If you’re running some command that takes a while, perhaps compressing a large file, you often want to know how far along it is. This is when the /proc/_pid_/io file is useful.

I might have a bzip2 process running with PID 3722, so /proc/3722/io might look like this:

rchar: 1791093411
wchar: 224891624
syscr: 1308428
syscw: 2992919
read_bytes: 250478592
write_bytes: 183074816
cancelled_write_bytes: 0

The most useful are “rchar” and “wchar” - the number of bytes read and written by this process.

The kernel documentation tells you more about this file.