timedate.html

Shall高级编程

CLASS="USERINPUT"
><B
>&#62;&#62; newfile</B
></TT
> (for ordinary
	      files).</P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="TIP"
><TABLE
CLASS="TIP"
WIDTH="90%"
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><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/tip.png"
HSPACE="5"
ALT="Tip"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>Before doing a <A
HREF="external.html#CPREF"
>cp -u</A
>
	      (<I
CLASS="FIRSTTERM"
>copy/update</I
>), use
	      <B
CLASS="COMMAND"
>touch</B
> to update the time stamp of files
	      you don't wish overwritten.</P
><P
>As an example, if the directory <TT
CLASS="FILENAME"
>/home/bozo/tax_audit</TT
> contains the
	      files <TT
CLASS="FILENAME"
>spreadsheet-051606.data</TT
>,
	      <TT
CLASS="FILENAME"
>spreadsheet-051706.data</TT
>, and
	      <TT
CLASS="FILENAME"
>spreadsheet-051806.data</TT
>, then
	      doing a <B
CLASS="COMMAND"
>touch spreadsheet*.data</B
>
	      will protect these files from being overwritten
	      by files with the same names during a
	      <B
CLASS="COMMAND"
>cp -u /home/bozo/financial_info/spreadsheet*data
	      /home/bozo/tax_audit</B
>.</P
></TD
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></TABLE
></DIV
></DD
><DT
><A
NAME="ATREF"
></A
><B
CLASS="COMMAND"
>at</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>at</B
> job control command executes
	      a given set of commands at a specified time. Superficially,
	      it resembles <A
HREF="system.html#CRONREF"
>cron</A
>, however,
	      <B
CLASS="COMMAND"
>at</B
> is chiefly useful for one-time execution
	      of a command set.</P
><P
><TT
CLASS="USERINPUT"
><B
>at 2pm January 15</B
></TT
> prompts for a set of
	      commands to execute at that time. These commands should be
	      shell-script compatible, since, for all practical
	      purposes, the user is typing in an executable shell
	      script a line at a time.	Input terminates with a <A
HREF="special-chars.html#CTLDREF"
>Ctl-D</A
>.</P
><P
>Using either the <TT
CLASS="OPTION"
>-f</TT
> option or input
	      redirection (<SPAN
CLASS="TOKEN"
>&#60;</SPAN
>), <B
CLASS="COMMAND"
>at</B
>
	      reads a command list from a file. This file is an
	      executable shell script, though it should, of course,
	      be non-interactive. Particularly clever is including the
	      <A
HREF="extmisc.html#RUNPARTSREF"
>run-parts</A
> command in
	      the file to execute a different set of scripts.</P
><P
>	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="SCREEN"
> <TT
CLASS="PROMPT"
>bash$ </TT
><TT
CLASS="USERINPUT"
><B
>at 2:30 am Friday &#60; at-jobs.list</B
></TT
>
 <TT
CLASS="COMPUTEROUTPUT"
>job 2 at 2000-10-27 02:30</TT
>
 	      </PRE
></TD
></TR
></TABLE
>
	    </P
></DD
><DT
><A
NAME="BATCHREF"
></A
><B
CLASS="COMMAND"
>batch</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>batch</B
> job control command is similar to
	      <B
CLASS="COMMAND"
>at</B
>, but it runs a command list when the system
	      load drops below <TT
CLASS="LITERAL"
>.8</TT
>. Like
	      <B
CLASS="COMMAND"
>at</B
>, it can read commands from a file with the
	      <TT
CLASS="OPTION"
>-f</TT
> option.</P
><P
><A
NAME="BATCHPROCREF"
></A
></P
><TABLE
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BORDER="1"
CELLPADDING="5"
><TR
><TD
><DIV
CLASS="SIDEBAR"
><A
NAME="AEN9868"
></A
><P
>The concept of <I
CLASS="FIRSTTERM"
>batch processing</I
>
              dates back to the era of mainframe computers. It means
              running a set of commands without user intervention.</P
></DIV
></TD
></TR
></TABLE
></DD
><DT
><A
NAME="CALREF"
></A
><B
CLASS="COMMAND"
>cal</B
></DT
><DD
><P
>Prints a neatly formatted monthly calendar to
	      <TT
CLASS="FILENAME"
>stdout</TT
>. Will do current year or a large
	      range of past and future years.</P
></DD
><DT
><A
NAME="SLEEPREF"
></A
><B
CLASS="COMMAND"
>sleep</B
></DT
><DD
><P
>This is the shell equivalent of a <I
CLASS="FIRSTTERM"
>wait
	      loop</I
>. It pauses for a specified number of
	      seconds, doing nothing. It can be useful for timing or
	      in processes running in the background, checking for
	      a specific event every so often (polling), as in <A
HREF="debugging.html#ONLINE"
>Example 29-6</A
>.  <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;sleep 3     # Pauses
   2&nbsp;	      3 seconds.</PRE
></TD
></TR
></TABLE
>
	    </P
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>sleep</B
> command defaults to
	      seconds, but minute, hours, or days may also be specified.
