File copy progress for any running process

Problem

Suppose you have a file copy in progress (or some other long time -taking file operation). It doesn’t have a progress bar, and you’re left wondering how long it’s progressed, and you can’t gauge it by comparing source and target file sizes for some reason.

(Credit: xkcd)

Solution

Turns out, Linux makes this task pretty easy, all you got to do is inspect is the file handle’s position.

Let’s simulate reading my entire SSD content:

$ sudo cat /dev/nvme0n1 > /dev/null

In my case cat process’s PID is 350448. I found it by running $ ps aux | grep /dev/nvme0n1, be sure to ignore the sudo one since it’s not the one doing the reading!

We can read the process' open file handles from /proc/<pid>/fd:

$ ls -al /proc/350448/fd
lrwx------ 1 root root 64 Jan  1 20:44 0 -> /dev/pts/1
l-wx------ 1 root root 64 Jan  1 20:44 1 -> /dev/null
lrwx------ 1 root root 64 Jan  1 20:44 2 -> /dev/pts/1
lr-x------ 1 root root 64 Jan  1 20:44 3 -> /dev/nvme0n1

The numbered files are file descriptors. We are interested in file descriptor info for file descriptor #3 at /proc/<pid>/fdinfo/<fdnum> (it is the one with /dev/nvme0n1 open):

$ cat /proc/350448/fdinfo/3
pos:	39948648448
flags:	0100000
mnt_id:	27

The position indicates it’s read 37.2 GB.

Caveats

Buffering

Please note that this is approximate, because if you’re copying a file, you don’t know how large read buffer it has. I.e. it has read that much, but depending on buffering the operation might not have yet written/processed/etc all that data.

Doesn’t work for special files

Please note that this position trick doesn’t work for special files like /dev/zero or /dev/urandom, their position curiously seems to be 0 even after you’ve read much data.