ch40_12.htm

the unix power tools

>The month of the year.</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>1-12</TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>weekday</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>The day of the week.</TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
>1-7  (1 = Monday) <EM
CLASS="emphasis"
>BSD</EM
></TD
></TR
><TR
CLASS="row"
VALIGN="TOP"
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
></TD
><TD
CLASS="entry"
ROWSPAN="1"
COLSPAN="1"
></TD
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CLASS="entry"
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>0-6  (0=Sunday) <EM
CLASS="emphasis"
>System V</EM
></TD
></TR
></TBODY
></TABLE
><P
CLASS="para"
>These fields can contain a single number, a pair of numbers
separated by a dash (indicating a range of numbers), a comma-separated list
of numbers and ranges, or an asterisk (a wildcard that represents all valid
values for that field).</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45109"
></A
>If the first character in an entry is a hash mark (#), <EM
CLASS="emphasis"
>cron</EM
> will
treat the entry as a comment and ignore it. This is an easy way to
temporarily disable an entry without permanently deleting it.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45114"
></A
>Here are some example <EM
CLASS="emphasis"
>crontab</EM
> entries (shown in System V format):</P
><P
CLASS="para"
><TABLE
CLASS="screen.co"
BORDER="1"
><TR
><TH
VALIGN="TOP"
><PRE
CLASS="calloutlist"
>
&#13;

<A
CLASS="co"
HREF="ch45_21.htm"
TITLE="45.21 n&gt;&amp;m: Swap Standard Output and Standard Error "
>2&gt;&amp;1</A
> 


