NAME
ntpdate - set the date and time via NTP
Disclaimer: The functionality of this program is now available in the ntpd program. See the -q command
line option in the ntpd - Network Time Protocol (NTP) daemon page. After a suitable period of mourning,
the ntpdate program is to be retired from this distribution
SYNOPSIS
ntpdate [ -46bBdqsuv ] [ -a key ] [ -e authdelay ] [ -k keyfile ] [ -o version ] [ -p samples ] [ -t
timeout ] [ -U user_name ] server [ ... ]
DESCRIPTION
ntpdate sets the local date and time by polling the Network Time Protocol (NTP) server(s) given as the
server arguments to determine the correct time. It must be run as root on the local host. A number of
samples are obtained from each of the servers specified and a subset of the NTP clock filter and selec-
tion algorithms are applied to select the best of these. Note that the accuracy and reliability of ntp-
date depends on the number of servers, the number of polls each time it is run and the interval between
runs.
ntpdate can be run manually as necessary to set the host clock, or it can be run from the host startup
script to set the clock at boot time. This is useful in some cases to set the clock initially before
starting the NTP daemon ntpd. It is also possible to run ntpdate from a cron script. However, it is
important to note that ntpdate with contrived cron scripts is no substitute for the NTP daemon, which
uses sophisticated algorithms to maximize accuracy and reliability while minimizing resource use.
Finally, since ntpdate does not discipline the host clock frequency as does ntpd, the accuracy using
ntpdate is limited.
Time adjustments are made by ntpdate in one of two ways. If ntpdate determines the clock is in error
more than 0.5 second it will simply step the time by calling the system settimeofday() routine. If the
error is less than 0.5 seconds, it will slew the time by calling the system adjtime() routine. The lat-
ter technique is less disruptive and more accurate when the error is small, and works quite well when
ntpdate is run by cron every hour or two.
ntpdate will decline to set the date if an NTP server daemon (e.g., ntpd) is running on the same host.
When running ntpdate on a regular basis from cron as an alternative to running a daemon, doing so once
every hour or two will result in precise enough timekeeping to avoid stepping the clock.
Note that in contexts where a host name is expected, a -4 qualifier preceding the host name forces DNS
resolution to the IPv4 namespace, while a -6 qualifier forces DNS resolution to the IPv6 namespace.
If NetInfo support is compiled into ntpdate, then the server argument is optional if ntpdate can find a
time server in the NetInfo configuration for ntpd.
http://bashconsole.org/man.8.ntpdate
