Why I can not redirect the standard output of dd command

nicksu · 01-18-2016, 09:02 AM

I can not figure it out why it writes nothing in my /tmp/result file while I execute like this:

Code:

MyUbuntu:/etc/systemd# dd if=/dev/zero of=/dev/null bs=1M count=10 1>/tmp/result
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.00576801 s, 1.8 GB/s
MyUbuntu:/etc/systemd# cat /tmp/result 
MyUbuntu:/etc/systemd# dd if=/dev/zero of=/dev/null bs=1M count=10 2>/tmp/result
MyUbuntu:/etc/systemd# cat /tmp/result 
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.00580062 s, 1.8 GB/s

I thought it should be come out nothing in the terminal and write all the output to the file /tmp/test , but it didn't .While the 2>/tmp/result did .

It is wierd , can somebody help me out ?

TobiSGD · 01-18-2016, 09:06 AM

If you leave out the of= option dd will by default write the copied data to stdout, so that it can be piped into other commands. Writing the status output also to stdout would interfere with parsing the copied data, so the developers have decided to write the status output to stderr instead. This why redirecting stdout (1) does not give you the wanted result, but redirecting stderr (2) will.