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How do I break a large, +4GB file into smaller files of about 500MB each.

And how do I re-assemble them again to get the original file?

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2 Answers 2

up vote 30 down vote accepted

You can use split and cat.

For example something like

$ split --bytes 500M --numeric-suffixes --suffix-length=3 foo foo.

(where the input filename is foo and the last argument is the output prefix)

The same command with short options:

$ split -b 100k -d -a 3 foo foo.

The split commands generate pieces named: foo.000, foo.001 ...

For re-assembling the generated pieces again you can use e.g.:

$ cat foo.* > foo_2

(assuming that the shell sorts the results of shell globbing - and the number of parts does not exceed the system dependent limit of arguments)

You can compare the result via:

$ cmp foo foo_2
$ echo $?

(which should output 0)

Alternatively, you can use a combination of find/sort/xargs to re-assemble the pieces:

$ find -maxdepth 1 -type f -name 'foo.*'  | sort | xargs cat > foo_3
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Try this command: man split cat md5sum –  Kevin M Sep 4 '10 at 19:13
When assembling, I recommend cat foo.{000..NNN} where NNN is the last expected piece. That way you get an error message if one of the pieces is missing. But note that -d to get numeric suffixes is specific to GNU split; on other platforms you have to make do with, foo.aab, etc. –  Gilles Oct 17 '10 at 11:16
And bear in mind that, for split, KB = 1000, K = 1024, MB = 1000*1000, M = 1024*1024 etc. –  Zorawar Nov 29 '12 at 18:05
Shouldn't this ... cat > foo_3 be ... cat >>foo_3? –  alk Jul 8 at 12:10
@alk, no, it should not. The part that xargs sees as arguments (and thus potentially forks/execs multiple times) is cat. The part > foo_3 is interpreted by the shell (the shell creates the output redirection for the xargs process). Thus, everything is ok. –  maxschlepzig Jul 8 at 16:09

You can also do this with Archive Manager if you prefer a GUI. Look under 'Save->Other Options->Split into volumes of'.

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i tagged it 'command-line', but thanks for the answer :) –  Stefan Sep 5 '10 at 19:25

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