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NAME | DESCRIPTION | STANDARDS | NOTES | SEE ALSO | COLOPHON |
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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),
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 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
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 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
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)).
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 IDs:
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.
setgroups(2)
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
prctl(2).
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 information.
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), 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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
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man-pages@man7.org
Linux man-pages 6.15 2025-05-17 credentials(7)
Pages that refer to this page: renice(1), access(2), execve(2), fork(2), getgid(2), getgroups(2), getpid(2), getresuid(2), getrlimit(2), getsid(2), getuid(2), intro(2), keyctl(2), kill(2), openat2(2), PR_SET_DUMPABLE(2const), ptrace(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), setpgid(2), setresuid(2), setreuid(2), setsid(2), setuid(2), wait(2), euidaccess(3), id_t(3type), initgroups(3), intro(3), killpg(3), lttng-ust(3), sd_bus_creds_get_pid(3), tcgetpgrp(3), proc_sys_fs(5), capabilities(7), cgroup_namespaces(7), landlock(7), namespaces(7), nptl(7), path_resolution(7), pid_namespaces(7), unix(7), user_namespaces(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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