id 命令可以显示真实有效的用户 ID(UID) 和组 ID(GID)。UID 是对一个用户的单一身份标识。组 ID（GID）则对应多个UID。
Usage: id [OPTION]... [USERNAME]
Print user and group information for the specified USERNAME,
or (when USERNAME omitted) for the current user.
-a ignore, for compatibility with other versions 忽略该命令,仅为了与其他版本兼容
-Z, --context print only the security context of the current user 仅输出当前用户的security context//id: --context (-Z) works only on an SELinux-enabled kernel
-g, --group print only the effective group ID 显示用户所属群组的ID
-G, --groups print all group IDs 显示用户所属附加群组的ID
-n, --name print a name instead of a number, for -ugG 显示用户，所属群组或附加群组的名称
-r, --real print the real ID instead of the effective ID, with -ugG 显示实际ID
-u, --user print only the effective user ID 显示用户ID
--help display this help and exit 显示帮助
--version output version information and exit 显示版本信息
Without any OPTION, print some useful set of identified information.
Report id bugs to bug-coreutils@gnu.org
GNU coreutils home page: <http://www.gnu.org/software/coreutils/>
General help using GNU software: <http://www.gnu.org/gethelp/>
Report id translation bugs to <http://translationproject.org/team/>
For complete documentation, run: info coreutils 'id invocation'
