Man Linux: Main Page and Category List


       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(2), 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.

   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 file
          system 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)

       *  File  system  user  ID  and  file  system 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
          file system user (group) ID to the same  value.   Consequently,  the
          file  system  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 file system 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),  and

       *  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.1-2001.  The real, effective, and saved set user and
       groups  IDs,  and  the  supplementary  group  IDs,  are  specified   in
       POSIX.1-2001.   The  file  system  user  and  group  IDs  are  a  Linux


       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),
       etc.)  is carried through to all of the POSIX threads in a process.


       bash(1),  csh(1),  ps(1),  access(2), execve(2), faccessat(2), fork(2),
       getpgrp(2),  getpid(2),  getppid(2),  getsid(2),  kill(2),   killpg(2),
       setegid(2),    seteuid(2),    setfsgid(2),    setfsuid(2),   setgid(2),
       setgroups(2),  setresgid(2),   setresuid(2),   setuid(2),   waitpid(2),
       euidaccess(3),      initgroups(3),      tcgetpgrp(3),     tcsetpgrp(3),
       capabilities(7), path_resolution(7), unix(7)


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