How does the progress tool work?
To collect the information this tool scans the /proc directory for coreutils commands, and then it searches for opened files in fd and fdinfo directories and then reports their status for the largest files. This is a very light tool and compatible with virtually any command.
man progress
Progress – Tool to monitor progress of basic command in Linux
Progress is a command-line-based tiny tool which is formerly known as a Coreutils Progress Viewer. This tool is written in the C language. This command looks for the Coreutils basic command like cp, mv, dd, tar, gzip, cat, etc. which are currently running on the system and display the percentage of copied data. This tool can also show the estimated time and throughput and provide a “top-like” mode. This is a very light tool. Progress tool is available for Linux, FreeBSD, and macOS.