How do I create a crontab through a script

Question

I need to add a cron job thru a script I run to set up a server  I am currently using Ubuntu  I can use crontab -e but that will open an editor to edit the current crontab  I want to do this programmatically   Is it possible to do so

User · Answer

For user crontabs  including root   you can do something like   crontab -l -u user   cat - filename   crontab -u user -   where the file named  filename  contains items to append  You could also do text manipulation using sed or another tool in place of cat  You should use the crontab command instead of directly modifying the file   A similar operation would be     crontab -l -u user  echo  crontab spec       crontab -u user -   If you are modifying or creating system crontabs  those may be manipulated as you would ordinary text files  They are stored in the  etc cron d   etc cron hourly   etc cron daily   etc cron weekly   etc cron monthly directories and in the files  etc crontab and  etc anacrontab

User · Answer

As a correction to those suggesting crontab -l   crontab -  This does not work on every system  For example  I had to add a job to the root crontab on dozens of servers running an old version SUSE  don t ask why   Old SUSEs prepend comment lines to the output of crontab -l  making crontab -l   crontab - non-idempotent  Debian recognizes this problem in the crontab manpage and patched its version of Vixie Cron to change the default behaviour of crontab -l    To edit crontabs programmatically on systems where crontab -l adds comments  you can try the following   EDITOR cat crontab -e  gt  old crontab  cat old crontab new job   crontab -  EDITOR cat tells crontab to use cat as an editor  not the usual default vi   which doesn t change the file  but instead copies it to stdout  This might still fail if crontab - expects input in a format different from what crontab -e outputs  Do not try to replace the final crontab - with crontab -e - it will not work

User · Answer

Here is how to modify cron a entry without directly editing the cron file  which is frowned upon    crontab -l -u  lt user gt    sed  s find replace g    crontab -u  lt user gt  -   If you want to remove a cron entry  use this   crontab -l -u  lt user gt    sed   find d    crontab -u  lt user gt  -   I realize this is not what gaurav was asking for  but why not have all the solutions in one place

User · Answer

Well  etc crontab just an ascii file so the simplest is to just   echo    15           root     date   gt  gt   etc crontab   which will add a job which will email you every 15 mins   Adjust to taste  and test via grep or other means whether the line was already added to make your script idempotent   On Ubuntu et al  you can also drop files in  etc cron   which is easier to do and test for---plus you don t mess with  system  config files such as  etc crontab

User · Answer

In Ubuntu and many other distros  you can just put a file into the  etc cron d directory containing a single line with a valid crontab entry  No need to add a line to an existing file   If you just need something to run daily  just put a file into  etc cron daily  Likewise  you can also drop files into  etc cron hourly   etc cron monthly  and  etc cron weekly

User · Answer

Crontab files are simply text files and as such can be treated like any other text file   The purpose of the crontab command is to make editing crontab files safer   When edited through this command  the file is checked for errors and only saved if there are none   crontab  path to file  can be used to specify a crontab stored in a file   Like crontab -e  this will only install the file if it is error free   Therefore  a script can either directly write cron tab files  or write them to a temporary file and load them with the crontab  path to temp file  command   Writing directly saves having to write a temporary file  but it also avoids the safety check

User · Answer

Here s a one-liner that doesn t use require the new job to be in a file    crontab -l 2 gt  dev null  echo    5          path to job -with args     crontab -   The 2 gt  dev null is important so that you don t get the no crontab for username message that some  nixes produce if there are currently no crontab entries

User · Answer

Cron jobs usually are stored in a per-user file under  var spool cron  The simplest thing for you to do is probably just create a text file with the job configured  then copy it to the cron spool folder and make sure it has the right permissions  600

User · Answer

Even more simple answer to you question would be   echo  0 1        root test sh    tee -a  var spool cron root   You can setup cronjobs on remote servers as below      bin bash servers  srv1 srv2 srv3 srv4 srv5  for i in  servers   do   echo  0 1        root test sh    ssh  i   tee -a  var spool cron root  done   In Linux  the default location of the crontab file is  var spool cron   Here you can find the crontab files of all users  You just need to append your cronjob entry to the respective user s file  In the above example  the root user s crontab file is getting appended with a cronjob to run  root test sh every day at 1 AM

User · Answer

I don t have enough reputation to comment  so I m adding at as an answer  feel free to add it as as comment next to his answer   Joe Casadonte s one-liner is perfect  except if you run with set -e  i e  if your script is set to fail on error  and if there are no cronjobs yet  In that case  the one-liner will NOT create the cronjob  but will NOT stop the script  The silent failure can be very misleading   The reason is that crontab -l returns with a 1 return code  causing the subsequent command  the echo  not to be executed    thus the cronjob is not created  But since they are executed as a subprocess  because of the parenthesis  they don t stop the script    Interestingly  if you run the same command again  it will work  once you have executed crontab - once  crontab -l still outputs nothing  but it doesn t return an error anymore  you don t get the no crontab for  lt user gt  message anymore   So the subsequent echo is executed and the crontab is created   In any case  if you run with set -e  the line must be    crontab -l 2 gt  dev null    true  echo    5          path to job -with args     crontab -

User · Answer

It is an approach to incrementally add the cron job     ssh USER NAME  PRODUCT IP nohup  echo    2         ping -c2 PRODUCT NAME com  gt  gt   var www html test html    crontab -u USER NAME -

User · Answer

I have written a crontab deploy tool in python  https   github com monklof deploycron  pip install deploycron   Install your crontab is very easy  this will merge the crontab into the system s existing crontab   from deploycron import deploycron deploycron content            echo hello  gt   tmp hello

[linux] How do I create a crontab through a script

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