procfs

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Reason: Use Template:ic. (Discuss in Talk:Procfs)

The proc filesystem, also called procfs, is usually mounted at /proc and contains information about the running system:

Processes, the most prominent use
Kernel information and parameters
System metrics, such as CPU usage

Also see proc(5).

Content

Kernel & system information

There are many files under /proc which provide a lot of information about the system as well as the kernel. There are too many to cover them all here, but some of them are listed below with brief information about what they are.

/proc/cpuinfo - informations about CPU
/proc/meminfo - information about the physical memory
/proc/vmstats - information about the virtual memory
/proc/mounts - information about the mounts(mount)
/proc/filesystems - information about active filesystems
/proc/uptime - current system uptime
/proc/cmdline - kernel command line

Processes

Inside /proc/<pid> is stored information about every process currently running. Below is an example showing some of the PIDs currently running

$ ls -l /proc

total 0
dr-xr-xr-x  9 root    root                  0 Sep  8 18:17 1
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 10
dr-xr-xr-x  9 daemonx daemonx               0 Sep  9 03:02 1057
dr-xr-xr-x  9 daemonx daemonx               0 Sep  8 18:18 1077
dr-xr-xr-x  9 daemonx daemonx               0 Sep  9 03:02 1087
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 11
dr-xr-xr-x  9 daemonx daemonx               0 Sep  9 03:02 1103
dr-xr-xr-x  9 daemonx daemonx               0 Sep  9 03:02 1107
dr-xr-xr-x  9 daemonx daemonx               0 Sep  9 03:02 1159
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 12
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 124
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 125
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 127
dr-xr-xr-x  9 root    root                  0 Sep  9 03:02 128
...

Lets take for example pid 1057 and see what's inside

$ ls -l /proc/1057

total 0
dr-xr-xr-x 2 daemonx daemonx 0 Sep  9 03:12 attr
-rw-r--r-- 1 daemonx daemonx 0 Sep  9 03:12 autogroup
-r-------- 1 daemonx daemonx 0 Sep  9 03:12 auxv
-r--r--r-- 1 daemonx daemonx 0 Sep  9 03:12 cgroup
--w------- 1 daemonx daemonx 0 Sep  9 03:12 clear_refs
-r--r--r-- 1 daemonx daemonx 0 Sep  9 03:12 cmdline
-rw-r--r-- 1 daemonx daemonx 0 Sep  9 03:12 comm
-rw-r--r-- 1 daemonx daemonx 0 Sep  9 03:12 coredump_filter
-r--r--r-- 1 daemonx daemonx 0 Sep  9 03:12 cpuset
lrwxrwxrwx 1 daemonx daemonx 0 Sep  9 03:12 cwd -> /home/daemonx
-r-------- 1 daemonx daemonx 0 Sep  9 03:12 environ
lrwxrwxrwx 1 daemonx daemonx 0 Sep  9 03:12 exe -> /usr/lib/gvfsd-metadata
dr-x------ 2 daemonx daemonx 0 Sep  9 03:12 fd
dr-x------ 2 daemonx daemonx 0 Sep  9 03:12 fdinfo
-rw-r--r-- 1 daemonx daemonx 0 Sep  9 03:12 gid_map
-r-------- 1 daemonx daemonx 0 Sep  9 03:12 io
-r--r--r-- 1 daemonx daemonx 0 Sep  9 03:12 latency
-r--r--r-- 1 daemonx daemonx 0 Sep  9 03:12 limits
-rw-r--r-- 1 daemonx daemonx 0 Sep  9 03:12 loginuid
dr-x------ 2 daemonx daemonx 0 Sep  9 03:12 map_files
-r--r--r-- 1 daemonx daemonx 0 Sep  9 03:12 maps
-rw------- 1 daemonx daemonx 0 Sep  9 03:12 mem
...

Some of the fields:

cmdline - arguments used to start program
cwd - current working directory for the process
environ - environment variables inside the process
fd/ - directory containing open file descriptors for the process
exe - symbolic link to process executable
maps - memory mapping of the process
mem - virtual memory of the process

Usage

To read from proc file, we can use cat: cat /proc/cmdline (run as the regular user). To write to the file, we can use echo: echo 1 > /proc/sys/kernel/sysrq (run as the root user).