414 lines
6.5 KiB
HTML
414 lines
6.5 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
|
|
<HTML
|
|
><HEAD
|
|
><TITLE
|
|
>Showing and setting time</TITLE
|
|
><META
|
|
NAME="GENERATOR"
|
|
CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK
|
|
REL="HOME"
|
|
TITLE="Linux System Administrators Guide"
|
|
HREF="index.html"><LINK
|
|
REL="UP"
|
|
TITLE="Keeping Time"
|
|
HREF="keeping-time.html"><LINK
|
|
REL="PREVIOUS"
|
|
TITLE="The hardware and software clocks"
|
|
HREF="hw-sw-clocks.html"><LINK
|
|
REL="NEXT"
|
|
TITLE="When the clock is wrong"
|
|
HREF="clock-wrong.html"></HEAD
|
|
><BODY
|
|
CLASS="SECT1"
|
|
BGCOLOR="#FFFFFF"
|
|
TEXT="#000000"
|
|
LINK="#0000FF"
|
|
VLINK="#840084"
|
|
ALINK="#0000FF"
|
|
><DIV
|
|
CLASS="NAVHEADER"
|
|
><TABLE
|
|
SUMMARY="Header navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TH
|
|
COLSPAN="3"
|
|
ALIGN="center"
|
|
>Linux System Administrators Guide: </TH
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="left"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="hw-sw-clocks.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="80%"
|
|
ALIGN="center"
|
|
VALIGN="bottom"
|
|
>Chapter 14. Keeping Time</TD
|
|
><TD
|
|
WIDTH="10%"
|
|
ALIGN="right"
|
|
VALIGN="bottom"
|
|
><A
|
|
HREF="clock-wrong.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"></DIV
|
|
><DIV
|
|
CLASS="SECT1"
|
|
><H1
|
|
CLASS="SECT1"
|
|
><A
|
|
NAME="SHOWING-SETTING-TIME"
|
|
></A
|
|
>14.3. Showing and setting time</H1
|
|
><P
|
|
> In Linux, the system time zone is determined
|
|
by the symbolic link <TT
|
|
CLASS="FILENAME"
|
|
>/etc/localtime</TT
|
|
>.
|
|
This link points to a time zone data file that describes
|
|
the local time zone. The time zone data files are located at
|
|
either <TT
|
|
CLASS="FILENAME"
|
|
>/usr/lib/zoneinfo</TT
|
|
> or
|
|
<TT
|
|
CLASS="FILENAME"
|
|
>/usr/share/zoneinfo</TT
|
|
> depending on what distribution
|
|
of Linux you use.</P
|
|
><P
|
|
> For example, on a SuSE system located in New Jersey the
|
|
<TT
|
|
CLASS="FILENAME"
|
|
>/etc/localtime</TT
|
|
> link would point to
|
|
<TT
|
|
CLASS="FILENAME"
|
|
>/usr/share/zoneinfo/US/Eastern</TT
|
|
>. On a Debian system
|
|
the <TT
|
|
CLASS="FILENAME"
|
|
>/etc/localtime</TT
|
|
> link would point to
|
|
<TT
|
|
CLASS="FILENAME"
|
|
>/usr/lib/zoneinfo/US/Eastern</TT
|
|
>.</P
|
|
><P
|
|
> If you fail to find the <TT
|
|
CLASS="FILENAME"
|
|
>zoneinfo</TT
|
|
>
|
|
directory in either the <TT
|
|
CLASS="FILENAME"
|
|
>/usr/lib</TT
|
|
> or
|
|
<TT
|
|
CLASS="FILENAME"
|
|
>/usr/share</TT
|
|
> directories, either do a
|
|
<B
|
|
CLASS="COMMAND"
|
|
>find /usr -print | grep zoneinfo</B
|
|
> or consult
|
|
your distribution's documentation.
|
|
</P
|
|
><P
|
|
> 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
|
|
<TT
|
|
CLASS="FUNCTION"
|
|
>tzset</TT
|
|
> manual page. </P
|
|
><P
|
|
> The <B
|
|
CLASS="COMMAND"
|
|
>date</B
|
|
> command shows the current date and
|
|
time.
|
|
|
|
For example:
|
|
|
|
<TABLE
|
|
BORDER="1"
|
|
BGCOLOR="#E0E0E0"
|
|
WIDTH="100%"
|
|
><TR
|
|
><TD
|
|
><FONT
|
|
COLOR="#000000"
|
|
><PRE
|
|
CLASS="SCREEN"
|
|
><TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
> <TT
|
|
CLASS="USERINPUT"
|
|
><B
|
|
>date</B
|
|
></TT
|
|
>
|
|
<TT
|
|
CLASS="COMPUTEROUTPUT"
|
|
>Sun Jul 14 21:53:41 EET DST 1996</TT
|
|
>
|
|
<TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
></PRE
|
|
></FONT
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
>
|
|
|
|
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).
