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

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 

Answer 1

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.