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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 get-
sid(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 collec-
tion of processes that share the same process group ID; the shell cre-
ates 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 pro-
cess 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 ses-
sion 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
of a process group, including kill(2), killpg(2), getpriority(2), set-
priority(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
* 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 filesys-
tem 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 corre-
sponding effective IDs that were set when the program was executed
(see execve(2)). A set-user-ID program can assume and drop privi-
leges 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
* 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
* 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_setaffin-
ity(2), sched_setscheduler(2), sched_setparam(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.1-2001. The real, effective, and saved set user and
groups IDs, and the supplementary group IDs, are specified in
POSIX.1-2001. 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.
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), sete-
gid(2), seteuid(2), setfsgid(2), setfsuid(2), setgid(2), setgroups(2),
setresgid(2), setresuid(2), setuid(2), waitpid(2), euidaccess(3), init-
groups(3), tcgetpgrp(3), tcsetpgrp(3), capabilities(7), path_resolu-
tion(7), signal(7), unix(7)
Linux 2013-12-27 CREDENTIALS(7)