<A
CLASS="co"
HREF="ch40_14.htm"
TITLE="40.14 Including Standard Input Within a cron Entry "
>\%</A
> &#13;</PRE
></TH
><TD
VALIGN="TOP"
><PRE
CLASS="screen"
>
0,15,30,45 * * * *  (echo -n '   '; date; echo &quot;&quot;) &gt;/dev/console
0,10,20,30,40,50 7-18 * * * /usr/lib/atrun
7 0 * * *  find / -name &quot;*.bak&quot; -type f -atime +7 -exec rm {} \;
12 4 * * *  /bin/sh /usr/adm/ckdsk &gt;/usr/adm/disk.log 2&gt;&amp;1
22 2 * * *  /bin/sh /usr/adm/ckpwd 2&gt;&amp;1 | mail root
30 3 * * 1 /bin/csh -f /usr/lib/uucp/uu.weekly &gt;/dev/null 2&gt;&amp;1
12 5 15-21 * * test `date +\%a` = Mon &amp;&amp; /usr/local/etc/mtg-notice
#30 2 * * 0,6  /usr/lib/newsbin/news.weekend</PRE
></TD
></TR
></TABLE
></P
><P
CLASS="para"
>The first entry displays the date on the console terminal every fifteen
minutes (on the quarter hour); notice that multiple commands are enclosed in
parentheses in order to redirect their output as a group. (Technically, this
says to run the commands together in a
<SPAN
CLASS="link"
>subshell (<A
CLASS="linkend"
HREF="ch13_07.htm"
TITLE="The () Subshell Operators "
>13.7</A
>)</SPAN
>.)
The second entry runs
<EM
CLASS="emphasis"
>/usr/lib/atrun</EM
> every ten minutes from 7:00 a.m. to 6:00 p.m. daily.
The third entry runs a <EM
CLASS="emphasis"
>find</EM
> command at seven minutes after midnight
to remove all <EM
CLASS="emphasis"
>.bak</EM
> files
not accessed in seven days.
[To cut wear and tear and load on your disk, try to
<SPAN
CLASS="link"
>combine <EM
CLASS="emphasis"
>find</EM
> jobs (<A
CLASS="linkend"
HREF="ch23_22.htm"
TITLE="Using find to Clear Out Unneeded Files "
>23.22</A
>)</SPAN
>.
Also, as article
<A
CLASS="xref"
HREF="ch40_05.htm"
TITLE="Avoiding Other at and cron Jobs "
>40.5</A
>
explains, try not to schedule your jobs at often-chosen times like 1:00
a.m., 2:00 a.m., and so on; pick oddball times like 4:12 a.m. <EM
CLASS="emphasis"
>-JP</EM
>&nbsp;]</P
><P
CLASS="para"
>The fourth and fifth lines run a shell script every day, at 4:12 a.m. and
2:22 a.m., respectively. The shell to execute the script is specified
explicitly on the command line in both cases; the system default shell,
usually the Bourne shell, is used if none is explicitly specified. Both
lines' entries redirect standard output and standard error, sending it to a
file in one case and mailing it to <EM
CLASS="emphasis"
>root</EM
> in the other.</P
><P
CLASS="para"
>The sixth entry executes a C shell script named <EM
CLASS="emphasis"
>uu.weekly</EM
>, stored in
<EM
CLASS="emphasis"
>/usr/lib/uucp</EM
>, at 3:30 a.m. on Monday mornings. Notice that the command
format&nbsp;- specifically the output redirection&nbsp;- is for the Bourne shell even
though the script itself will be run under the C shell.
The seventh entry runs on the third Monday of every month; there's more
explanation below.
The final entry
would run the command <EM
CLASS="emphasis"
>/usr/lib/newsbin/news.weekend</EM
> at 2:30 a.m. on
Saturday and Sunday mornings if it were not disabled with a <CODE
CLASS="literal"
>#</CODE
>.
(<CODE
CLASS="literal"
>#</CODE
> can also be used to add comments to your <EM
CLASS="emphasis"
>crontab</EM
>.)</P
><P
CLASS="para"
>The fourth through sixth entries illustrate three output-handling
alternatives: redirecting it to a file, piping it through mail, and
discarding it to
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>/dev/null</EM
> (<A
CLASS="linkend"
HREF="ch13_14.htm"
TITLE="What Can You Do with an Empty File? "
>13.14</A
>)</SPAN
>.
If no output redirection is performed, the
output is sent via mail to the user who ran the command.</P
><P
CLASS="para"
>The <EM
CLASS="emphasis"
>cmd</EM
> field can be any UNIX command or group of commands (properly
separated with semicolons). The entire <EM
CLASS="emphasis"
>crontab</EM
> entry can be arbitrarily
long, but it must be a single physical line in the file.</P
><P
CLASS="para"
>One problem with the <EM
CLASS="emphasis"
>crontab</EM
> syntax is that it lets you specify
any day of the month, and any day of the week; but it doesn't let you
construct cases like &quot;the third Monday of every month.&quot;
You might think that the <EM
CLASS="emphasis"
>crontab</EM
> entry:</P
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>12 5 15-21 * 1 <CODE
CLASS="replaceable"
><I
>your-command</I
></CODE
></PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>would do the trick, but it won't; this <EM
CLASS="emphasis"
>crontab</EM
> entry runs your
command on every Monday, plus the 15th through the 21st of each month.
[1]
An answer from Greg Ubben is shown in the seventh entry.
He uses the
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>test</EM
> (<A
CLASS="linkend"
HREF="ch44_20.htm"
TITLE="test: Testing Files and Strings "
>44.20</A
>)</SPAN
>
and
<SPAN
CLASS="link"
><EM
CLASS="emphasis"
>date</EM
> (<A
CLASS="linkend"
HREF="ch51_10.htm"
TITLE="The date Command "
>51.10</A
>)</SPAN
>
commands to compare the name of today (like <EM
CLASS="emphasis"
>Tue</EM
>)
to the day we want the entry to be executed (here, <CODE
CLASS="literal"
>Mon</CODE
>).
This entry will be run between the 15th and 21st of each month, but
the <EM
CLASS="emphasis"
>mtg-notice</EM
> command will only run on the Monday during that
period.
The shell's
<SPAN
CLASS="link"
><CODE
CLASS="literal"
>&amp;&amp;</CODE
> operator (<A
CLASS="linkend"
HREF="ch44_09.htm"
TITLE="Testing Your Success "
>44.9</A
>)</SPAN
>
runs the <EM
CLASS="emphasis"
>mtg-notice</EM
> command only when the previous test succeeds.
Greg actually writes the entry as shown here, testing for failure of the
<EM
CLASS="emphasis"
>test</EM
> command:</P
><BLOCKQUOTE
CLASS="footnote"
><P
CLASS="para"
>[1] This strange behavior seems to be a System V peculiarity that somehow
infected the rest of the world.
&quot;True&quot; BSD systems behave the way we explained earlier.
However, SunOS 4.X systems have incorporated System V's behavior; and, 
with the advent of Solaris, there are relatively few true commercial
BSD systems left in the world. </P
></BLOCKQUOTE
><P
CLASS="para"
><BLOCKQUOTE
CLASS="screen"
><PRE
CLASS="screen"
>12 5 15-21 * * test `date +\%a` != Mon || /usr/local/etc/mtg-notice</PRE
></BLOCKQUOTE
></P
><P
CLASS="para"
>He did it in that &quot;backwards&quot; way so the <EM
CLASS="emphasis"
>cron</EM
> job's exit status would
be 0 (success) in the case when it doesn't execute <EM
CLASS="emphasis"
>mtg-notice</EM
>.
You may need that technique, too.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45173"
></A
>The <EM
CLASS="emphasis"
>cron</EM
> command starts the <EM
CLASS="emphasis"
>cron</EM
> program. It has no options.
Once started, <EM
CLASS="emphasis"
>cron</EM
> never terminates. It is normally started
automatically by one of the system initialization scripts.
<EM
CLASS="emphasis"
>cron</EM
> reads the <EM
CLASS="emphasis"
>crontab</EM
> file(s) every minute to see whether 
there have been changes. Therefore, any change to its schedule will take
effect within one minute.</P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>AF</SPAN
>, <SPAN
CLASS="authorinitials"
>JP</SPAN
></P
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