4.2 About the /proc Virtual File System

The files in the /proc directory hierarchy contain information about your system hardware and the processes that are running on the system. You can change the configuration of the kernel by writing to certain files that have write permission.

The name of the proc file system stems from its original purpose on the Oracle Solaris operating system, which was to allow access by debugging tools to the data structures inside running processes. Linux added this interface and extended it to allow access to data structures in the kernel. Over time, /proc became quite disordered and the sysfs file system was created in an attempt to tidy it up. For more information, see Section 4.3, “About the /sys Virtual File System”.

Files under the /proc directory are virtual files that the kernel creates on demand to present a browsable view of the underlying data structures and system information. As such, /proc is an example of a virtual file system. Most virtual files are listed as zero bytes in size, but they contain a large amount of information when viewed.

Virtual files such as /proc/interrupts, /proc/meminfo, /proc/mounts, and /proc/partitions provide a view of the system’s hardware. Others, such as /proc/filesystems and the files under /proc/sys provide information about the system's configuration and allow this configuration to be modified.

Files that contain information about related topics are grouped into virtual directories. For example, a separate directory exists in /proc for each process that is currently running on the system, and the directory's name corresponds to the numeric process ID. /proc/1 corresponds to the init process, which has a PID of 1.

You can use commands such as cat, less, and view to examine virtual files within /proc. For example, /proc/cpuinfo contains information about the system's CPUs:

# cat /proc/cpuinfo
processor         : 0
vendor_id         : GenuineIntel
cpu family        : 6
model             : 42
model name        : Intel(R) Core(TM) i5-2520M CPU @ 2.50GHz
stepping          : 7
cpu MHz           : 2393.714
cache size        : 6144 KB
physical id       : 0
siblings          : 2
core id           : 0
cpu cores         : 2
apicid            : 0
initial apicid    : 0
fpu               : yes
fpu_exception     : yes
cpuid level       : 5
wp                : yes

Certain files under /proc require root privileges for access or contain information that is not human-readable. You can use utilities such as lspci, free, and top to access the information in these files. For example, lspci lists all PCI devices on a system:

# lspci
00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
00:01.1 IDE interface: Intel Corporation 82371AB/EB/MB PIIX4 IDE (rev 01)
00:02.0 VGA compatible controller: InnoTek Systemberatung GmbH VirtualBox Graphics Adapter
00:03.0 Ethernet controller: Intel Corporation 82540EM Gigabit Ethernet Controller (rev 02)
00:04.0 System peripheral: InnoTek Systemberatung GmbH VirtualBox Guest Service
00:05.0 Multimedia audio controller: Intel Corporation 82801AA AC'97 Audio Controller (rev 01)
00:06.0 USB controller: Apple Inc. KeyLargo/Intrepid USB
00:07.0 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 08)
00:0b.0 USB controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB2 EHCI Controller
00:0d.0 SATA controller: Intel Corporation 82801HM/HEM (ICH8M/ICH8M-E) SATA Controller [AHCI mode]
        (rev 02)