expression.
-s, –separator=separator Use SEPARATOR as the record separator.

To reverse a file byte by byte, use (in bash, ksh, or sh):
tac -r -s ‘.\|
‘ file