How to create a cron job using Bash automatically without the interactive editor?

You may be able to do it on-the-fly

crontab -l | { cat; echo "0 0 0 0 0 some entry"; } | crontab -

This works since crontab -l lists the current crontab jobs, cat prints it (from standard input), echo prints the new command and crontab - adds all the printed stuff into the crontab file. You can see the effect by doing a new crontab -l.

Note: if the user has no existing crontab, you may see this message:

no crontab for <username>

Anyway, it still works, even if you see that message.

This shorter one requires no temporary file, it is immune to multiple insertions, and it lets you change the schedule of an existing entry.

Say you have these:

croncmd="/home/me/myfunction myargs > /home/me/myfunction.log 2>&1"
cronjob="0 */15 * * * $croncmd"

To add it to the crontab, with no duplication:

( crontab -l | grep -v -F "$croncmd" ; echo "$cronjob" ) | crontab -

To remove it from the crontab whatever its current schedule:

( crontab -l | grep -v -F "$croncmd" ) | crontab -

Notes:

• grep -F matches the string literally, as we do not want to interpret it as a regular expression
• We also ignore the time scheduling and only look for the command. This way; the schedule can be changed without the risk of adding a new line to the crontab

Thanks everybody for your help. Piecing together what I found here and elsewhere I came up with this:

The Code

command="php $INSTALL/indefero/scripts/gitcron.php"
job="0 0 * * 0 $command"
cat <(fgrep -i -v "$command" <(crontab -l)) <(echo "$job") | crontab -

I couldn't figure out how to eliminate the need for the two variables without repeating myself.

command is obviously the command I want to schedule. job takes $command and adds the scheduling data. I needed both variables separately in the line of code that does the work.

Details

1. Credit to duckyflip, I use this little redirect thingy (<(*command*)) to turn the output of crontab -l into input for the fgrep command.
2. fgrep then filters out any matches of $command (-v option), case-insensitive (-i option).
3. Again, the little redirect thingy (<(*command*)) is used to turn the result back into input for the cat command.
4. The cat command also receives echo "$job" (self explanatory), again, through use of the redirect thingy (<(*command*)).
5. So the filtered output from crontab -l and the simple echo "$job", combined, are piped ('|') over to crontab - to finally be written.
6. And they all lived happily ever after!

In a nutshell:

This line of code filters out any cron jobs that match the command, then writes out the remaining cron jobs with the new one, effectively acting like an "add" or "update" function. To use this, all you have to do is swap out the values for the command and job variables.

EDIT (fixed overwriting):

cat <(crontab -l) <(echo "1 2 3 4 5 scripty.sh") | crontab -

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.

So, in Debian, Ubuntu, and many similar Debian based distros...

There is a cron task concatenation mechanism that takes a config file, bundles them up and adds them to your cron service running.

You can put a file under the /etc/cron.d/somefilename where somefilename is whatever you want.

echo "0,15,30,45 * * * * ntpdate -u time.nist.gov" | sudo tee /etc/cron.d/vmclocksync

Let's disassemble this:

echo - a vehicle to create output on std out. printf, cat... would work as well

" - use a doublequote at the beginning of your string, you're a professional

0,15,30,45 * * * * - the standard cron run schedule, this one runs every 15 minutes

ntpdate -u time.nist.gov - the actual command I want to run

" - because my first double quotes needs a buddy to close the line being output

sudo - because you need elevated privileges to change cron configs under the /etc directory

tee - tee accepts stdin then outputs to both stdout and appends to a file specified (instead of overwrites*)

/etc/cron.d/vmclocksync - vmclocksync is the filename I've chosen, it goes in /etc/cron.d/

* You can decide for yourself if possible destruction with overwrite is right or possible duplicates with appending are for you. Alternatively, you could do something convoluted or involved to check if the file name exists, if there is anything in it, and whether you are adding any kind of duplicate-- but, I have stuff to do and I can't do that for you right now.

