Find and Update cron entry, via script

Question

I'm trying to add a piece of code to my script that will update its own entry in crontab. So far, I've been working with with the following:

crontab -u root -l | grep -w "$VAR" | crontab -u root - && { crontab -l -u root 2>/dev/null; echo "0 */2 * * * /root/$VAR/script > /dev/null 2>&1"; } | crontab -u root -

The objective is to have this run on several scripts and all of them will find and update its own line, based on $VAR.

For example:

script1

#!/bin/bash
VAR="home1"

crontab -u root -l | grep -w "$VAR" | crontab -u root - && { crontab -l -u root 2>/dev/null; echo "0 */2 * * * /root/$VAR/script > /dev/null 2>&1"; } | crontab -u root -

echo "Hi"

exit 0

script2

#!/bin/bash
VAR="home2"

crontab -u root -l | grep -w "$VAR" | crontab -u root - && { crontab -l -u root 2>/dev/null; echo "0 */2 * * * /root/$VAR/script > /dev/null 2>&1"; } | crontab -u root -

echo "Hi"

exit 0

What I'm trying to add is have these scripts create the following crontab:

0 */2 * * * /root/home1/script > /dev/null 2>&1

0 */2 * * * /root/home2/script > /dev/null 2>&1

For some reason the last running overwrites all entries.

Any tips? Thank you!

Answer 1

Using a file or standard input to configure crontab newly creates (overwrites) the contents rather than appends to whatever might already exist.

One easy solution would be to write each of your entries to a single file:

crontab -u root -l | grep -w "$VAR" | crontab -u root - && \
    { crontab -l -u root 2>/dev/null; echo "0 */2 * * * /root/$VAR/script > /dev/null 2>&1"; } \
    >> cron-entry-file

Then after you've collected all the entries do this just once:

crontab -u root cron-entry-file

If you are generating your entries in different scripts and/or at different times then each time you would need to do: crontab -l > cron-entry-file to preserve current settings, then follow the steps above. (Hopefully someone comes along with a more graceful solution but this brute-force approach should work.)

---

Edit

Since your scenario is not quite as straightforward as I thought when originally writing the above consider manipulating the crontab file directly. It's just a regular file (under /var/cron/tabs/{username} for example). Might want to read this first: https://serverfault.com/questions/347318/is-it-bad-to-edit-cron-file-manually

Answer 2

I found that the best way to have a script update cron entries is to direct them to a separate file and have cron feed off of that. With the example I provided in the question, I ended up doing the following:

echo "0 */2 * * * root /root/$VAR/script > /dev/null 2>&1" > /etc/cron.d/$VAR

This will create the file named after the variable on /etc/cron.d/ and will follow the schedule defines in the file itself. Cleaner than managing crontab, as recommended in this thread.