Pipe stderr and stdout separately in bash/sh

Question

I've been trying to use the following command on my server dd if=/dev/zero bs=1M count=1024 | md5sum

The output:

1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 2.92245 s, 367 MB/s
cd573cfaace07e7949bc0c46028904ff  -

How do I let it show the speed (367 MB/s) as output only? The status is printed to stderr.

I'm currently using awk but it showed the md5 hash.

Helps are appreciated :)

Do you *need* the `md5sum`? Or is `of=/dev/null` also adequate? — Martin Tournoij, Feb 22 '14 at 02:07
I'm not sure what is the purpose of this command. You surely won't test MD5 calculation speed, but rather speed of reading from /dev/zero (or any other source). Are you sure `| md5sum` is at all relevant? — mike.dld, Feb 22 '14 at 02:38
Or, is there a way to show the second last output? I'm pretty sure that there is a way. — user2248259, Feb 22 '14 at 02:44
The problem isn't so much in showing the output, the problem is piping the stdout to `md5sum`, while piping the `stderr` to something else (ie. `awk`, or whatever). This is surprisingly complicated to do. — Martin Tournoij, Feb 22 '14 at 02:49
@Carpetsmoker +1. Quite easy with temporary file, but yeah, complicated without it. — mike.dld, Feb 22 '14 at 02:51

score 4 · Answer 1 · answered Feb 22 '14 at 04:00

First, a function to simulate your command

simulation() { 
  echo "1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 2.92245 s, 367 MB/s" >&2
  echo "cd573cfaace07e7949bc0c46028904ff  -"
}

$ simulation >/dev/null
1024+0 records in
1024+0 records out
1073741824 bytes (1.1 GB) copied, 2.92245 s, 367 MB/s

$ simulation 2>/dev/null
cd573cfaace07e7949bc0c46028904ff  -

Then, the solution: redirecting stderr to a process substitution that displays the desired output back to stderr, capturing stdout in a variable.

$ md5sum=$( simulation 2> >(sed -n '/MB\/s/ {s/.*, //p; q}' >&2) )
367 MB/s

$ echo $md5sum
cd573cfaace07e7949bc0c46028904ff -

To clarify, you need to replace `simulation` with just the `dd` command, and put the `| md5sum` right *before* the last closing parenthesis, ie: `>&2) | md5sum )` — Martin Tournoij, Feb 22 '14 at 04:06

Pipe stderr and stdout separately in bash/sh

1 Answers1