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

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

User · Answer

2 gt  dev null crontab -l   echo  quot 0 3        usr local bin certbot-auto renew quot     crontab -     cat  lt  crontab -l 2 gt  dev null   lt  echo  quot 0 3        usr local bin certbot-auto renew quot     crontab -      write out current crontab crontab -l  gt  mycron 2 gt  dev null   echo new cron into cron file echo  quot 0 3        usr local bin certbot-auto renew quot   gt  gt  mycron   install new cron file crontab mycron  rm mycron

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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

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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   gt  gt   cron path   cron job to run every 1 hour  echo  0   1       command to be executed   gt  gt   cron path

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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   gt  gt   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

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echo  0         docker system prune --force  gt  dev null 2 gt  amp 1    sudo tee  etc cron daily dockerprune

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You can probably change the default editor to ed and use a heredoc to edit   EDITOR ed export EDITOR  crontab -e  lt  lt  EOF  gt  a  gt            Myscript  gt            AnotherScript  gt            MoreScript  gt     gt  w  gt  q  gt  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  lt  lt  EOF  gt   Myscript   gt  d  gt     gt  w  gt  q  gt  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  -

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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  lt  fgrep -i -v   command   lt  crontab -l    lt  echo   job     crontab -   addtocrontab  0 0 1       echo hello

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For testing when your cron will be executed I recommend https   crontab guru  It shows the result of your setup in human readable format  As well you can hit random button and you will get examples

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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  gt   etc cron d  lt job gt   lt  lt  EOF SHELL  bin bash  PATH  sbin  bin  usr sbin  usr bin  MAILTO root HOME     01          lt user gt   lt command gt  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

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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  lt  crontab -l      crontab -l gt   TMPC   echo   JOB  gt  gt   TMPC   crontab   TMPC

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script function to add cronjobs  check duplicate entries useable expressions        cronjob creator                 usage  cronjob creator   lt interval gt     lt command gt      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   gt    CRONIN      echo   1  2   gt  gt   CRONIN     crontab   CRONIN      rm  CRONIN fi     tested        cronjob creator sh    10           echo  this is a test   gt  export file    crontab  -l     10         echo  this is a test   gt  export file   source   my brain

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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

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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

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CRON  1 2 3 4 5  root bin backup sh   cat  lt   crontab -l   grep -v    CRON    lt   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   lt interval gt     lt command gt    lt add remove gt   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   gt  export file  add   crontab  -l     10         echo  this is a test   gt  export file

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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  gt   home me myfunction log 2 gt  amp 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

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EDIT  fixed overwriting    cat  lt  crontab -l   lt  echo  1 2 3 4 5 scripty sh     crontab -

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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  lt  crontab -l   grep -v      cron    lt  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

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You can add to the crontab as follows    write out current crontab crontab -l  gt  mycron  echo new cron into cron file echo  00 09     1-5 echo hello   gt  gt  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

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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

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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  lt  fgrep -i -v   command   lt  crontab -l    lt  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   Credit to duckyflip  I use this little redirect thingy   lt   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   Again  the little redirect thingy   lt   command    is used to turn the result back into input for the cat command  The cat command also receives echo   job   self explanatory   again  through use of the redirect thingy   lt   command     So the filtered output from crontab -l and the simple echo   job   combined  are piped       over to crontab - to finally be written  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

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For a nice quick and dirty creation replacement of a crontab from with a BASH script  I used this notation   crontab  lt  lt EOF 00 09     1-5 echo hello EOF

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

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