credentials(7)                                    Miscellaneous Information Manual                                   credentials(7)

       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), set‐
       priority(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  ses‐
       sion 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 cre‐
       ates 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 control‐
       ling terminal of at most one session.

       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 jobs 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), get‐
       priority(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 group 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) (getresgid(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 setfsgid(2).

       •  Supplementary group IDs.  This is a set of additional group IDs that are used for permission checks when accessing  files
          and  other shared resources.  Before Linux 2.6.4, a process can be a member of up to 32 supplementary groups; since Linux
          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 supple‐
          mentary group IDs using getgroups(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 de‐
       scribed 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_set‐
          param(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)).

   Modifying process user and group IDs
       Subject to rules described in the relevant manual pages, a process can use the following APIs to modify its user  and  group

       setuid(2) (setgid(2))
              Modify the process's real (and possibly effective and saved-set) user (group) IDs.

       seteuid(2) (setegid(2))
              Modify the process's effective user (group) ID.

       setfsuid(2) (setfsgid(2))
              Modify the process's filesystem user (group) ID.

       setreuid(2) (setregid(2))
              Modify the process's real and effective (and possibly saved-set) user (group) IDs.

       setresuid(2) (setresgid(2))
              Modify the process's real, effective, and saved-set user (group) IDs.

              Modify the process's supplementary group list.

       Any changes to a process's effective user (group) ID are automatically carried over to the process's filesystem user (group)
       ID.  Changes to a process's effective user or group ID can also affect the process "dumpable"  attribute,  as  described  in

       Changes to process user and group IDs can affect the capabilities of the process, as described in capabilities(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.

       Various  fields in the /proc/pid/status file show the process credentials described above.  See proc(5) for further informa‐

       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), facces‐
       sat(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), euidac‐
       cess(3), initgroups(3), killpg(3), tcgetpgrp(3), tcgetsid(3), tcsetpgrp(3), group(5), passwd(5), shadow(5), capabilities(7),
       namespaces(7),  path_resolution(7),  pid_namespaces(7),  pthreads(7),  signal(7),  system_data_types(7), unix(7), user_name‐
       spaces(7), sudo(8)

Linux man-pages 6.03                                         2023-02-05                                              credentials(7)