631 lines
9.4 KiB
HTML
631 lines
9.4 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<HTML
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><HEAD
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>Scheduling Commands to run in the background</TITLE
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>Prev</A
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ALIGN="LEFT"
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WIDTH="100%"></DIV
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><DIV
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CLASS="CHAPTER"
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><H1
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><A
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NAME="SCHEDULING"
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></A
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>Chapter 18. Scheduling Commands to run in the background</H1
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><P
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>There are two main tools used to perform scheduled tasks, <EM
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>at</EM
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> and <EM
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>cron</EM
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>. You may also like to try <A
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HREF="http://anacron.sourceforge.net"
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TARGET="_top"
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>anacron</A
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> if your computer does not run continuously, as cron will only work if your computer is left on (anacron can catch up with the scheduled tasks the next time the computer is on...).</P
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><P
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></P
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><DIV
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CLASS="VARIABLELIST"
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><DL
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><DT
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>at</DT
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><DD
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><P
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>'at' executes a command once on a particular day, at a particular time. <EM
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>at</EM
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> will add a particular command to be executed.</P
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><P
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>Examples:</P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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>at 21:30</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>You then type the commands you want executed then press the end-of-file key (normally
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<B
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CLASS="KEYCAP"
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>CTRL</B
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>-<B
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CLASS="KEYCAP"
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>D</B
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>
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). Also try:</P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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>at now + time</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>This will run at the current time + the hours/mins/seconds you specify (use<EM
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> at now + 1 hour </EM
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>to have command(s) run in 1 hour from now...)</P
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><P
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>You can also use the <EM
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>-f</EM
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> option to have at execute a particular file (a shell script).</P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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>at -f shell_script now + 1 hour</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>This would run the shell script 1 hour from now.</P
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></DD
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><DT
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>atq</DT
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><DD
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><P
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>Will list jobs currently in queue for the user who executed it, if root executes at it will list all jobs in queue for the at daemon. Doesn't need or take any options.</P
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></DD
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><DT
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>atrm</DT
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><DD
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><P
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>Will remove a job from the 'at' queue.</P
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><P
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>Command syntax:</P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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>atrm job_no</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>Will delete the job “job_no” (use <EM
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>atq</EM
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> to find out the number of the job)</P
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></DD
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><DT
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>cron<EM
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></EM
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></DT
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><DD
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><P
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>cron can be used to schedule a particular function to occur every minute, hour, day, week, or month. </P
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><P
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>It's normal to use the crontab to perform the editing functions as this automates the process for the cron daemon and makes it easier for normal users to use cron.</P
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><DIV
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CLASS="TIP"
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><P
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></P
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><TABLE
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CLASS="TIP"
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WIDTH="90%"
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BORDER="0"
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><TR
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><TD
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WIDTH="25"
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ALIGN="CENTER"
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VALIGN="TOP"
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><IMG
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SRC="../images/tip.gif"
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HSPACE="5"
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ALT="Tip"></TD
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><TH
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ALIGN="LEFT"
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VALIGN="CENTER"
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><B
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>Anacron</B
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></TH
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></TR
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><TR
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><TD
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> </TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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><P
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><EM
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>anacron</EM
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> is another tool designed for systems which are not always on, such as home computers</P
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><P
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>While <EM
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>cron </EM
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>will not run if the computer is off, <EM
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>anacron </EM
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>will simply run the command when the computer is next on (it catches up with things).</P
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></TD
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></TR
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></TABLE
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></DIV
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><P
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></P
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></DD
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><DT
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>crontab</DT
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><DD
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><P
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><EM
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>crontab</EM
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> is used to edit, read and remove the files which the cron daemon reads. </P
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><P
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>Options for crontab (use<EM
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> crontab -option(s)</EM
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>):</P
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><P
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></P
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><UL
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><LI
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><P
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><EM
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>-e</EM
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> --- to edit the file.</P
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></LI
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><LI
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><P
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><EM
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>-l </EM
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>--- to list the contents of the file.</P
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></LI
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><LI
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><P
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><EM
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>-u username </EM
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>--- use the <EM
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>-u</EM
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> with a username argument to work with another users crontab file.</P
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></LI
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></UL
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><P
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>When using <EM
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> crontab -e </EM
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>you have a number of fields (6) what they mean is listed below:</P
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><P
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><DIV
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CLASS="INFORMALTABLE"
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><A
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NAME="AEN11094"
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></A
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><P
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></P
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><TABLE
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BORDER="1"
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CLASS="CALSTABLE"
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><TBODY
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><TR
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>Field</P
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></TD
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>Allowed Values</P
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></TD
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></TR
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><TR
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>minute</P
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></TD
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>0-59</P
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></TD
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></TR
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><TR
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>hour</P
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></TD
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>0-23</P
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></TD
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></TR
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><TR
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>day of month</P
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></TD
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>1-31</P
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></TD
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></TR
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><TR
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>month</P
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></TD
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>1-12 (or names, see below)</P
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></TD
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></TR
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><TR
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>day of week</P
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></TD
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><TD
