NAME

       proc - process information, system information, and sysctl pseudo-filesystem

DESCRIPTION

       The  proc  filesystem  is  a  pseudo-filesystem which provides an interface to kernel data
       structures.  It is commonly mounted at /proc.  Typically, it is mounted  automatically  by
       the system, but it can also be mounted manually using a command such as:

           mount -t proc proc /proc

       Most  of  the  files  in  the  proc filesystem are read-only, but some files are writable,
       allowing kernel variables to be changed.

   Mount options
       The proc filesystem supports the following mount options:

       hidepid=n (since Linux 3.3)
              This option controls who can access the information in /proc/pid directories.   The
              argument, n, is one of the following values:

              0   Everybody  may  access  all  /proc/pid  directories.   This  is the traditional
                  behavior, and the default if this mount option is not specified.

              1   Users may not access files and subdirectories inside any /proc/pid  directories
                  but their own (the /proc/pid directories themselves remain visible).  Sensitive
                  files such as /proc/pid/cmdline and /proc/pid/status are now protected  against
                  other  users.   This makes it impossible to learn whether any user is running a
                  specific program (so long as the program doesn't otherwise reveal itself by its
                  behavior).

              2   As  for  mode  1,  but in addition the /proc/pid directories belonging to other
                  users become invisible.  This means that /proc/pid entries  can  no  longer  be
                  used  to  discover  the  PIDs on the system.  This doesn't hide the fact that a
                  process with a specific PID value exists (it can be learned by other means, for
                  example,  by "kill -0 $PID"), but it hides a process's UID and GID, which could
                  otherwise be learned by employing  stat(2)  on  a  /proc/pid  directory.   This
                  greatly  complicates  an attacker's task of gathering information about running
                  processes (e.g., discovering whether  some  daemon  is  running  with  elevated
                  privileges,  whether  another  user  is running some sensitive program, whether
                  other users are running any program at all, and so on).

       gid=gid (since Linux 3.3)
              Specifies the ID  of  a  group  whose  members  are  authorized  to  learn  process
              information  otherwise  prohibited  by hidepid (i.e., users in this group behave as
              though /proc was mounted with hidepid=0).  This group should  be  used  instead  of
              approaches such as putting nonroot users into the sudoers(5) file.

   Overview
       Underneath /proc, there are the following general groups of files and subdirectories:

       /proc/pid subdirectories
              Each  one  of  these  subdirectories  contains  files  and  subdirectories exposing
              information about the process with the corresponding process ID.

              Underneath  each  of  the  /proc/pid  directories,  a  task  subdirectory  contains
              subdirectories  of the form task/tid, which contain corresponding information about
              each of the threads in the process, where tid  is  the  kernel  thread  ID  of  the
              thread.

              The  /proc/pid  subdirectories  are  visible  when  iterating  through  /proc  with
              getdents(2) (and thus are visible when one uses  ls(1)  to  view  the  contents  of
              /proc).

       /proc/tid subdirectories
              Each  one  of  these  subdirectories  contains  files  and  subdirectories exposing
              information about the thread with the corresponding thread  ID.   The  contents  of
              these directories are the same as the corresponding /proc/pid/task/tid directories.

              The  /proc/tid  subdirectories  are  not  visible when iterating through /proc with
              getdents(2) (and thus are not visible when one uses ls(1) to view the  contents  of
              /proc).

       /proc/self
              When  a process accesses this magic symbolic link, it resolves to the process's own
              /proc/pid directory.

       /proc/thread-self
              When a thread accesses this magic symbolic link, it resolves to the  process's  own
              /proc/self/task/tid directory.

       /proc/[a-z]*
              Various other files and subdirectories under /proc expose system-wide information.

       All of the above are described in more detail below.

NOTES

       Many  files  contain  strings  (e.g.,  the  environment  and command line) that are in the
       internal format, with subfields terminated by null bytes  ('\0').   When  inspecting  such
       files,  you  may  find  that  the  results  are  more readable if you use a command of the
       following form to display them:

           $ cat file | tr '\000' '\n'

SEE ALSO

       cat(1), dmesg(1), find(1), free(1), htop(1), init(1), ps(1), pstree(1), tr(1),  uptime(1),
       chroot(2), mmap(2), readlink(2), syslog(2), slabinfo(5), sysfs(5), hier(7), namespaces(7),
       time(7),  arp(8),  hdparm(8),  ifconfig(8),  lsmod(8),  lspci(8),  mount(8),   netstat(8),
       procinfo(8), route(8), sysctl(8)

       The      Linux      kernel      source      files:     Documentation/filesystems/proc.rst,
       Documentation/admin-guide/sysctl/fs.rst,      Documentation/admin-guide/sysctl/kernel.rst,
       Documentation/admin-guide/sysctl/net.rst, and Documentation/admin-guide/sysctl/vm.rst.