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.
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
$ 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
/proc/<pid>/fdinfo/<fdnum> (it is the one with
$ cat /proc/350448/fdinfo/3 pos: 39948648448 flags: 0100000 mnt_id: 27
The position indicates it’s read 37.2 GB.
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
their position curiously seems to be 0 even after you’ve read much data.