What is /proc
There is a directory named /proc on Linux. The files in this directory are virtual, for the Linux kernel to provide internal information; there is no file entity anywhere, like files on an SSD.Example of files in /proc
An example is as follows. The ls command is executed with -F option, directories and symbolic links are respectively shown with / and @ at the tail.$ ls -F /proc/ 1/ 73669/ crypto irq/ modules sys/ 3674/ 73681/ devices kallsyms mounts@ sysrq-trigger 3675/ 73687/ diskstats kcore mtrr sysvipc/ 38/ 91/ dma keys net@ thread-self@ 4771/ 92/ driver/ key-users pagetypeinfo timer_list 4777/ acpi/ dynamic_debug/ kmsg partitions tty/ 4778/ asound/ execdomains kpagecgroup pressure/ uptime 61/ buddyinfo fb kpagecount schedstat version 72/ bus/ filesystems kpageflags self@ vmallocinfo 73/ cgroups fs/ loadavg slabinfo vmstat 73632/ cmdline interrupts locks softirqs zoneinfo 73638/ consoles iomem meminfo stat 73639/ cpuinfo ioports misc swapsAs you can see at first glance, there are directories with only numbers (e.g. 1/, 3674/, etc.) and other files and directories (e.g. acpi/, buddyinfo, etc.). Numeric directories contain information on processes with process numbers of that value in the form of files under that directory. Other files and directories (files included in) contain information about the entire system.
Directories for process
Now, as an example, let's run the sleep command and start the process. The process is started with process ID 73717.$ sleep 3600 & [1] 73717Let's look at the /proc information about this process.
$ ls -F /proc/73717/ arch_status fd/ net/ setgroups attr/ fdinfo/ ns/ smaps autogroup gid_map numa_maps smaps_rollup auxv io oom_adj stack cgroup ksm_merging_pages oom_score stat clear_refs ksm_stat oom_score_adj statm cmdline limits pagemap status comm loginuid patch_state syscall coredump_filter map_files/ personality task/ cpu_resctrl_groups maps projid_map timens_offsets cpuset mem root@ timers cwd@ mountinfo sched timerslack_ns environ mounts schedstat uid_map exe@ mountstats sessionid wchanThere are many different files and directories, but I will list here two that I often use.
exe
Here is a link to the executable of the process. You can see that the executable of the process is /usr/bin/sleep.$ ls -l /proc/73717/exe lrwxrwxrwx 1 foo foo 0 Mar 19 11:06 /proc/73717/exe -> /usr/bin/sleep
fd
It contains a symbolic link to the file that the process is opening. The file name of the symbolic link is the descriptor number. In this case, we see that three files for standard I/O are open.$ ls -l /proc/73717/fd total 0 lrwx------ 1 foo foo 64 Mar 19 11:15 0 -> /dev/pts/5 lrwx------ 1 foo foo 64 Mar 19 11:15 1 -> /dev/pts/5 lrwx------ 1 foo foo 64 Mar 19 11:15 2 -> /dev/pts/5
Entire system information
Three system-wide files are also presented.partitions
Partitions detected by the Linux kernel are included. In the following, the NVMe storage partition is shown. When a USB storage device is inserted, sda, sda1, sda2, etc. will be added to the list. This is useful to check if the Linux kernel recognizes the connected storage device.$ cat /proc/partitions major minor #blocks name 259 0 1953514584 nvme0n1 259 1 498688 nvme0n1p1 259 2 19530752 nvme0n1p2 259 3 19530752 nvme0n1p3 259 4 19530752 nvme0n1p4 259 5 19530752 nvme0n1p5 259 6 1874891776 nvme0n1p6
uptime
The time since Linux was started is stored.$ uptime 11:22:23 up 9:19, 2 users, load average: 0.00, 0.00, 0.00
self
When a process refers to this file, it is a link to the directory for that process. In other words, different links point to different destinations depending on the process reading them.
In the following example, The link 73729 is the directory for the ls command itself which is reading /proc.
$ ls -l /proc/self lrwxrwxrwx 1 root root 0 Mar 19 02:02 /proc/self -> 73729
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