credentials(7) — Linux manual page


credentials(7)      Miscellaneous Information Manual      credentials(7)

NAME         top

       credentials - process identifiers

DESCRIPTION         top

   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

       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

       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 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),
       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

   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

       •  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
          supplementary 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 described in

       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

       •  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)).

   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).

STANDARDS         top

       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.

NOTES         top

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

       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

SEE ALSO         top

       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), 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_namespaces(7), sudo(8)

Linux man-pages 6.04           2023-03-30                 credentials(7)

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