Split a large file, without wasting disk space

FILE=file_name; CHUNK=$((64*1024*1024)); SIZE=$(stat -c "%s" $FILE); for ((i=0; i < $SIZE; i+=$CHUNK)); do losetup --find --show --offset=$i --sizelimit=$CHUNK $FILE; done
It's common to want to split up large files and the usual method is to use split(1). If you have a 10GiB file, you'll need 10GiB of free space. Then the OS has to read 10GiB and write 10GiB (usually on the same filesystem). This takes AGES. . The command uses a set of loop block devices to create fake chunks, but without making any changes to the file. This means the file splitting is nearly instantaneous. The example creates a 1GiB file, then splits it into 16 x 64MiB chunks (/dev/loop0 .. loop15). . Note: This isn't a drop-in replacement for using split. The results are block devices. tar and zip won't do what you expect when given block devices. . These commands will work: hexdump /dev/loop4 . gzip -9 < /dev/loop6 > part6.gz . cat /dev/loop10 > /media/usb/part10.bin
Sample Output
$ truncate -s 1G big_file
$ ls -lh big_file
-rw-r--r--. 1 flatcap flatcap 1.0G Oct  3 13:55 big_file

$ FILE=big_file; SIZE=$(stat -c "%s" $FILE); CHUNK=$((64*1024*1024)); for ((i=0; i < $SIZE; i+=$CHUNK)); do losetup --find --show --offset=$i --sizelimit=$CHUNK $FILE; done
/dev/loop0
/dev/loop1
/dev/loop2
/dev/loop3
/dev/loop4
/dev/loop5
/dev/loop6
/dev/loop7
/dev/loop8
/dev/loop9
/dev/loop10
/dev/loop11
/dev/loop12
/dev/loop13
/dev/loop14
/dev/loop15

5
By: flatcap
2014-10-03 13:18:19

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What Others Think

Sweet! This problem is exactly the kind of thing that most people just accept as an unavoidable irritation. They know there's a better way to split a huge file, but they don't know how to express it. (Or, even worse, some people are stuck using a GUI where creative expression of solutions is prohibited.) This is an excellent example of the power and flexibility of the command line. P.S. Kudos for being the first person on commandlinefu to use the losetup(8) command.
hackerb9 · 187 weeks and 3 days ago
Many thanks hackerb9 :-) Despite working with large virtual machine images for many years, I only recently figured out what I should have been doing.
flatcap · 187 weeks and 3 days ago

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