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
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.
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.
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"
echo "0 * * * * docker system prune --force >/dev/null 2>&1" | sudo tee /etc/cron.daily/dockerprune
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 ;)
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.
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")
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.
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:
You may be able to do it on-the-fly
crontab -l | { cat; echo "0 0 0 0 0 some entry"; } | crontab -
crontab -l
lists the current crontab jobs, cat
prints it, 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
.
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.
sudo echo "0,15,30,45 * * * * ntpdate -u time.nist.gov" >> /etc/cron.d/vmclocksync
Let's disassemble this:
sudo - because you need elevated privileges to change cron configs under the /etc directory
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
>> - the double redirect appends instead of overwrites*
/etc/cron.d/vmclocksync - vmclocksync is the filename I've chosen, it goes in /etc/cron.d/
* if we used the > redirect, we could guarantee we only had one task entry. But, we would be at risk of blowing away any other rules in an existing file. You can decide for yourself if possible destruction with > is right or possible duplicates with >> 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.
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
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
You can probably change the default editor to ed and use a heredoc to edit.
EDITOR=ed
export EDITOR
crontab -e << EOF
> a
> * * * * * Myscript
> * * * * * AnotherScript
> * * * * * MoreScript
> .
> w
> q
> EOF
Note the leading > in that code means the return/enter key is pressed to create a new line.
The a means APPEND so it will not overwrite anything.
The . means you're done editing.
The w means WRITE the changes.
The q means QUIT or exit ed.
you can check it out
crontab -l
You can delete an entry too.
EDITOR=ed
export EDITOR
crontab -e << EOF
> /Myscript/
> d
> .
> w
> q
> EOF
That will delete the crontab entry with Myscript in it.
The d means delete the pattern inside the / /.
No check it again
crontab -l
This solution works inside a script too less the > of course :-)
Thanks everybody for your help. Piecing together what I found here and elsewhere I came up with this:
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.
<(*command*)
) to turn the output of crontab -l
into input for the fgrep
command.fgrep
then filters out any matches of $command
(-v
option), case-insensitive (-i
option).<(*command*)
) is used to turn the result back into input for the cat
command.cat
command also receives echo "$job"
(self explanatory), again, through use of the redirect thingy (<(*command*)
).crontab -l
and the simple echo "$job"
, combined, are piped ('|') over to crontab -
to finally be written.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.
(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
For testing when your cron will be executed I recommend
It shows the result of your setup in human readable format. As well you can hit random button and you will get examples.
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
Source: Stackoverflow.com