How to add a new task in crontab using a script

Question

I need to use crontab to add new tasks to be done at a certain time. I need to add these tasks using a script. I will do something like:

echo "command" >> /etc/crontab

But I know that this crontab file is just for root user, so I need to use:

crontab -e

to open my own list of tasks. The problem is the previous command opens a temporary file and I can't locate it. (It has a random name in /tmp or something?) I will do something like:

echo "5 5 5 5 5 user command" | crontab -e

to write a new line in my own crontab list but this doesn't work.

Am I wrong in something? Does anyone has an idea for solving this? I'm using Ubuntu.

Answer 1

You are having difficulty with your current approach because crontab -e doesn't read from stdin. However, crontab without the -e switch, does read from stdin.

The most obvious way is to just append the new cron job to the existing crontab:

crontab -l > current_cron
cat >> current_cron << EOF
# new cron job here
EOF
crontab < current_cron
rm -f current_cron

The obvious issue with this approach is that there is no check to see if the cron job exists already. That's where it can get a little tricky.

One way to tackle that would be to have a "script managed section" in crontab, typically, at the bottom:

# DO NOT MODIFY THE SECTION BELOW MANUALLY.  IT'S MANAGED BY AUTOMATION.
#---script managed section---
"cron job 1"
"cron job 2"

Then, a script can manage this section - it is best to have a single script that manages all automatic cron jobs:

crontab -l > current_cron
sed '/---script managed section---/q' current_cron > new_cron
cat >> new_cron << EOF
# updated cron jobs here
EOF
crontab < new_cron
rm -f new_cron current_cron

If we need even more fine-grained control, then we can have a section for each managed cron job, with begin and end markers and make the script deal with a specific section each time.

However, there is a better idea for Ubuntu. We can completely do away with crontab and use /etc/cron.d instead. Here, we have one file for each cron job which follows the exact format of a cron job spec. Since there is a 1:1 correspondence between cron jobs and /etc/cron.d files, they are managed directly as files rather than through crontab command and hence, automation is much easier.

From man cron on Ubuntu:

In Debian, cron reads the files in the /etc/cron.d directory. cron treats the files in /etc/cron.d as in the same way as the /etc/crontab file (they follow the special format of that file, i.e. they include the user field). However, they are independent of /etc/crontab: they do not, for example, inherit environment variable settings from it. This change is specific to Debian.

Answer 2

crontab uses that temporary file to give you access to insert new commands. You don't get to edit your own file, or mess with the system's copy of the file. Instead, try creating your own file; then us **crontab -l to get that listed in the system crontab.

If you're stubborn, you could try to stream-feed vi commands to crontab -e, but that's doing things the hard way.

Is that enough to get you going?