mjhjmtu

I use this dd command for checking disk speed:

dd if=/dev/zero of=/path/file bs=1G count=1 oflag=direct

which gives back something like this:

1 oflag=direct
1+0 records in
1+0 records out
1073741824 bytes (1,1 GB, 1,0 GiB) copied, 8,52315 s, 126 MB/s

Now I would like to pipe this output, not the file dd is writing, but to a separate file.

I tried adding

>> /tmp/foo

or

| sudo tee /tmp/foo

to the dd command, but that just creates an empty file.

To be able to insert dd in a pipeline before or after another command, its informational messages are written to standard error rather than to standard output.

The OpenBSD manual for dd explicitly mentions this (but the Ubuntu manual seems to omit this fact, but mentions it in the more complete info page):

When finished, dd displays the
number of complete and partial input and output blocks and truncated
input records to the standard error output.

To redirect standard error from a command, use 2>filename. To append the standard error stream to an already existing file without truncating it, use 2>>filename.

For example:

dd if=/dev/zero of=/path/file bs=1G count=1 oflag=direct 2>dd.txt

Note that you mix appending output in the first of your examples (using >>) with truncating output in your second example (using tee). To append to a file with tee, use tee -a.

dd output is actually printing to stderr not stdout

You can redirect stderr to a file for your dd command as follows:

$ dd if=/dev/zero of=/path/file bs=1G count=1 oflag=direct
2>> /path/to/file