How to create a cron job using Bash automatically

Question

Does crontab have an argument for creating cron jobs without using the editor (crontab -e). If so, What would be the code create a cronjob from a Bash script?

Answer 1

You can add to the crontab as follows:

#write out current crontab
crontab -l > mycron
#echo new cron into cron file
echo "00 09 * * 1-5 echo hello" >> mycron
#install new cron file
crontab mycron
rm mycron

---

Cron line explaination

* * * * * "command to be executed"
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Source nixCraft.

Answer 2

No, there is no option in crontab to modify the cron files.

You have to: take the current cron file (crontab -l > newfile), change it and put the new file in place (crontab newfile).

If you are familiar with perl, you can use this module Config::Crontab.

LLP, Andrea

Answer 3

script function to add cronjobs. check duplicate entries,useable expressions * > "

cronjob_creator () {         
# usage: cronjob_creator '<interval>' '<command>'

  if [[ -z $1 ]] ;then
    printf " no interval specified\n"
elif [[ -z $2 ]] ;then
    printf " no command specified\n"
else
    CRONIN="/tmp/cti_tmp"
    crontab -l | grep -vw "$1 $2" > "$CRONIN"
    echo "$1 $2" >> $CRONIN
    crontab "$CRONIN"
    rm $CRONIN
fi
}

tested :

$ ./cronjob_creator.sh '*/10 * * * *' 'echo "this is a test" > export_file'
$ crontab  -l
$ */10 * * * * echo "this is a test" > export_file

source : my brain ;)

Answer 4

My preferred solution to this would be this:

(crontab -l | grep . ; echo -e "0 4 * * * myscript\n") | crontab -

This will make sure you are handling the blank new line at the bottom correctly. To avoid issues with crontab you should usually end the crontab file with a blank new line. And the script above makes sure it first removes any blank lines with the "grep ." part, and then add in a new blank line at the end with the "\n" in the end of the script. This will also prevent getting a blank line above your new command if your existing crontab file ends with a blank line.

Answer 5

There have been a lot of good answers around the use of crontab, but no mention of a simpler method, such as using cron.

Using cron would take advantage of system files and directories located at /etc/crontab, /etc/cron.daily,weekly,hourly or /etc/cron.d/:

cat > /etc/cron.d/<job> << EOF
SHELL=/bin/bash 
PATH=/sbin:/bin:/usr/sbin:/usr/bin 
MAILTO=root HOME=/  
01 * * * * <user> <command>
EOF

In this above example, we created a file in /etc/cron.d/, provided the environment variables for the command to execute successfully, and provided the user for the command, and the command itself. This file should not be executable and the name should only contain alpha-numeric and hyphens (more details below).

To give a thorough answer though, let's look at the differences between crontab vs cron/crond:

crontab -- maintain tables for driving cron for individual users

For those who want to run the job in the context of their user on the system, using crontab may make perfect sense.

cron -- daemon to execute scheduled commands

For those who use configuration management or want to manage jobs for other users, in which case we should use cron.

A quick excerpt from the manpages gives you a few examples of what to and not to do:

/etc/crontab and the files in /etc/cron.d must be owned by root, and must not be group- or other-writable. In contrast to the spool area, the files under /etc/cron.d or the files under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly may also be symlinks, provided that both the symlink and the file it points to are owned by root. The files under /etc/cron.d do not need to be executable, while the files under /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly and /etc/cron.monthly do, as they are run by run-parts (see run-parts(8) for more information).

Source: http://manpages.ubuntu.com/manpages/trusty/man8/cron.8.html

Managing crons in this manner is easier and more scalable from a system perspective, but will not always be the best solution.

Answer 6

Chances are you are automating this, and you don't want a single job added twice. In that case use:

__cron="1 2 3 4 5 /root/bin/backup.sh"
cat <(crontab -l) |grep -v "${__cron}" <(echo "${__cron}")

This only works if you're using BASH. I'm not aware of the correct DASH (sh) syntax.

Update: This doesn't work if the user doesn't have a crontab yet. A more reliable way would be:

(crontab -l ; echo "1 2 3 4 5 /root/bin/backup.sh") | sort - | uniq - | crontab - 

Alternatively, if your distro supports it, you could also use a separate file:

echo "1 2 3 4 5 /root/bin/backup.sh" |sudo tee /etc/crond.d/backup

Found those in another SO question.