credentials - process identifiers


   Process ID (PID)
   Each  process  has  a  unique  nonnegative  integer  identifier that is
   assigned when the process is created  using  fork(2).   A  process  can
   obtain  its  PID  using getpid(2).  A PID is represented using the type
   pid_t (defined in <sys/types.h>).

   PIDs are used in a range  of  system  calls  to  identify  the  process
   affected  by  the call, for example: kill(2), ptrace(2), setpriority(2)
   setpgid(2), setsid(2), sigqueue(3), and waitpid(2).

   A process's PID is preserved across an execve(2).

   Parent process ID (PPID)
   A process's parent process ID identifies the process that created  this
   process using fork(2).  A process can obtain its PPID using getppid(2).
   A PPID is represented using the type pid_t.

   A process's PPID is preserved across an execve(2).

   Process group ID and session ID
   Each process has a session ID and a process group ID, both  represented
   using  the  type  pid_t.   A  process  can  obtain its session ID using
   getsid(2), and its process group ID using getpgrp(2).

   A child created by fork(2) inherits its parent's session ID and process
   group  ID.   A  process's session ID and process group ID are preserved
   across an execve(2).

   Sessions and process groups are abstractions devised to  support  shell
   job  control.   A  process  group  (sometimes  called  a  "job")  is  a
   collection of processes that share the same process group ID; the shell
   creates  a new process group for the process(es) used to execute single
   command or pipeline (e.g., the two processes  created  to  execute  the
   command  "ls | wc"  are placed in the same process group).  A process's
   group membership can  be  set  using  setpgid(2).   The  process  whose
   process  ID  is  the  same as its process group ID is the process group
   leader for that group.

   A session is a collection of processes that share the same session  ID.
   All  of  the  members  of a process group also have the same session ID
   (i.e., all of the members of a process group always belong to the  same
   session,  so  that  sessions and process groups form a strict two-level
   hierarchy of processes.)  A new session is created when a process calls
   setsid(2),  which creates a new session whose session ID is the same as
   the PID of the process that  called  setsid(2).   The  creator  of  the
   session is called the session leader.

   All  of  the  processes in a session share a controlling terminal.  The
   controlling terminal is established when the session leader first opens
   a  terminal  (unless  the  O_NOCTTY  flag  is  specified  when  calling
   open(2)).  A terminal may be the controlling terminal of  at  most  one

   At  most  one of the jobs in a session may be the foreground job; other
   jobs in the session are background jobs.  Only the foreground  job  may
   read  from  the  terminal; when a process in the background attempts to
   read from the terminal, its process group is  sent  a  SIGTTIN  signal,
   which  suspends  the  job.   If  the  TOSTOP  flag has been set for the
   terminal (see termios(3)), then only the foreground job  may  write  to
   the  terminal;  writes from background job cause a SIGTTOU signal to be
   generated, which suspends the job.  When terminal keys that generate  a
   signal (such as the interrupt key, normally control-C) are pressed, the
   signal is sent to the processes in the foreground job.

   Various system calls and library functions may operate on  all  members
   of  a  process  group,  including  kill(2),  killpg(3), getpriority(2),
   setpriority(2),   ioprio_get(2),    ioprio_set(2),    waitid(2),    and
   waitpid(2).   See  also  the  discussion  of the F_GETOWN, F_GETOWN_EX,
   F_SETOWN, and F_SETOWN_EX operations in fcntl(2).

   User and group identifiers
   Each process has various associated user and groups IDs.  These IDs are
   integers,  respectively  represented  using  the  types uid_t and gid_t
   (defined in <sys/types.h>).

   On Linux, each process has the following user and group identifiers:

   *  Real user ID and real group ID.  These IDs determine  who  owns  the
      process.   A  process  can  obtain  its  real  user (group) ID using
      getuid(2) (getgid(2)).

   *  Effective user ID and effective group ID.  These IDs are used by the
      kernel  to determine the permissions that the process will have when
      accessing shared resources such as message  queues,  shared  memory,
      and  semaphores.  On most UNIX systems, these IDs also determine the
      permissions  when  accessing  files.   However,   Linux   uses   the
      filesystem  IDs described below for this task.  A process can obtain
      its effective user (group) ID using geteuid(2) (getegid(2)).

