uuidgen [option]

Create a new Universal Unique Identifier (UUID) and print it to standard output. The generated UUID consists of five hyphenseparated
groups of hex digits (e.g., 3cdfc61d-87d3-41b5-ba50-32870b33dc67). The default is to generate a random-based UUID, but this requires that a high-quality random-number generator be available on the system.

Options
-r Generate a random-based UUID.
-t Generate a time-based UUID.

usleep [microseconds]
usleep [option]

Sleep some number of microseconds (default is 1).

Options
-?, --help
Print help information and then exit.
--usage
Print brief usage message and then exit.
-v, --version
Print version information.

users [file]
users option

Print a space-separated list of each login session on the host. Note that this may include the same user multiple times. Consult file or, by default, /var/log/utmp or /var/log/wtmp.

Options
--help
Print usage information and exit.
--version
Print version information and exit.

usermod [options] user

System administration command. Modify user account information.

userdel [option] user

System administration command. Delete all entries for user in system account files.

Option
-f, --force
Remove the user even if they are currently logged in. Remove home directory and mail spool even if they are used by another user. Remove group too, if USERGROUPS_ENAB is set to yes in /etc/login.defs.
-r, --remove
Remove the home directory of user and any files contained in it.

useradd [options] [user]

System administration command. Create new user accounts or update default account information. Unless invoked with the -D option, user must be given. useradd will create new entries in system files. Home directories and initial files may also be created as needed.

uptime [option]

Print the current time, how long the system has been running, the number of users currently logged in (which may include the same
user multiple times), and system load averages. This output is also produced by the first line of w command output.

Option
-V Print version information and exit.

unlink filename
unlink option

Remove the specified file using the system unlink function.

Options
--help
Print help information and exit.
--version
Print version information and exit.

uniq [options] [file1 [file2]]

Remove duplicate adjacent lines from sorted file1 or from standard input, sending one copy of each line to file2 (or to standard
output). Often used as a filter. Specify only one of -d or -u. See also comm and sort.

unicode_stop

Take keyboard and console out of Unicode mode