procfs
The proc file system, also called procfs, is a pseudo file system that 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.
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 -
/proc/filesystems- information about filesystems that have been compiled into the kernel and whose kernel modules are currently loaded -
/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 is 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 (zero-delimited). -
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
You can interact with /proc contents as with regular files.
To read from a file:
$ cat /proc/cmdline
To write to a file:
# echo 1 > /proc/sys/kernel/sysrq