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I used gzip to compress an image which is quite huge still.

dd if=/dev/sda2 bs=1M | gzip -c -9 > sda2.dd.img.gz

then I changed the partitioning of the Drive because I wanted to install Linux. And when trying to decompress and write it to the former (smaller) partition

gunzip sda2.dd.img.gz >/dev/sda2

(using > instead of dd, as described here, its content is first written to another file (instead of directly to the partition as I expected).

The Problem: the Partition where the zipped file is on is to. small. I could use a external media but that will take more time. But would prefer to write the decompressed data to the partition directly.

So I decided to get more info about zipping itself first. but it left me even more confused (see my first response here) Can anyone give any hint, please ?

thanks in advance,

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  • 1
    It looks as if you’re restoring the data obtained from a partition to the disk rather than to a partition, or is that a typo in the text? Commented Feb 24 at 18:46
  • According to the man page, gunziponly works on files: it replaces each named file with its uncompressed version. If you want to write to stdout, use zcat. Commented Feb 24 at 23:03

2 Answers 2

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This

gunzip filename.gz

will write to filename.

This

gunzip filename.gz > output

will still write to filename (and produce an empty output file).

You probably meant to use

gunzip < filename.gz > output

which will read from stdin and write to stdout, so the redirections work as expected.

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  • thanks a lot for your answer, which worked for me today. and helped solving the problem your answer was very educational and helping! Commented Feb 27 at 14:06
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(using > instead of dd, as described here, its content is first written to another file (instead of directly to the partition as I expected).

No, that's not correct. > /dev/sdh makes the kernel directly write the decompressed data to /dev/sdh. However, gunzip file.gz decompresses to file, not standard output. As Frostschutz correctly points out (and as I missed), you want gunzip < file.gz > destination.

So, if it doesn't fit afterwards, the original partition /dev/sda<n> was simply larger than /dev/sdh. This is completely independent from any compression used!

Remarks:

  • you're reading from a partition, but you're writing to a complete device. That usually is not right. So, make sure you're trying to achieve the right thing!
  • your dd on the input is also not very helpful; you'd probably be getting better performance if you just did < /dev/sda<n> instead of dd if=/dev/sda<n> bs=1M |, i.e. 
    n=… gzip -c -9 < /dev/sda${n} > sda${n}.dd.img.gz
  • gzip -9 is relatively slow (pigz is faster and does exactly the same!), but still pretty bad at compression. Maybe try 
    # TO COMPRESS: n=… zstd -f -T0 -10 /dev/sda${n} > sda${n}.img.zstd # -f: allow reading from a block device directly # -T0: use all available CPU cores for compression # -10: compression "effort" 10. Should be still faster than `gzip -9`. # Goes up to 18, if you want to experiment. # TO UNPACK: zstd -f -d sda${n}.img.zstd > /dev/sdh # -f: allow direct writing to a block device # -d: decompress
    instead.
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  • "So, if it doesn't fit afterwards, the original partition /dev/sda<n> was simply larger than /dev/sdh. This is completely independent from any compression used!" Yes, that's correct. I first made a backup (of Windows) then reduced the size (with cfdisk) to install linux. And then after, the Windows didn't run any more so I wanted to reinstall it on the now smaller partition! About the mistake you are pointing out: I was not quite correct. The command I used was: gunzip sda<n>.dd.img.gz >/dev/sda<n> Commented Feb 24 at 21:54
  • So you mean I have to resize the Partition to it's original size, reinstall Windows, defrag it, resize the the Partition and make the backup again?.. ?? UH! Commented Feb 24 at 22:01
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    well, what did you expect when you made a byte-exact backup of something larger than what you'll restore it to? Commented Feb 24 at 22:50
  • As frostschutz correctly says, gunzip file.gz writes the decompressed data to 'file' not to stdout, so it cannot be redirected (or piped). gunzip <file.gz does write to stdout, as does gunzip -c file.gz, and as Paul_Pedant commented zcat file.gz (zcat is a script that just does gzip -dc while gunzip just does gzip -d). Commented Feb 25 at 6:09
  • @dave_thompson_085 aaargh, missed that. Fixed my answer. Commented Feb 25 at 11:46

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