Insert entry into crontab unless it already exists (as one-liner if possible)

Question

What's the preferred method to insert an entry into /etc/crontab unless it exists, preferably using a one-liner?

Here's my example entry I wish to place into /etc/crontab unless it already exists in there.

*/1 *  *  *  * some_user python /mount/share/script.py

I'm on CentOS 6.6 and so far I have this:

if grep "*/1 *  *  *  * some_user python /mount/share/script.py" /etc/crontab; then echo "Entry already in crontab"; else echo "*/1 *  *  *  * some_user python /mount/share/script.py" >> /etc/crontab; fi

score 23 · Accepted Answer · answered Dec 01 '14 at 11:24

23

You can do this:

grep 'some_user python /mount/share/script.py' /etc/crontab || echo '*/1 *  *  *  * some_user python /mount/share/script.py' >> /etc/crontab

If the line is absent, grep will return 1, so the right hand side of the or || will be executed.

answered Dec 01 '14 at 11:24

arco444

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I understand that we are not supposed to edit /etc/crontab directly these days - how would we do it with crontab -e? – tofutim Mar 10 '18 at 22:42
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(crontab -l | grep 'some_user python /mount/share/script.py') || { crontab -l; '*/1 * * * * some_user python /mount/share/script.py'; } | crontab - – PhilJ Mar 11 '20 at 22:14
@PhilJ why we use crontab -l before '*/1 * * * *....' ? – Estet Jan 26 '21 at 22:12
Does not work for me: "Permission denied". Going to explore options with `tee` – Bix Jul 15 '21 at 22:52
@PhilJ Getting an error `-bash: */1 * * * * some_user python /mount/share/script.py: No such file or directory` – Athena Wisdom Nov 16 '21 at 05:37
@Estet We use that to get the existing cronjobs. – T Hansda Feb 04 '22 at 17:39

score 0 · Answer 2 · edited Feb 27 '18 at 23:03

0

You can do it like this:

if grep "\*\/5 \* \* \* \* /usr/local/bin/test.sh" /var/spool/cron/root; then echo "Entry already in crontab"; else echo "*/5 * * * * /usr/local/bin/test.sh" >>  /var/spool/cron/root; fi

Or even more terse:

grep '\*\/12 \* \* \* \* /bin/yum makecache fast' /var/spool/cron/root \
    || echo '*/12 * * * * /bin/yum makecache fast' >> /var/spool/cron/root

edited Feb 27 '18 at 23:03

Jay Taylor

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answered Jun 27 '17 at 02:13

dleon

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You should not write to /var/spool/cron/root directly, it is specifically stated in the documentation, you have to go through crontab. The files in /var/spool/cron must include only properly formatted lines, which crontab checks before committing to disk. – Wadih M. Oct 10 '22 at 15:20

score 0 · Answer 3 · answered Oct 25 '17 at 08:52

0

Factoring out the filename & using the q & F options file="/etc/crontab"; grep -qF "some_user python /mount/share/script.py" "$file" || echo "*/1 * * * * some_user python /mount/share/script.py"

answered Oct 25 '17 at 08:52

karpada

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Insert entry into crontab unless it already exists (as one-liner if possible)

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