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I'm using the dd command to create a bootable usb from iso file:

sudo dd if=~/Desktop/ubuntu.iso of=/dev/sdx bs=1M

After pressing enter it momentarily exits and gives me:

915+0 records in 915+0 records out 959447040 bytes (959 MB) copied,
0.539375 s, 1.8 GB/s

So it's like running in background because I can see that the flash drive is working. Eventually it will stop copying and I can remove the drive successfully but the question is why doesn't dd command wait for copying to finish. Why does it run in background. And how can I make it wait?

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    Have you tried sync?
    – choroba
    Jul 9, 2013 at 15:32
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    @choroba Does it make sense to run 'sync' if the system's already writing data to the flash drive? Jul 9, 2013 at 16:03
  • I am not sure, byt sync might wait.
    – choroba
    Jul 9, 2013 at 16:05
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    dd also has some sync options, for example conv=fsync. That said, I never had to use it with /dev/sd* drives myself. If you literally used /dev/sdx my guess would've been you had a useless 959MB file in /dev (ramdisk) now... Jul 9, 2013 at 16:07
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    @frostschutz I used /sdc for my flash drive Jul 9, 2013 at 16:23

2 Answers 2

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Despite popular belief, dd is a perfectly ordinary command, it isn't more low-level than cat or cp. Your command reads from the disk cache and writes to the disk buffers like any other command.

In order to make sure that the data is fully written to the physical media, you need to call sync. The command sync flushes all output buffers to the disk(s). When the sync command returns, the data has been fully written.

sudo dd if=~/Desktop/ubuntu.iso of=/dev/sdx bs=1M; sync

Most of the time, you don't need to call sync, because unmounting a filesystem does the same job. When the umount command returns, or when you get a confirmation message after clicking “Eject”, the buffers have been written to the disk. Here, you're directly writing to the disk without going through a mounted filesystem, so you need to explicitly flush the buffer.

Note that instead of dd, you could use tee. This has two advantages: there's less risk of inverting the source and destination due to a typo, and it's probably slightly faster.

<~/Desktop/ubuntu.iso sudo tee /dev/sdx >/dev/null; sync
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    How about the "Eject" or "Safely Remove" buttons/icons. Wouldn't they also call sync before giving the "OK to remove" notification?
    – user68186
    Jul 9, 2013 at 16:11
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    You likely want to use sudo tee /dev/sdx >/dev/null, otherwise the copying process would still be very slow due to data being written to console output.
    – Lekensteyn
    Jul 9, 2013 at 16:26
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    @user68186 They don't call the sync command, but they do the same job under the hood. Jul 9, 2013 at 16:30
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    This answer is wrong. Without conv=fdatasync, the dd command does not wait to finish until the data is written to the disk, as the OP requested. Furthermore, the sync command schedules a sync operation, but it immediately returns; it does not wait to return until after the data are written to the disk.
    – vy32
    Jun 26, 2016 at 17:52
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    @vy32 you're right dd won't wait for the cache to be flushed without conv=fdatasync etc. but on non-ancient Linux you're incorrect about sync not waiting. You can see coreutils' sync(8) making the sync(2) syscall. The sync(2) man-page says "Before version 1.3.20 Linux did not wait for I/O to complete before returning." so sync(8) won't return until the disk acknowledges the writes (or an error is triggered).
    – Anon
    Oct 9, 2019 at 5:56
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Try this:

sudo dd if=~/Desktop/ubuntu.iso of=/dev/sdx conv=fdatasync bs=1m

The conv=fdatasync tells dd to use the special options to make sure that the data get written to the physical device.

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