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;sleep 3 h   # Pauses 3 hours!</PRE
></TD
></TR
></TABLE
>
            </P
></TD
></TR
></TABLE
></DIV
><DIV
CLASS="NOTE"
><TABLE
CLASS="NOTE"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/note.png"
HSPACE="5"
ALT="Note"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <A
HREF="system.html#WATCHREF"
>watch</A
> command may
	      be a better choice than <B
CLASS="COMMAND"
>sleep</B
> for running
	      commands at timed intervals.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="USLEEPREF"
></A
><B
CLASS="COMMAND"
>usleep</B
></DT
><DD
><P
><I
CLASS="FIRSTTERM"
>Microsleep</I
> (the
	      <I
CLASS="FIRSTTERM"
>u</I
> may be read as the Greek
	      <I
CLASS="FIRSTTERM"
>mu</I
>, or <I
CLASS="FIRSTTERM"
>micro-</I
>
	      prefix). This is the same as <B
CLASS="COMMAND"
>sleep</B
>,
	      above, but <SPAN
CLASS="QUOTE"
>"sleeps"</SPAN
> in microsecond
	      intervals. It can be used for fine-grained timing,
	      or for polling an ongoing process at very frequent
	      intervals.</P
><P
>  
	      <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;usleep 30     # Pauses 30 microseconds.</PRE
></TD
></TR
></TABLE
>
	    </P
><P
>This command is part of the Red Hat
	    <I
CLASS="FIRSTTERM"
>initscripts / rc-scripts</I
> package.</P
><DIV
CLASS="CAUTION"
><TABLE
CLASS="CAUTION"
WIDTH="90%"
BORDER="0"
><TR
><TD
WIDTH="25"
ALIGN="CENTER"
VALIGN="TOP"
><IMG
SRC="common/caution.png"
HSPACE="5"
ALT="Caution"></TD
><TD
ALIGN="LEFT"
VALIGN="TOP"
><P
>The <B
CLASS="COMMAND"
>usleep</B
> command does not
	      provide particularly accurate timing, and is therefore
	      unsuitable for critical timing loops.</P
></TD
></TR
></TABLE
></DIV
></DD
><DT
><A
NAME="HWCLOCKREF"
></A
><B
CLASS="COMMAND"
>hwclock</B
>, <A
NAME="CLOCKREF"
></A
><B
CLASS="COMMAND"
>clock</B
></DT
><DD
><P
>The <B
CLASS="COMMAND"
>hwclock</B
> command accesses or
	      adjusts the machine's hardware clock. Some options
	      require <I
CLASS="FIRSTTERM"
>root</I
> privileges. The
	      <TT
CLASS="FILENAME"
>/etc/rc.d/rc.sysinit</TT
> startup file
	      uses <B
CLASS="COMMAND"
>hwclock</B
> to set the system time
	      from the hardware clock at bootup.</P
><P
>The <B
CLASS="COMMAND"
>clock</B
> command is a synonym for
	      <B
CLASS="COMMAND"
>hwclock</B
>.</P
></DD
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