|
|
<B
|
|
CLASS="COMMAND"
|
|
>date</B
|
|
> can also show the universal time:
|
|
|
|
<TABLE
|
|
BORDER="1"
|
|
BGCOLOR="#E0E0E0"
|
|
WIDTH="100%"
|
|
><TR
|
|
><TD
|
|
><FONT
|
|
COLOR="#000000"
|
|
><PRE
|
|
CLASS="SCREEN"
|
|
><TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
> <TT
|
|
CLASS="USERINPUT"
|
|
><B
|
|
>date -u</B
|
|
></TT
|
|
>
|
|
<TT
|
|
CLASS="COMPUTEROUTPUT"
|
|
>Sun Jul 14 18:53:42 UTC 1996</TT
|
|
>
|
|
<TT
|
|
CLASS="PROMPT"
|
|
>$</TT
|
|
></PRE
|
|
></FONT
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
>
|
|
|
|
<B
|
|
CLASS="COMMAND"
|
|
>date</B
|
|
> is also used to set the kernel's software
|
|
clock:
|
|
|
|
<TABLE
|
|
BORDER="1"
|
|
BGCOLOR="#E0E0E0"
|
|
WIDTH="100%"
|
|
><TR
|
|
><TD
|
|
><FONT
|
|
COLOR="#000000"
|
|
><PRE
|
|
CLASS="SCREEN"
|
|
><TT
|
|
CLASS="PROMPT"
|
|
>#</TT
|
|
> <TT
|
|
CLASS="USERINPUT"
|
|
><B
|
|
>date 07142157</B
|
|
></TT
|
|
>
|
|
<TT
|
|
CLASS="COMPUTEROUTPUT"
|
|
>Sun Jul 14 21:57:00 EET DST 1996</TT
|
|
>
|
|
<TT
|
|
CLASS="PROMPT"
|
|
>#</TT
|
|
> <TT
|
|
CLASS="USERINPUT"
|
|
><B
|
|
>date</B
|
|
></TT
|
|
>
|
|
<TT
|
|
CLASS="COMPUTEROUTPUT"
|
|
>Sun Jul 14 21:57:02 EET DST 1996</TT
|
|
>
|
|
<TT
|
|
CLASS="PROMPT"
|
|
>#</TT
|
|
></PRE
|
|
></FONT
|
|
></TD
|
|
></TR
|
|
></TABLE
|
|
>
|
|
|
|
See the <B
|
|
CLASS="COMMAND"
|
|
>date</B
|
|
> 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. </P
|
|
><P
|
|
>Beware of the <B
|
|
CLASS="COMMAND"
|
|
>time</B
|
|
> 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.</P
|
|
><P
|
|
> <B
|
|
CLASS="COMMAND"
|
|
>date</B
|
|
> only shows or sets the software
|
|
clock. The <B
|
|
CLASS="COMMAND"
|
|
>clock</B
|
|
> 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 <B
|
|
CLASS="COMMAND"
|
|
>date</B
|
|
>, and then the hardware clock with
|
|
<B
|
|
CLASS="COMMAND"
|
|
>clock -w</B
|
|
>. </P
|
|
><P
|
|
> The <TT
|
|
CLASS="OPTION"
|
|
>-u</TT
|
|
> option to <B
|
|
CLASS="COMMAND"
|
|
>clock</B
|
|
>
|
|
tells it that the hardware clock is in universal time.
|
|
You <EM
|
|
>must</EM
|
|
> use the <TT
|
|
CLASS="OPTION"
|
|
>-u</TT
|
|
>
|
|
option correctly. If you don't, your computer will be quite
|
|
confused about what the time is. </P
|
|
><P
|
|
> The clocks should be changed with care. Many parts of a
|
|
Unix system require the clocks to work correctly. For example,
|
|
the <B
|
|
CLASS="COMMAND"
|
|
>cron</B
|
|
> 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 <B
|
|
CLASS="COMMAND"
|
|
>cron</B
|
|
> wanted to run all the periodic
|
|
commands for twenty years all at once. Current versions of
|
|
<B
|
|
CLASS="COMMAND"
|
|
>cron</B
|
|
> can handle this correctly, but you should
|
|
still be careful. Big jumps or backward jumps are more dangerous
|
|
than smaller or forward ones. </P
|
|
></DIV
|
|
><DIV
|
|
CLASS="NAVFOOTER"
|
|
><HR
|
|
ALIGN="LEFT"
|
|
WIDTH="100%"><TABLE
|
|
SUMMARY="Footer navigation table"
|
|
WIDTH="100%"
|
|
BORDER="0"
|
|
CELLPADDING="0"
|
|
CELLSPACING="0"
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="hw-sw-clocks.html"
|
|
ACCESSKEY="P"
|
|
>Prev</A
|
|
></TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="index.html"
|
|
ACCESSKEY="H"
|
|
>Home</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="clock-wrong.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
|
></TR
|
|
><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
|
>The hardware and software clocks</TD
|
|
><TD
|
|
WIDTH="34%"
|
|
ALIGN="center"
|
|
VALIGN="top"
|
|
><A
|
|
HREF="keeping-time.html"
|
|
ACCESSKEY="U"
|
|
>Up</A
|
|
></TD
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="right"
|
|
VALIGN="top"
|
|
>When the clock is wrong</TD
|
|
></TR
|
|
></TABLE
|
|
></DIV
|
|
></BODY
|
|
></HTML
|
|
> |