procfs Linux ( /proc )

What is the function of procfs?

Created in RAM when the computer boots.

proc

anything you write under proc will be temporary.

/proc is a real-time api to the kernel. You can set kernel variables, or retrieve kernel information. procfs is analogically a file system simulation to be able to communicate with the kernel via the file system.

Each process has its own directory under /proc with the process id as the name. In this directory you can find all kind of information the kernel has for the particular process.

Other directories allow to either get kernel statistics (like /proc/meminfo) or information (like /proc/cpuinfo) or you can configure certain things (like /proc/sys/vm/swappiness allows you to change how the kernel will use the swap space).

Exploring /proc File System in Linux

Today, we are going to take a look inside the /proc directory and develop a familiarity with it. The /proc directory is present on all Linux systems, regardless of flavor or architecture.

One misconception that we have to immediately clear up is that the /proc directory is NOT a real File System, in the sense of the term. It is a Virtual File System. Contained within the procfs are information about processes and other system information. It is mapped to /proc and mounted at boot time.

Linux proc file system

First, lets get into the /proc directory and have a look around:

# cd /proc

The first thing that you will notice is that there are some familiar sounding files, and then a whole bunch ofnumbered directories. The numbered directories represent processes, better known as PIDs, and within them, a command that occupies them. The files contain system information such as memory (meminfo), CPU information (cpuinfo), and available filesystems.

Read Also:  Linux Free Command to Check Physical Memory and Swap Memory

Let’s take a look at one of the files first:

# cat /proc/meminfo
Sample Output

which returns something similar to this:

MemTotal:         604340 kB
MemFree:           54240 kB
Buffers:           18700 kB
Cached:           369020 kB
SwapCached:            0 kB
Active:           312556 kB
Inactive:         164856 kB
Active(anon):      89744 kB
Inactive(anon):      360 kB
Active(file):     222812 kB
Inactive(file):   164496 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:             0 kB
SwapFree:              0 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:         89724 kB
Mapped:            18012 kB
Shmem:               412 kB
Slab:              50104 kB
SReclaimable:      40224 kB
...

As you can see, /proc/meminfo contains a bunch of information about your system’s memory, including the total amount available (in kb) and the amount free on the top two lines.

Running the cat command on any of the files in /proc will output their contents. Information about any files is available in the man page by running:

# man 5 /proc/

I will give you quick rundown on /proc’s files:

  1. /proc/cmdline – Kernel command line information.
  2. /proc/console – Information about current consoles including tty.
  3. /proc/devices – Device drivers currently configured for the running kernel.
  4. /proc/dma – Info about current DMA channels.
  5. /proc/fb – Framebuffer devices.
  6. /proc/filesystems – Current filesystems supported by the kernel.
  7. /proc/iomem – Current system memory map for devices.
  8. /proc/ioports – Registered port regions for input output communication with device.
  9. /proc/loadavg – System load average.
  10. /proc/locks – Files currently locked by kernel.
  11. /proc/meminfo – Info about system memory (see above example).
  12. /proc/misc – Miscellaneous drivers registered for miscellaneous major device.
  13. /proc/modules – Currently loaded kernel modules.
  14. /proc/mounts – List of all mounts in use by system.
  15. /proc/partitions – Detailed info about partitions available to the system.
  16. /proc/pci – Information about every PCI device.
  17. /proc/stat – Record or various statistics kept from last reboot.
  18. /proc/swap – Information about swap space.
  19. /proc/uptime – Uptime information (in seconds).
  20. /proc/version – Kernel version, gcc version, and Linux distribution installed.

Within /proc’s numbered directories you will find a few files and links. Remember that these directories’ numbers correlate to the PID of the command being run within them. Let’s use an example. On my system, there is a folder name /proc/12:

# cd /proc/12
# ls
Sample Output
attr        coredump_filter  io         mounts      oom_score_adj  smaps    wchan
autogroup   cpuset           latency    mountstats  pagemap        stack
auxv        cwd              limits     net         personality    stat
cgroup      environ          loginuid   ns          root           statm
clear_refs  exe              maps       numa_maps   sched          status
cmdline     fd               mem        oom_adj     schedstat      syscall
comm        fdinfo           mountinfo  oom_score   sessionid      task

If I run:

# cat /proc/12/status

I get the following:

