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i was asked to write shell script to perform the following task:

update cronentry from

\*/15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out 

to

\*/30 2,4,6 * * * /bin/sh /opt/myscript/myscript2.sh > /tmp/myscript2.check.out 

before edit take a backup of the cronentry

i have tried the following script and need to check if it will work or does require any modification - please help!!

Shell Script to update cronentry(backup before edit)

#!/bin/sh
crontab -l > my_crontab.backup

crontab -l | grep -v "*/30 2,4,6 * * * /bin/sh /opt/myscript/myscript2.sh > /tmp/myscript2.check.out" >*/15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out

echo "*/30 2,4,6 * * * /bin/sh /opt/myscript/myscript2.sh > /tmp/myscript2.check.out" >>*/15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out

crontab */15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out

1 Answer 1

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You wish to delete a particular crontab entry and replace it with another.

You may dump the existing crontab schedule to a text file using

crontab -l >crontab-old.txt

Assuming that you know that the existing crontab entry is exactly

*/15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out 

(with no additional spaces anywhere) you may filter that out from the crontab-old.txt file using

grep -v -x -F '*/15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out' crontab-old.txt >crontab-new.txt

To add the new schedule to the crontab-new.txt file, you could do

cat <<'END_ENTRY' >>crontab-new.txt
*/30 2,4,6 * * * /bin/sh /opt/myscript/myscript2.sh > /tmp/myscript2.check.out 
END_ENTRY

And finally, you load the new schedule with

crontab crontab-new.txt

You now have crontab-old.txt with the old schedule and crontab-new.txt with the new schedule, and you have loaded the new schedule.

You should obviously look at the files in-between each step above to make sure the correct edits are made to them, if you are running this manually, or while you are developing a script to do it.

In a script, this may look like

#!/bin/sh

crontab -l >crontab-old.txt

grep -v -x -F '*/15 * * * * /bin/sh /opt/myscript/myscript.sh > /tmp/myscript.check.out' crontab-old.txt >crontab-new.txt

cat <<'END_ENTRY' >>crontab-new.txt
*/30 2,4,6 * * * /bin/sh /opt/myscript/myscript2.sh > /tmp/myscript2.check.out 
END_ENTRY

crontab crontab-new.txt

The grep command uses -F to match the given pattern as a string rather than as a regular expression. The pattern contains * characters which are special in regular expressions, so using -F allows us to match the string without special handling of these. I'm also using -x which forces successful matches to match across a full line, from start to finish, rather than parts of lines.

Also related:

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  • Thanks for the input
    – ravi
    Commented Aug 8, 2019 at 13:46

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