Using /proc on Linux

/proc on Linux is a pseudo file system that provides an interface to the kernel data structures. It is very useful for troubleshooting and the following highlights some that I have found helpful. The man page can be accessed with full details on Arch Linux with :

# man 5 proc

/proc/cpu

• A collection of CPU and system architecture dependent items
• Useful to find the CPU type, number of cores and flags
• lscpu uses this file to obtain its information
  

  $ cat /proc/cpu
  

/proc/cmdline

• Arguments passed to the Linux kernel at boot time
• Useful to see what options were used by e.g. grub on boot
  

  $ cat /proc/cmdline
  

/proc/loadavg

• The first three fields in this file are load average figures giving the number of jobs in the run queue (state R) or waiting for disk I/O (state D) averaged over 1, 5, and 15 minutes.
• Same numbers used by uptime
  

  $ uptime && cat /proc/loadavg
   20:37:08 up  1:36,  8 users,  load average: 0.51, 2.17, 2.56
   0.51 2.17 2.56 6/681 11570
  

• The fourth field consists of two numbers separated by a slash (/)
• The first of these is the number of currently runnable kernel scheduling entities (processes, threads)
• The value after the slash is the number of kernel scheduling entities that currently exist on the system.
• The fifth field is the PID of the process that was most recently created on the system.

/proc/meminfo

• Reports statistics about memory usage on the system
• Used by free to report the amount of free and used memory (both physical and swap) on the system as well as the shared memory and buffers used by the kernel
  

   $ cat /proc/meminfo
  

/proc/[pid]/cmdline

• Everything in Linux is a file and each process has a directory under /proc for its /pid/
• As pid 1 is the init script we can
  

  $ cat /proc/1/cmdline
  /sbin/init
  

• This read-only file holds the complete command line for the process

/proc/[pid]/fd/

• A subdirectory containing one entry for each file which the process has open, named by its file descriptor
• Its a symbolic link to the actual file