How do I set up cron to run a file just once at a specific time

Question

How do I set up cron to run a file just once at a specific time  One of the alternatives is at but it is not accessible to all users on standard hosting plans  Therefore I was wondering whether there is way to do it using cron

User · Accepted Answer

Try this out to execute a command on 30th March 2011 at midnight:

0 0 30 3 ? 2011  /command

WARNING: As noted in comments, the year column is not supported in standard/default implementations of cron. Please refer to TomOnTime answer below, for a proper way to run a script at a specific time in the future in standard implementations of cron.

User · Answer

For those who is not able to access install at in environment  can use custom script     bin bash if      -lt 2    then echo  quot  quot  echo  quot Syntax Error  quot  echo  quot Usage   0  lt shell script gt   lt datetime gt  quot  echo  quot  lt datetime gt  format   Y m d H M quot  echo  quot Example   0  home user scripts server backup sh 202008142350 quot  echo  quot  quot  exit 1 fi  while true  do   t   date   Y m d H M           if    t -eq  2    then      bin bash  1     echo DONE   date       break    fi    sleep 1  done  Let s name the script as run1time sh Example could be something like  nohup bash run1time sh  path to your script sh 202008150300  amp

User · Answer

You could put a crontab file in  etc cron d which would run a script that would run your command and then delete the crontab file in  etc cron d   Of course  that means your script would need to run as root

User · Answer

You really want to use at   It is exactly made for this purpose   echo  usr bin the command options   at now   1 day   However if you don t have at  or your hosting company doesn t provide access to it  you can have a cron job include code that makes sure it only runs once   Set up a cron entry with a very specific time   0 0 2 12    home adm bin the command options   Next  home adm bin the command needs to either make sure it only runs once       bin bash  COMMAND  home adm bin the command DONEYET    COMMAND  alreadyrun   export PATH  usr bin  PATH  if    -f  DONEYET     then   exit 1 fi touch   DONEYET     Put the command you want to run exactly once here  echo  You will only get this once     mail -s  Greetings   me example com

User · Answer

at is the correct way     If you don t have the at command in the machine and you also don t have install privilegies on it  you can put something like this on cron  maybe with the crontab command          5    path to comand to execute   usr bin crontab -l    usr bin grep -iv command to execute    usr bin crontab -    it will execute your command one time and remove it from cron after that

User · Answer

Your comment suggests you re trying to call this from a programming language   If that s the case  can your program fork a child process that calls sleep then does the work     What about having your program calculate the number of seconds until the desired runtime  and have it call shell exec  sleep   secondsToWait    myCommandToRun

User · Answer

You really want to use at  It is exactly made for this purpose   echo  usr bin the command options   at now   1 day   However if you don t have at  or your hosting company doesn t provide access to it  you could make a self-deleting cron entry   Sadly  this will remove all your cron entries  However  if you only have one  this is fine   0 0 2 12   crontab -r    home adm bin the command options   The command crontab -r removes your crontab entry  Luckily the rest of the command line will still execute   WARNING  This is dangerous   It removes ALL cron entries   If you have many  this will remove them all  not just the one that has the  crontab -r  line

[linux] How do I set up cron to run a file just once at a specific time?

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