How do I suppress dd output?

Question

I have a bash script that creates some file using dd. The problem is dd throws a great amount of output that is going to mess with the output of my script. Searching around I've found a solution:

dd if=boot1h of="/dev/r$temp1" >& /dev/null

Is there an alternative, or is redirecting to /dev/null the only way?

Answer 1

Add status=none:

dd if=boot1h of="/dev/r$temp1" status=none

From the dd (coreutils) 8.21 docs:

'status=LEVEL'
     Transfer information is normally output to stderr upon receipt of
     the 'INFO' signal or when 'dd' exits.  Specifying LEVEL will adjust
     the amount of information printed, with the last LEVEL specified
     taking precedence.

     'none'
          Do not print any informational or warning messages to stderr.
          Error messages are output as normal.

     'noxfer'
          Do not print the final transfer rate and volume statistics
          that normally make up the last status line.

     'progress'
          Print the transfer rate and volume statistics on stderr, when
          processing each input block.  Statistics are output on a
          single line at most once every second, but updates can be
          delayed when waiting on I/O.

Answer 2

From the dd(1) man page:

   status=noxfer
          suppress transfer statistics

thus:

dd if=boot1h of="/dev/r$temp1" status=noxfer

This still outputs the

0+1 records in
0+1 records out

garbage when dd exits, so redirecting to a data sink really is your only option.

Answer 3

For future reference:

To suppress dd output completely redirect stderr to /dev/null like so:

dd if=/dev/urandom of=sample.txt bs=5MB count=1 2> /dev/null

This works nicely if you want to, for example, time the process using the time command in bash and assign the result to a variable, without getting any of the output that dd produces.

reference: http://www.unix.com/shell-programming-and-scripting/131624-how-suppress-dd-output.html

Answer 4

With any Unix application or command, you can suppress all output with

cmd >/dev/null 2>&1

The first bit redirects the standard output (unit number 1) to /dev/null. But you need the second part to ALSO redirect the error output (unit number 2) to the same place as number 1.

In UNIX, STDIN=0, STDOUT=1 and STDERR=2

---

Answer 5

Something like this should also work for you with recent versions of BASH and ZSH:

dd if=/path/to/file of=/path/to/another_file bs=1M count=1 &> /dev/null

P.S. This is just an example I ran...

Answer 6

If I understand correctly what you are trying to do, are you putting that sudo command into the script and expecting the script to prompt for your password when it runs there? In that case you are just doing things the complicated way.

A cleaner solution is to write the script in the usual way (i.e without sudo) and run it as the superuser. The reason behind this is, if the script needs superuser access, then just give it the access (why wait until a certain command?). In the script, to check if it is being run as root do something like this:

if [ "$(id -u)" != "0" ]; then
    echo "This script must be run as root" 1>&2
    exit 1
fi

Answer 7

you can redirect the output to an regular file i.e.:

 dd if=boot1h of="/dev/r$temp1" >& /tmp/dd.log

Answer 8

Per the man page, using dd status=none won't get rid of error messages.

If there is a single error message that is expected/desirable, you can use grep to both check that execution was as expected and eat the output. In this example, I obliterate a partition by overwriting with zeros so that mke2fs won't question the need to reformat it.

Running dd without a count specifier will always result in a "No space left on device" error message and nonzero exit value. By searching for the expected error message with grep, the expected behavior returns no error while any unexpected behavior returns an error.

# desired behavior: erase to the end of partition and return zero as exit code.
$ dd if=/dev/zero of=/dev/mmcblk2p7 bs=1M 2>&1 | grep -q 'No space left on device'
$ echo $?
0

# Nonzero exit code is returned if our expected error message doesn't appear. 
$ dd if=/dev/zero of=/dev/mmcblk2p7 bs=1M 2>&1 | grep -q 'Frobulators are not block-aligned'
$ echo $?
1

The redirect of stderr to stdout is necessary because most dd output goes to stderr while grep operates on stdout. Use grep's -q flag to control visibility of the output.