Cron Job Examples and Tip

Submitted by nanodano on Tue, 08/04/2020 - 02:32

Introduction

Cron jobs are useful for scheduling commands to run periodically. For example:

Every 15 minutes
Once per month

In this guide I will show you several common cron tasks and tips that I use including:

Common scheduling examples
Chaining together multiple programs
Redirecting output
Checking logs
Running a script from a Python virtual environment

Cron is good for programs that run and then complete, like generating a report, but cron is not for services that stay running forever like a web server. If you want to have a program that continuously runs and you want to ensure the program stays running even if it crashes or the server restarts, you want to setup a service. Check out my tutorial on how to do that: Creating Systemd Service Files.

Cron is available in Linux and Mac operating systems, but not Windows.

Video

How to edit cron jobs

You can use crontab -e to edit cron jobs. Note that each user has their own crontab. This means if you run it without sudo, you are editing your users crontab and with sudo you are editing Root's crontab.

You can use -u <username> to specify which users crontab to edit. This is useful if you want to edit a system user's crontab. Read more about How to Create a Secure Linux System User

# Edit current user's crontab
crontab -e
# Edit root crontab
sudo crontab -e
# Edit a specific user's crontab
sudo crontab -u some_user -e

You can list current cron jobs with:

sudo crontab -u some_user -l

Cron schedules edited with crontab -e are stored in /var/spool/cron but you should never modify them directly.

You can also add cron jobs in the /etc/cron.* directories, but personally, I prefer the crontab -e method.

Tips

Always use full path to files and executables
Make sure your script is executable (chmod +x)
If you need to perform several steps, put it in a shell script and call that script
Use quotation marks for anything with spaces
Do not run jobs as root unless absolutely necessary. Use the lowest privilege user possible.

Cron format

A crontab entry has a specific format where the first five columns represent the schedule and the rest of the line is considered the command. You can use spaces or tabs to format a line. Comment lines start with #.

# Month, Hour, Day of Month, Month, Day of Week, Command
#   M     H        DoM         M      DoW        Command
# Example:
    *     *         *          *       *         /some/command --with-args

Common schedules

Here are some common examples that I have used myself. You can find a ton of useful examples for scheduling at crontab.guru.

Run every minute

The * symbol means to run on every occurrence. So in this case * * * * * means:

on every minute
of every hour
on each day of the month
for each month
and every day of the week

# Run every minute
* * * * *    /script/to/run

Run every 15 minutes

*/15 * * * *   /thing/to/execute

Run nightly at 1:30 AM

30  1  *  *  *     /path/to/command

Run weekly on Sunday at 4:30 AM

For the Day of Week field (the 5th column) Sunday = 0 and Saturday = 6.

30 4 * * 0    /path/to/script

Run monthly on the 1st at 2:15 AM

First of the month is Day of Month of 1 (column 3).

15 2 1 * *    /path/to/executable

Run a command from a Python virtual environment

To use a Python virtual environment to run Python, pip, or a script that was installed by a package.

There is no need to "activate" the virtual environment or create a separate script to activate and then run. You can simply invoke the Python interpreter directly from the virtual environment using its full path. It will automatically use the packages associated with that virtual environment location.

To learn more, check out my Python Virtual Environments Tutorial.

* * * * *    /path/to/venv/bin/python /path/to/script.py

Pass environment variables

You can set environment variables directly in a command like this example.

* * * * *    MY_ENV_VAR=123 ANOTHER_VAR="hi" /path/to/script.sh

Stringing together multiple commands

This example will run multiple commands in a row. If any script fails (returns a non-zero exit code), it will exit and not run the next command. The && will only continue is the previous script executed successfully and returned 0.

* * * * *    /script1 && /script2 && /script3

Redirect output

You may want to override the default output settings for a cron job to ensure you capture output to a file or to discard output so it does not write to disk. I will demonstrate some examples of each.

To learn more about redirecting output in general, check out my tutorial, STDIN, STDOUT, STDERR, Piping, and Redirecting.

Redirect stderr to stdout

If you cron is only logging standard output, but not standard error, you can redirect stderr to stdout.

# Redirect stderr to stdout
* * * * *     /script 2>&1

Redirect output to a log file

Depending on the logging situation of your server, output from cron execution may not be logged at all. In those cases you may want to redirect output to a file. You can also redirect standard error to standard output to ensure everything is captured.

# Redirect output to a specific file 
* * * * *     /script > /my/log.txt 2>&1

Discard output

On the other hand, you may have thorough logging of the cron execution output, but you may want to discard all of it instead of filling up disk space. In these situations you can redirect to /dev/null.

# Redirect output to /dev/null if you do not want the output to go anywhere
* * * * *     /script > /dev/null 2>&1

View cron logs

It may differ by Linux distribution, but there are a couple places to look:

The journalctl output from systemd's cron.service
Log files in /var/log/ like /var/log/cron

To check journalctl:

# View systemd logging
journalctl -u cron
# Tail systemd log for cron
journalctl -u cron -f

To view the log file in /var/log/:

# View cron log file
less /var/log/cron
# Tail cron log file
tail -f /var/log/cron

Conclusion

After reading this, you should understand how to setup basic cron jobs for