[Tutorial] Schedule Command for CentOS using `crontab`

Published: July 31, 2024 by SingularityKChen (Last updated: July 31, 2024 )

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Tutorial 26

Schedule Command for CentOS using crontab

Schedule Command for CentOS using `crontab`

Introduction

crontab is a command-line utility that allows you to schedule tasks to run at specific times or intervals. You can use crontab to run scripts, commands, or other programs automatically at a specified time.

Install and Enable `crontab`

# Install `crontab`
sudo yum install cronie
# Start and enable `crontab`
sudo systemctl start crond
sudo systemctl enable crond

Then, check the status of crond:

sudo systemctl status crond

Start and Edit Crontab Schedule

crontab -e

After editing the crontab file, save and exit the editor. The cron daemon will automatically reload the crontab file with the following message:

crontab: installing new crontab

Crontab Syntax

# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of week (0 - 6) (Sunday to Saturday)
# │ │ │ │ │
# │ │ │ │ │
# * * * * * command to execute

Crontab Examples

# Run a command every minute
* * * * * /path/to/command

# Run a command every 5 minutes
*/5 * * * * /path/to/command

# Run a command every hour
0 * * * * /path/to/command

# Run a command every day at 3:30 AM
30 3 * * * /path/to/command

# Run a command every week on Sunday at 3:30 AM

30 3 * * 0 /path/to/command

# Run a command every month on the 1st at 3:30 AM
30 3 1 * * /path/to/command

Crontab Examples with Output

# Redirect output to a file
* * * * * /path/to/command > /path/to/output.log

# Redirect output to a file and append
* * * * * /path/to/command >> /path/to/output.log

# Redirect output to a file and error to another file
* * * * * /path/to/command > /path/to/output.log 2> /path/to/error.log

# Redirect output and error to the same file
* * * * * /path/to/command > /path/to/output.log 2>&1

Crontab Examples with Environment Variables

# Set environment variables
* * * * * export VAR=value; /path/to/command

# Set environment variables from a file
* * * * * source /path/to/env.file; /path/to/command

Check Crontab Execution Result

You can check your email for the output of the crontab command when you see the following message:

You have new mail in /var/spool/mail/xxx

You can check the mail content by using the following command:

mail
# or
cat /var/spool/mail/xxx

Schedule Command for CentOS using crontab