Cron: Difference between revisions
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#!/bin/bash | |||
# resources | |||
#SBATCH cpus-per-task=1 | |||
#SBATCH mem-per-cpu=4G | |||
#SBATCH array=-1%1 | |||
#SBATCH partition=express | |||
# job name, error and output files | |||
#SBATCH --job-name=cron-job | |||
#SBATCH error=err_%j_4a.log | |||
#SBATCH output=out_%j_%4a.log | |||
#SBATCH ntasks=1 | |||
# email address to request notifications when the job is complete or if it fails | |||
#SBATCH --mail-type=FAIL | |||
#SBATCH --mail-user=$USER@uab.edu | |||
# redirects the output to a .txt file | |||
echo "Hello World" > hello.txt | |||
</pre> | </pre> |
Revision as of 16:54, 16 March 2021
Cron
What Is Cron?
Cron is a unix time based job scheduler tool used to schedule command execution at a specified time interval. These jobs are referred to as cronjobs, and are a great way of automating tasks or scripts so that they can be executed at a specific time.
Overview
A crontab file houses instructions to the cron daemon of the basic form: "run this command at this time on this date". Each user can have their own crontab file.
Crontab Syntax
crontab [ -u user ] file crontab [ -u user ] { -l | -r | -e } crontab [ -u user ] [ -i ] { -e | -l | -r } crontab [ -u user ] [ -l | -r | -e ] [-i] [-s]
Cron Expression Table
Cron actions are driven by a crontab file, also known as a "cron table". The user's crontab file is a configuration file that specifies shell commands to run at a given schedule.
Every crontab file consists of two parts: a schedule and a command. Here is a look into the scheduling syntax:
# ┌───────────── minute (0 - 59) # │ ┌───────────── hour (0 - 23) # │ │ ┌───────────── day of the month (1 - 31) # │ │ │ ┌───────────── month (1 - 12) # │ │ │ │ ┌───────────── day of the week (0 - 6) (Sunday to Saturday; # │ │ │ │ │ 7 is also Sunday on some systems) # │ │ │ │ │ # │ │ │ │ │ # * * * * * <command to execute>
A crontab file has five fields; each field is represented by an asterisk to determine the data and time of a certain task.
Crontab Characters
Asterisk (*) - defines all the scheduling parameters.
Comma (,) - maintains two or more execution times of a single command.
Hyphen (-) - determines the range of time when setting several execution times of a single command.
Slash (/) - creates predetermined intervals of time in a specific range.
Last (L) - determines the last day of the week given in. a month. Example: 4L means the last Thursday.
Weekday (W) - determines the day of the week, followed by a number ranging from 1 to 5. Example: 1#2 means the second Monday
Question mark (?) - to leave blank
Crontab Options
To install, update, or edit a job in crontab, use the -e option.
$ crontab -e
To list crontab entires, use the -l option
$ crontab -l
To remove ALL jobs from crontab, use the -r option
$ crontab -r
To remove confirm removing a job from crontab, use the -i option
$ crontab -i -r
To add SELINUX security to a crontab file,
$ crontab -s
To edit another user's crontab file, use the user -u option and specify the username
$ crontab -u username -e
To list other user crontab entries
$ crontab -u username -l
NOTE: By default, cron uses vim to edit the crontab file. If you're not familiar with Vi/Vim, use the following command in Cheaha to go through a brief tutorial:
$ vimtutor
Rudimentary Examples
Hello World Using Cron
Bash Script to Submit Sbatch job to Slurm
#!/bin/bash # resources #SBATCH cpus-per-task=1 #SBATCH mem-per-cpu=4G #SBATCH array=-1%1 #SBATCH partition=express # job name, error and output files #SBATCH --job-name=cron-job #SBATCH error=err_%j_4a.log #SBATCH output=out_%j_%4a.log #SBATCH ntasks=1 # email address to request notifications when the job is complete or if it fails #SBATCH --mail-type=FAIL #SBATCH --mail-user=$USER@uab.edu # redirects the output to a .txt file echo "Hello World" > hello.txt
Crontab File
#!/usr/bin/env MAILTO=$USER@uab.edu # submit script to queuing system # runs cronjob.sh every minute of every day * * * * * /cm/shared/apps/slurm/18.08.9/bin/sbatch /data/user/$USER/Hello_Cron/cronjob.sh
The first line in the cron configuration file (crontab file) is the shebang. When this file is executed, if the file content beings with #!, the kernel executes the file specified on the #! line and passes the original file as an argument. Essentially, the shebang tells the terminal which program to use to run your scripts if your script is an executable file.
#!/usr/bin/env