14.3. Showing and setting time
In Linux, the system time zone is determined
by the symbolic link /etc/localtime.
This link points to a time zone data file that describes
the local time zone. The time zone data files are located at
either /usr/lib/zoneinfo or
/usr/share/zoneinfo depending on what distribution
of Linux you use.
For example, on a SuSE system located in New Jersey the
/etc/localtime link would point to
/usr/share/zoneinfo/US/Eastern. On a Debian system
the /etc/localtime link would point to
/usr/lib/zoneinfo/US/Eastern.
If you fail to find the zoneinfo
directory in either the /usr/lib or
/usr/share directories, either do a
find /usr -print | grep zoneinfo or consult
your distribution's documentation.
What happens when you have a users located in a different
timezone? A user can change his private time zone by setting the
TZ environment variable. If it is unset, the system time zone
is assumed. The syntax of the TZ variable is described in the
tzset manual page.
The date command shows the current date and
time.
For example:
$ date
Sun Jul 14 21:53:41 EET DST 1996
$ |
That time is Sunday, 14th of July, 1996, at about ten before
ten at the evening, in the time zone called ``EET DST''
(which might be East European Daylight Savings Time).
date can also show the universal time:
$ date -u
Sun Jul 14 18:53:42 UTC 1996
$ |
date is also used to set the kernel's software
clock:
# date 07142157
Sun Jul 14 21:57:00 EET DST 1996
# date
Sun Jul 14 21:57:02 EET DST 1996
# |
See the
date manual page for more details;
the syntax is a bit arcane. Only root can set the time.
While each user can have his own time zone, the clock is the
same for everyone.
Beware of the time command. This is not
used to get the system time. Instead it's used to time how long
something takes. Refer the the time man page.
date only shows or sets the software
clock. The clock commands synchronizes
the hardware and software clocks. It is used when the system
boots, to read the hardware clock and set the software clock.
If you need to set both clocks, you first set the software clock
with date, and then the hardware clock with
clock -w.
The -u option to clock
tells it that the hardware clock is in universal time.
You must use the -u
option correctly. If you don't, your computer will be quite
confused about what the time is.
The clocks should be changed with care. Many parts of a
Unix system require the clocks to work correctly. For example,
the cron daemon runs commands periodically.
If you change the clock, it can be confused of whether
it needs to run the commands or not. On one early Unix
system, someone set the clock twenty years into the future,
and cron wanted to run all the periodic
commands for twenty years all at once. Current versions of
cron can handle this correctly, but you should
still be careful. Big jumps or backward jumps are more dangerous
than smaller or forward ones.
