Provided by: manpages-posix_2017a-2_all 
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
id — return user identity
SYNOPSIS
id [user]
id -G [-n] [user]
id -g [-nr] [user]
id -u [-nr] [user]
DESCRIPTION
If no user operand is provided, the id utility shall write the user and group IDs and the
corresponding user and group names of the invoking process to standard output. If the
effective and real IDs do not match, both shall be written. If multiple groups are
supported by the underlying system (see the description of {NGROUPS_MAX} in the System
Interfaces volume of POSIX.1‐2017), the supplementary group affiliations of the invoking
process shall also be written.
If a user operand is provided and the process has appropriate privileges, the user and
group IDs of the selected user shall be written. In this case, effective IDs shall be
assumed to be identical to real IDs. If the selected user has more than one allowable
group membership listed in the group database, these shall be written in the same manner
as the supplementary groups described in the preceding paragraph.
OPTIONS
The id utility shall conform to the Base Definitions volume of POSIX.1‐2017, Section 12.2,
Utility Syntax Guidelines.
The following options shall be supported:
-G Output all different group IDs (effective, real, and supplementary) only, using
the format "%u\n". If there is more than one distinct group affiliation, output
each such affiliation, using the format " %u", before the <newline> is output.
-g Output only the effective group ID, using the format "%u\n".
-n Output the name in the format "%s" instead of the numeric ID using the format
"%u".
-r Output the real ID instead of the effective ID.
-u Output only the effective user ID, using the format "%u\n".
OPERANDS
The following operand shall be supported:
user The login name for which information is to be written.
STDIN
Not used.
INPUT FILES
None.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of id:
LANG Provide a default value for the internationalization variables that are unset or
null. (See the Base Definitions volume of POSIX.1‐2017, Section 8.2,
Internationalization Variables for the precedence of internationalization
variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all the other
internationalization variables.
LC_CTYPE Determine the locale for the interpretation of sequences of bytes of text data
as characters (for example, single-byte as opposed to multi-byte characters in
arguments).
LC_MESSAGES
Determine the locale that should be used to affect the format and contents of
diagnostic messages written to standard error and informative messages written
to standard output.
NLSPATH Determine the location of message catalogs for the processing of LC_MESSAGES.
ASYNCHRONOUS EVENTS
Default.
STDOUT
The following formats shall be used when the LC_MESSAGES locale category specifies the
POSIX locale. In other locales, the strings uid, gid, euid, egid, and groups may be
replaced with more appropriate strings corresponding to the locale.
"uid=%u(%s) gid=%u(%s)\n", <real user ID>, <user-name>,
<real group ID>, <group-name>
If the effective and real user IDs do not match, the following shall be inserted
immediately before the '\n' character in the previous format:
" euid=%u(%s)"
with the following arguments added at the end of the argument list:
<effective user ID>, <effective user-name>
If the effective and real group IDs do not match, the following shall be inserted directly
before the '\n' character in the format string (and after any addition resulting from the
effective and real user IDs not matching):
" egid=%u(%s)"
with the following arguments added at the end of the argument list:
<effective group-ID>, <effective group name>
If the process has supplementary group affiliations or the selected user is allowed to
belong to multiple groups, the first shall be added directly before the <newline> in the
format string:
" groups=%u(%s)"
with the following arguments added at the end of the argument list:
<supplementary group ID>, <supplementary group name>
and the necessary number of the following added after that for any remaining supplementary
group IDs:
",%u(%s)"
and the necessary number of the following arguments added at the end of the argument list:
<supplementary group ID>, <supplementary group name>
If any of the user ID, group ID, effective user ID, effective group ID, or
supplementary/multiple group IDs cannot be mapped by the system into printable user or
group names, the corresponding "(%s)" and name argument shall be omitted from the
corresponding format string.
When any of the options are specified, the output format shall be as described in the
OPTIONS section.
STDERR
The standard error shall be used only for diagnostic messages.
OUTPUT FILES
None.
EXTENDED DESCRIPTION
None.
EXIT STATUS
The following exit values shall be returned:
0 Successful completion.
>0 An error occurred.
CONSEQUENCES OF ERRORS
Default.
The following sections are informative.
APPLICATION USAGE
Output produced by the -G option and by the default case could potentially produce very
long lines on systems that support large numbers of supplementary groups. (On systems
with user and group IDs that are 32-bit integers and with group names with a maximum of 8
bytes per name, 93 supplementary groups plus distinct effective and real group and user
IDs could theoretically overflow the 2048-byte {LINE_MAX} text file line limit on the
default output case. It would take about 186 supplementary groups to overflow the
2048-byte barrier using id -G). This is not expected to be a problem in practice, but in
cases where it is a concern, applications should consider using fold -s before post-
processing the output of id.
EXAMPLES
None.
RATIONALE
The functionality provided by the 4 BSD groups utility can be simulated using:
id -Gn [ user ]
The 4 BSD command groups was considered, but it was not included because it did not
provide the functionality of the id utility of the SVID. Also, it was thought that it
would be easier to modify id to provide the additional functionality necessary to systems
with multiple groups than to invent another command.
The options -u, -g, -n, and -r were added to ease the use of id with shell commands
substitution. Without these options it is necessary to use some preprocessor such as sed
to select the desired piece of information. Since output such as that produced by:
id -u -n
is frequently wanted, it seemed desirable to add the options.
FUTURE DIRECTIONS
None.
SEE ALSO
fold, logname, who
The Base Definitions volume of POSIX.1‐2017, Chapter 8, Environment Variables, Section
12.2, Utility Syntax Guidelines
The System Interfaces volume of POSIX.1‐2017, getgid(), getgroups(), getuid()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .