I’ve been using Linux for years, but I’ve never really known was /proc was or why certain commands used it. I’m not sure why I’ve never looked it up in the past, but I recently did and I thought I’d share.
/proc isn’t a “real” directory, in the sense that it doesn’t exist on disk. It’s not an in memory file system either. It’s a virtual file system that exposes information from the Linux kernel. /proc uses the procfs file system driver and is mounted to /proc at boot time. A lot of system utilities are simply wrappers around reading files from /proc.
For example, run the following command on your Linux machine (it may look familiar):
cat /proc/cpuinfo
The above command should spit out all sorts of information about your CPU. Very useful.
/proc contains all sorts of info, so I recommend looking around. /proc/meminfo contains more info about the kernel’s memory usage than you even knew existed. /proc/cmdline shows the options that Grub used to boot the kernel.
Process Information
I find the process specific information very useful on a day-to-day basis. Process information is available under /proc/<pid>.
For example, /proc/<pid>/limits will show you the ulimits of that process as they are right now.
/proc/<pid>/oom_adj is a writable file that can adjust priorities for the out-of-memory killer, to prevent your process from being killed.
/proc/<pid>/status is the human readable status of that process.
Writing to /proc
You can also write to some proc files to change configuration options in the Linux kernel. For example, you can manipulate swappiness via /proc/sys/vm/swappiness.
Next time you see a command that redirects output to /proc, you’ll know it’s changing a kernel configuration option.
Just remember, /proc represents the current kernel configuration, and changes will not persist through a reboot.
Useful files in /proc
There are many useful files in /proc, so I recommend just looking around. However, the following are among the more useful.
/proc/cmdline
Kernel command line arguments
/proc/cpuinfo
CPU information such as make, model, cores, etc
/proc/devices
List of devices (block/character devices)
/proc/filesystems
List of supported filesystem drivers
/proc/meminfo
Information about memory usage, both physical and swap
/proc/modules
Kernel modules that are currently loaded
/proc/mounts
List of mounted filesystems, mount points, and mount arguments
/proc/swaps
Swap space devices and utilization
/proc/sys
Lots of kernel parameters, most of which are writable to allow changes to kernel parameters without rebooting/recompiling
/proc/version
Kernel version
A great source of information is the Linux Documentation Project, see http://www.tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html