Name:	xenwatch
State:	S (sleeping)
Tgid:	12
Pid:	12
PPid:	2
TracerPid:	0
Uid:	0	0	0	0
Gid:	0	0	0	0
FDSize:	64
Groups:
Threads:	1
SigQ:	1/4592
SigPnd:	0000000000000000
ShdPnd:	0000000000000000
SigBlk:	0000000000000000
SigIgn:	ffffffffffffffff
SigCgt:	0000000000000000
CapInh:	0000000000000000
CapPrm:	ffffffffffffffff
CapEff:	ffffffffffffffff
CapBnd:	ffffffffffffffff
Cpus_allowed:	1
Cpus_allowed_list:	0
Mems_allowed:	00000000,00000001
Mems_allowed_list:	0
voluntary_ctxt_switches:	84
nonvoluntary_ctxt_switches:	0

So, what does this mean? Well, the important part is at the top. We can see from the status file that this process belongs to xenwatch. Its current state is sleeping, and its process ID is 12, obviously. We also can see who is running this, as UID and GID are 0, indicating that this process belongs to the root user.

In any numbered directory, you will have a similar file structure. The most important ones, and their descriptions, are as follows:

  1. cmdline – command line of the process
  2. environ – environmental variables
  3. fd – file descriptors
  4. limits – contains information about the limits of the process
  5. mounts – related information

You will also notice a number of links in the numbered directory:

  1. cwd – a link to the current working directory of the process
  2. exe – link to the executable of the process
  3. root – link to the work directory of the process

This should get you started with familiarizing yourself with the /proc directory. It should also provide insight to how a number of commands obtain their info, such as uptime, lsof, mount, and ps, just to name a few.

cat /proc/cpuinfo Which gave me

processor   : 0
vendor_id   : GenuineIntel
cpu family  : 6
model       : 69
model name  : Intel(R) Core(TM) i7-4500U CPU @ 1.80GHz
stepping    : 1
microcode   : 0x17
cpu MHz     : 774.000
cache size  : 4096 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2
apicid      : 0
initial apicid  : 0
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 fma cx16 xtpr pdcm pcid sse4_1 sse4_2 movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
bogomips    : 3591.40
clflush size    : 64
cache_alignment : 64
address sizes   : 39 bits physical, 48 bits virtual
power management:

processor   : 1
vendor_id   : GenuineIntel
cpu family  : 6
model       : 69
model name  : Intel(R) Core(TM) i7-4500U CPU @ 1.80GHz
stepping    : 1
microcode   : 0x17
cpu MHz     : 1600.000
cache size  : 4096 KB
physical id : 0
siblings    : 4
core id     : 0
cpu cores   : 2
apicid      : 1
initial apicid  : 1
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 fma cx16 xtpr pdcm pcid sse4_1 sse4_2 movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
bogomips    : 3591.40
clflush size    : 64
cache_alignment : 64
address sizes   : 39 bits physical, 48 bits virtual
power management:

processor   : 2
vendor_id   : GenuineIntel
cpu family  : 6
model       : 69
model name  : Intel(R) Core(TM) i7-4500U CPU @ 1.80GHz
stepping    : 1
microcode   : 0x17
cpu MHz     : 800.000
cache size  : 4096 KB
physical id : 0
siblings    : 4
core id     : 1
cpu cores   : 2
apicid      : 2
initial apicid  : 2
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 fma cx16 xtpr pdcm pcid sse4_1 sse4_2 movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
bogomips    : 3591.40
clflush size    : 64
cache_alignment : 64
address sizes   : 39 bits physical, 48 bits virtual
power management:

processor   : 3
vendor_id   : GenuineIntel
cpu family  : 6
model       : 69
model name  : Intel(R) Core(TM) i7-4500U CPU @ 1.80GHz
stepping    : 1
microcode   : 0x17
cpu MHz     : 774.000
cache size  : 4096 KB
physical id : 0
siblings    : 4
core id     : 1
cpu cores   : 2
apicid      : 3
initial apicid  : 3
fpu     : yes
fpu_exception   : yes
cpuid level : 13
wp      : yes
flags       : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf eagerfpu pni pclmulqdq dtes64 monitor ds_cpl vmx est tm2 ssse3 fma cx16 xtpr pdcm pcid sse4_1 sse4_2 movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm ida arat epb xsaveopt pln pts dtherm tpr_shadow vnmi flexpriority ept vpid fsgsbase tsc_adjust bmi1 avx2 smep bmi2 erms invpcid
bogomips    : 3591.40
clflush size    : 64
cache_alignment : 64
address sizes   : 39 bits physical, 48 bits virtual
power management:

 /proc/uptime

This file contains information detailing how long the system has been on since its last restart. The output of /proc/uptime is quite minimal:

350735.47 234388.90

The first number is the total number of seconds the system has been up. The second number is how much of that time the machine has spent idle, in seconds.

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Posted on June 16, 2015, in Linux (Ubuntu/CentOS). Bookmark the permalink. Leave a comment.

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