The Linux kernel makes a special filesystem available, which is mounted under /proc on Debian systems. This is a “pseudo-filesystem” because it doesn’t really exist on any of your physical devices.
The proc filesystem contains information about the system and running processes. Some of the “files” in /proc are reasonably understandable to humans (try typing cat /proc/meminfo or cat /proc/cpuinfo); others are arcane collections of numbers. Often, system utilities use these to gather information and present it to you in a more understandable way.
People frequently panic when they notice one file in particular— /proc/kcore —which is generally huge. This is (more or less) a copy of the contents of your computer’s memory. It’s used to debug the kernel. It doesn’t actually exist anywhere, so don’t worry about its size.
If you want to know about all the things in /proc, type man 5 proc.
13.2.4 Large-Scale Copying
Sometimes you may want to copy one directory to another location. Maybe you’re adding a new hard disk and you want to copy /usr/local to it. There are several ways you can do this.
The first is to use cp. The command cp -a will tell cp to do a copy preserving all the information it can. So, you might use
cp -a /usr/local /destination
However, there are some things that cp -a won’t catch[[1]]. So, the best way to do a large copy job is to chain two tar commands together, like so:
[1] Sparse files and hard links are two examples.