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pv is much smarter than you think. If instead of `cat|pv` you just use `pv filename` or even `pv <file`, it'll work out the file or block device size on its own.

If you forget to use pv and want a nice progress bar for an already executing process, use `pv -d <pid>`. That'll display progress bars for every open file. Works even for things like installers and servers, where you wouldn't be able to use a pv pipeline anyway.

By the way there is a tool called "progress" which will scan all running processes of (supported) tools like gzip, cat, grep etc., and report progress of their file operations.

If you want to emulate this by hand, first get the fd number of the file of interest by `ls -l /proc/PID/fd/` and then `cat /proc/PID/fdinfo/NUMBER`. There is a line called "pos", which is the position in the file.

I did not know about `pv -d`, that's a great feature!

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