For a nice quick and dirty creation/replacement of a crontab from with a BASH script, I used this notation:

crontab <<EOF
00 09 * * 1-5 echo hello
EOF

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.

echo "0 * * * * docker system prune --force >/dev/null 2>&1" | sudo tee /etc/cron.daily/dockerprune

A variant which only edits crontab if the desired string is not found there:

CMD="/sbin/modprobe fcpci"
JOB="@reboot $CMD"
TMPC="mycron"
grep "$CMD" -q <(crontab -l) || (crontab -l>"$TMPC"; echo "$JOB">>"$TMPC"; crontab "$TMPC")

(2>/dev/null crontab -l ; echo "0 3 * * * /usr/local/bin/certbot-auto renew") | crontab -

cat <(crontab -l 2>/dev/null) <(echo "0 3 * * * /usr/local/bin/certbot-auto renew") | crontab -

#write out current crontab

crontab -l > mycron 2>/dev/null

#echo new cron into cron file

echo "0 3 * * * /usr/local/bin/certbot-auto renew" >> mycron

#install new cron file

crontab mycron

rm mycron

If you're using the Vixie Cron, e.g. on most Linux distributions, you can just put a file in /etc/cron.d with the individual cronjob.

This only works for root of course. If your system supports this you should see several examples in there. (Note the username included in the line, in the same syntax as the old /etc/crontab)

It's a sad misfeature in cron that there is no way to handle this as a regular user, and that so many cron implementations have no way at all to handle this.

Bash script for adding cron job without the interactive editor. Below code helps to add a cronjob using linux files.

#!/bin/bash

cron_path=/var/spool/cron/crontabs/root

#cron job to run every 10 min.
echo "*/10 * * * * command to be executed" >> $cron_path

#cron job to run every 1 hour.
echo "0 */1 * * * command to be executed" >> $cron_path

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

add -w parameter to grep exact command, without -w parameter adding the cronjob "testing" cause deletion of cron job "testing123"

script function to add/remove cronjobs. no duplication entries :

cronjob_editor () {         
# usage: cronjob_editor '<interval>' '<command>' <add|remove>

if [[ -z "$1" ]] ;then printf " no interval specified\n" ;fi
if [[ -z "$2" ]] ;then printf " no command specified\n" ;fi
if [[ -z "$3" ]] ;then printf " no action specified\n" ;fi

if [[ "$3" == add ]] ;then
    # add cronjob, no duplication:
    ( crontab -l | grep -v -F -w "$2" ; echo "$1 $2" ) | crontab -
elif [[ "$3" == remove ]] ;then
    # remove cronjob:
    ( crontab -l | grep -v -F -w "$2" ) | crontab -
fi 
} 
cronjob_editor "$1" "$2" "$3"

tested :

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

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.

Here is a bash function for adding a command to crontab without duplication

function addtocrontab () {
  local frequency=$1
  local command=$2
  local job="$frequency $command"
  cat <(fgrep -i -v "$command" <(crontab -l)) <(echo "$job") | crontab -
}
addtocrontab "0 0 1 * *" "echo hello"

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

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 ;)

Say you're logged in as the user "ubuntu", but you want to add a job to a different user's crontab, like "john", for example. You can do the following:

(sudo crontab -l -u john; echo "* * * * * command") | awk '!x[$0]++' | sudo crontab -u john -

Source for most of this solution: https://www.baeldung.com/linux/create-crontab-script

I was having tons of issues trying to add a job to another user's crontab. It kept duplicating crontabs, or just flat-out deleting them. After some testing, though, I'm confident this line of code will append a new job to a specified user's crontab, non-destructively, including not creating a job that already exists.

I wanted to find an example like this, so maybe it helps:

COMMAND="/var/lib/postgresql/backup.sh"
CRON="0 0 * * *"
USER="postgres"
CRON_FILE="postgres-backup"
# At CRON times, the USER will run the COMMAND
echo "$CRON $USER $COMMAND" | sudo tee /etc/cron.d/$CRON_FILE
echo "Cron job created. Remove /etc/cron.d/$CRON_FILE to stop it."