   *  Saved set-user-ID and saved set-group-ID.  These  IDs  are  used  in
      set-user-ID  and  set-group-ID  programs  to  save  a  copy  of  the
      corresponding effective IDs that  were  set  when  the  program  was
      executed (see execve(2)).  A set-user-ID program can assume and drop
      privileges by switching its effective user ID back and forth between
      the  values  in  its  real  user  ID  and  saved  set-user-ID.  This
      switching  is  done  via  calls  to  seteuid(2),   setreuid(2),   or
      setresuid(2).   A  set-group-ID program performs the analogous tasks
      using setegid(2),  setregid(2),  or  setresgid(2).   A  process  can
      obtain  its  saved  set-user-ID  (set-group-ID)  using  getresuid(2)

   *  Filesystem user ID and filesystem group ID (Linux-specific).   These
      IDs,  in  conjunction  with  the  supplementary  group IDs described
      below, are used to determine permissions for  accessing  files;  see
      path_resolution(7) for details.  Whenever a process's effective user
      (group) ID is changed, the kernel  also  automatically  changes  the
      filesystem  user  (group)  ID  to the same value.  Consequently, the
      filesystem IDs normally have the same values  as  the  corresponding
      effective  ID, and the semantics for file-permission checks are thus
      the same on Linux as on other UNIX systems.  The filesystem IDs  can
      be  made to differ from the effective IDs by calling setfsuid(2) and

   *  Supplementary group IDs.  This is a set of additional group IDs that
      are used for permission checks when accessing files and other shared
      resources.  On Linux kernels before 2.6.4, a process can be a member
      of  up to 32 supplementary groups; since kernel 2.6.4, a process can
      be  a  member  of  up  to  65536  supplementary  groups.   The  call
      sysconf(_SC_NGROUPS_MAX)  can  be  used  to  determine the number of
      supplementary groups of which a process may be a member.  A  process
      can  obtain  its  set of supplementary group IDs using getgroups(2),
      and can modify the set using setgroups(2).

   A child process created by fork(2) inherits copies of its parent's user
   and  groups  IDs.  During an execve(2), a process's real user and group
   ID and supplementary group IDs are preserved; the effective  and  saved
   set IDs may be changed, as described in execve(2).

   Aside  from  the  purposes  noted  above, a process's user IDs are also
   employed in a number of other contexts:

   *  when determining the permissions for sending signals (see kill(2));

   *  when determining  the  permissions  for  setting  process-scheduling
      parameters  (nice  value,  real time scheduling policy and priority,
      CPU    affinity,     I/O     priority)     using     setpriority(2),
      sched_setaffinity(2),    sched_setscheduler(2),   sched_setparam(2),
      sched_setattr(2), and ioprio_set(2);

   *  when checking resource limits (see getrlimit(2));

   *  when checking the limit on the number of inotify instances that  the
      process may create (see inotify(7)).


   Process IDs, parent process IDs, process group IDs, and session IDs are
   specified in POSIX.1.  The real, effective,  and  saved  set  user  and
   groups  IDs, and the supplementary group IDs, are specified in POSIX.1.
   The filesystem user and group IDs are a Linux extension.


   The POSIX threads specification requires that credentials are shared by
   all  of  the threads in a process.  However, at the kernel level, Linux
   maintains separate user and group credentials  for  each  thread.   The
   NPTL  threading implementation does some work to ensure that any change
   to user or group credentials (e.g., calls to  setuid(2),  setresuid(2))
   is  carried  through  to  all  of  the POSIX threads in a process.  See
   nptl(7) for further details.


   bash(1),  csh(1),  groups(1),  id(1),  newgrp(1),  ps(1),   runuser(1),
   setpriv(1),  sg(1), su(1), access(2), execve(2), faccessat(2), fork(2),
   getgroups(2), getpgrp(2), getpid(2),  getppid(2),  getsid(2),  kill(2),
   setegid(2),    seteuid(2),    setfsgid(2),    setfsuid(2),   setgid(2),
   setgroups(2),  setpgid(2),   setresgid(2),   setresuid(2),   setsid(2),
   setuid(2),   waitpid(2),   euidaccess(3),   initgroups(3),   killpg(3),
   tcgetpgrp(3),    tcsetpgrp(3),    group(5),    passwd(5),    shadow(5),
   capabilities(7),  namespaces(7), path_resolution(7), pid_namespaces(7),
   pthreads(7), signal(7), unix(7), user_namespaces(7), sudo(8)


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   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at


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