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WIDTH="50%"
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ALIGN="CENTER"
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VALIGN="TOP"
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><P
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>0-7 (0 or 7 is Sun, or use three letter names)</P
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></TD
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></TR
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></TBODY
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></TABLE
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><P
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></P
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></DIV
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></P
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><P
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>There are also a number of shortcut methods for common tasks, including:<A
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NAME="AEN11136"
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HREF="#FTN.AEN11136"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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></P
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><P
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></P
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><UL
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><LI
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><P
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><EM
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>@reboot</EM
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> --- run command at reboot </P
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></LI
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><LI
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><P
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><EM
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>@yearly</EM
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> --- same as 0 0 1 1 * </P
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></LI
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><LI
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><P
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><EM
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>@annually</EM
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> --- same as @yearly </P
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></LI
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><LI
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><P
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><EM
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>@monthly</EM
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> --- same as 0 0 1 * * </P
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></LI
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><LI
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><P
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><EM
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>@weekly</EM
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> --- same as 0 0 * * 0 </P
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></LI
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><LI
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><P
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><EM
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>@daily</EM
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> --- same as 0 0 * * * </P
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></LI
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><LI
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><P
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><EM
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>@midnight</EM
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> --- same as @daily </P
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></LI
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><LI
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><P
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><EM
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>@hourly</EM
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> --- same as 0 * * * * </P
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></LI
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></UL
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><P
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><A
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NAME="AEN11169"
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HREF="#FTN.AEN11169"
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><SPAN
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CLASS="footnote"
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>[2]</SPAN
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></A
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></P
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><P
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>Note that * (asterisk) is used to mean anything (similar to the wildcard). For example if you leave the day part (the 5th place) with an asterisk it would mean everyday.</P
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><P
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>Lists are allowed. A list is a set of numbers (or ranges) separated by commas. Examples: ``1,2,5,9'', ``0-4,8-12”.</P
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><P
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>Step values can be used in conjunction with ranges. Following a range with ``/<number>'' specifies skips of the number's value through the range. For example, ``0-23/2'' can be used in the hours field to specify command execution every other hour (the alternative in the V7 standard is ``0,2,4,6,8,10,12,14,16,18,20,22''). Steps are also permitted after an asterisk, so if you want to say ``every two hours'', just use ``*/2''.</P
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><P
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>When writing a crontab entry you simply type in six fields separated by spaces, the first five are those listed in the table (using numbers or letters and numbers as appropriate), the 6th field is the command to be executed and any options, cron will read everything up until the newline.</P
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><P
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>Example:</P
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><TABLE
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BORDER="1"
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BGCOLOR="#E0E0E0"
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WIDTH="90%"
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><TR
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><TD
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><FONT
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COLOR="#000000"
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><PRE
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CLASS="SCREEN"
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>5 4 * * sun echo "run at 5 after 4 every sunday"</PRE
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></FONT
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></TD
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></TR
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></TABLE
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><P
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>This would run the echo command with the string shown at 4:05 every Sunday.</P
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></DD
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></DL
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></DIV
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><P
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></P
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></DIV
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><H3
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CLASS="FOOTNOTES"
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>Notes</H3
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><TABLE
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BORDER="0"
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CLASS="FOOTNOTES"
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WIDTH="100%"
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><TR
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="5%"
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><A
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NAME="FTN.AEN11136"
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HREF="scheduling.html#AEN11136"
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><SPAN
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CLASS="footnote"
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>[1]</SPAN
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></A
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></TD
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><TD
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ALIGN="LEFT"
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VALIGN="TOP"
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WIDTH="95%"
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><P
|
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>This information has come from (without editing) a post on the LinuxChix techtalk mailing list, please see [16] in the <A
|
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HREF="references.html"
|
|
><I
|
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>Bibliography</I
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></A
|
|
> for further information.</P
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></TD
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></TR
|
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><TR
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><TD
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ALIGN="LEFT"
|
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VALIGN="TOP"
|
|
WIDTH="5%"
|
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><A
|
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NAME="FTN.AEN11169"
|
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HREF="scheduling.html#AEN11169"
|
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><SPAN
|
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CLASS="footnote"
|
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>[2]</SPAN
|
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></A
|
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></TD
|
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><TD
|
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ALIGN="LEFT"
|
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VALIGN="TOP"
|
|
WIDTH="95%"
|
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><P
|
|
>This information comes from the cron manual page with small additions (no changes to original content), refer to [13] in the <A
|
|
HREF="references.html"
|
|
><I
|
|
>Bibliography</I
|
|
></A
|
|
> for further information.</P
|
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></TD
|
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></TR
|
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></TABLE
|
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><DIV
|
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CLASS="NAVFOOTER"
|
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><HR
|
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ALIGN="LEFT"
|
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WIDTH="100%"><TABLE
|
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SUMMARY="Footer navigation table"
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WIDTH="100%"
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BORDER="0"
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CELLPADDING="0"
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CELLSPACING="0"
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><TR
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><TD
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WIDTH="33%"
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ALIGN="left"
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VALIGN="top"
|
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><A
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HREF="working-with-ms-dos.html"
|
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ACCESSKEY="P"
|
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>Prev</A
|
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></TD
|
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><TD
|
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WIDTH="34%"
|
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ALIGN="center"
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VALIGN="top"
|
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><A
|
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HREF="index.html"
|
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ACCESSKEY="H"
|
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>Home</A
|
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></TD
|
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><TD
|
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WIDTH="33%"
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|
ALIGN="right"
|
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VALIGN="top"
|
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><A
|
|
HREF="miscellaneous.html"
|
|
ACCESSKEY="N"
|
|
>Next</A
|
|
></TD
|
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></TR
|
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><TR
|
|
><TD
|
|
WIDTH="33%"
|
|
ALIGN="left"
|
|
VALIGN="top"
|
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>Working with MS-DOS files</TD
|
|
><TD
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WIDTH="34%"
|
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ALIGN="center"
|
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VALIGN="top"
|
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> </TD
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><TD
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WIDTH="33%"
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ALIGN="right"
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VALIGN="top"
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>Miscellaneous</TD
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></TR
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></TABLE
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></DIV
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></BODY
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></HTML
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> |