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I am running the shred command to securely wipe some storage.

I would like to see its progress, but have lost the ssh terminal that was running the command.

Is there a way that I can use to see the verbose output of the shred command from a new ssh terminal?

I have tried to hook into and view the output from /proc/pid/fd/0 1 2 etc but it just shows the random characters from /dev/urandom. I tried using strace, but the same file descriptors were available, with the same results. Shred shows 0, 1, 2, 3, and 4 fd, but they are all showing the random output.

MTIA, waiting patiently with stethoscope against drive...

miller the gorilla
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1 Answers1

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On Linux (since OP mentioned /proc/pid/fd/0 which is a Linux thing), for any tool that is doing sequential operations, the method below in 3 steps can be used:

  1. Figure out the PID of the tool

    eg:

    # pgrep ^shred$
699

  2. Note for this PID the file descriptor(s) of interest that is reading or writing

    For example with ls -l /proc/<pid>/fd/ or stat -c %N /proc/<pid>/fd/<FD> or lsof ... ...

    Example:

    # ls -l /proc/699/fd/
total 0
lrwx------ 1 root root 64 May 24 11:40 0 -> /dev/pts/6
lrwx------ 1 root root 64 May 24 11:40 1 -> /dev/pts/6
lrwx------ 1 root root 64 May 24 11:40 2 -> /dev/pts/6
l-wx------ 1 root root 64 May 24 11:40 3 -> /dev/loop7

    Here the FD of interest is 3. It could have been FD 4 (like appears to be OP's case in this comment) or any other, depending on the command, the version of this command or its options when invoked.

  3. Query the kernel, using /proc/<pid>/fdinfo/<fd> about additional information:

    /proc/[pid]/fdinfo/ (since Linux 2.6.22)

    [...] The files in this directory are readable only by the owner of the process.

    (or by root or at least a process with the adequate capability.)

    The contents of each file can be read to obtain information about the corresponding file descriptor. [...]

    The fields are as follows:

    pos This is a decimal number showing the file offset.
    [...]

    For example:

    # cat /proc/699/fdinfo/3
pos: 286785536
flags:   0140001
mnt_id:  830
ino: 39

    or:

    # awk '/^pos:/ { print $2 }' /proc/699/fdinfo/3
286785536

    So in this example shred is currently writing at position 286785536 ( ~ 273MiB).

This method doesn't require any cooperation from the tool: it doesn't have to be run in a terminal, be run in verbose mode, have any text output anywhere, log anything anywhere, to support handling a special signal (like dd does) to dump information, nor be ptrace()-ed: the kernel provides the current information through /proc.

A.B
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  • Thanks, but I have tried examining the file descriptors. I have the following: ```lrwx------ 1 root root 64 May 23 11:06 0 -> '/dev/pts/0 (deleted)' lrwx------ 1 root root 64 May 23 11:06 1 -> '/dev/pts/0 (deleted)' lrwx------ 1 root root 64 May 23 11:06 2 -> '/dev/pts/0 (deleted)' lr-x------ 1 root root 64 May 23 11:06 3 -> /dev/urandom l-wx------ 1 root root 64 May 23 16:45 4 -> /dev/sda``` It might be because I tried reading with strace... – miller the gorilla May 24 '23 at 18:20
  • So it's obviously FD number 4 in your case: /dev/sda . Did you read the last part of my answer, with `fdinfo` (not just `fd`)? – A.B May 24 '23 at 18:21
  • I missed the fdinfo, as you suggested, but I get only the following: ```pos: 0 flags: 02004001 mnt_id: 13 ino: 27936``` and it doesn't seem to change. The other fdinfos are the same. – miller the gorilla May 24 '23 at 18:50
  • Considering the example I provided was with really using the `shred` command and observing what it's doing, I'm a bit surprised your shred command is doing nothing and didn't even start doing anything (pos: 0). Did you use cat /proc/PID/fdinfo/4 (with the correct pid and the fd number 4, not 3)? – A.B May 24 '23 at 18:51
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    I think there is a problem on your system, but I'd rather not help debug, because I don't want to risk giving the wrong command that would affect something else. – A.B May 24 '23 at 18:58
  • According to top, shred is working away, and I used the correct pid, and tried all the fdinfos, which read almost exactly the same, the fdinfo in the above comment is 4. Its a raspberry py 2b, so running a 32 bit debian, I don't know if that makes any difference. – miller the gorilla May 24 '23 at 18:59
  • Yes, now I take a closer look, there is some system error/misconfiguration. I will reinstall. Thanks for your help. – miller the gorilla May 24 '23 at 19:42