Welcome to another exciting lesson on system automation! Today, we will delve into scheduling tasks automatically using cron. If you often find yourself needing to run specific tasks at regular intervals without manual intervention, then cron is the tool for you.
Cron is a time-based job scheduler in Unix-like operating systems. Users can set up and maintain automated jobs using cron syntax. While there are alternatives like at and systemd timers, cron is widely used for its simplicity and flexibility.
Let's get started!
Cron allows you to schedule tasks, known as "cron jobs," to be executed at specific times or intervals. The configuration of these jobs is stored in files called "crontabs" (cron tables). Each user on a system can have their own crontab, and there's also a system-wide crontab managed by the root user. Suppose you want to run a script that backs up your computer every night. Instead of manually running the script every night, you can use cron to run the script at any time.
Cron uses a simple and powerful syntax to define job schedules. The basic format for a cron job is:
1* * * * * command_to_execute 2│ │ │ │ │ 3│ │ │ │ └──── Day of the week (0 - 7) (both 0 and 7 mean Sunday) 4│ │ │ └────── Month (1 - 12) 5│ │ └──────── Day of the month (1 - 31) 6│ └────────── Hour (0 - 23) 7└──────────── Minute (0 - 59)
Each of these fields can be specified as:
- A single number:
5means "exactly at five". - A range of numbers:
1-5means "1 to 5". - A list of numbers:
1,2,3means "1 or 2 or 3". - An asterisk (
*) wildcard means "every" possible value of this field. /: Specifies step values. For example,*/5in the minutes field means "every 5 minutes".
Here are some examples:
Run a job every day at 5 AM
Plain text10 5 * * * /path/to/script.sh
Run a job every Monday at 3 PM
Plain text10 15 * * 1 /path/to/script.sh
Run a job every 15 minutes
Plain text1*/15 * * * * /path/to/script.sh
Run a job every day at midnight
Plain text10 0 * * * /path/to/nightly_backup.sh
Run a job at 10 PM on the 1st of every month
Plain text10 22 1 * * /path/to/monthly_job.sh
Let's create a script that we will later schedule to run every minute. The print_time.sh script simply prints the current date and time to time.txt. While you can use relative paths in cron jobs, it's important to ensure that the cron job's working directory is set correctly. Using absolute paths is generally more reliable, but if you need to use relative paths, specify the working directory with cd within the cron job to avoid any issues.
Here's the cron job script:
print_time.sh
Bash1#!/bin/bash 2TARGET_DIR="/usercode/FILESYSTEM" 3OUTPUT_FILE="$TARGET_DIR/time.txt" 4echo $(date) >> $OUTPUT_FILE
Whenever the script is run, the output of the $(date) command is written to /usercode/FILESYSTEM/time.txt.
To schedule a cron job, we need to add it to the crontab file. The crontab -l command displays the contents of the user's crontab file, showing all the scheduled tasks and their corresponding schedules. To add a cron job we use this syntax:
Plain text1(crontab -l; echo "<command>" ) | crontab -
crontab -llists the current user's cron jobs.echo "<command>"creates a new line with the specified cron command- The parentheses group the two commands together so that they are treated as a single unit.
| crontab -pipelines the combined output of the grouped commands to the crontab command.
In essence, this command appends a new cron job to the existing list of cron jobs for the user, ensuring that any existing jobs are not overwritten but retained along with the new addition.
Now let's write another script called schedule_task.sh to set up and schedule the print_time.sh script as a cron job.
We need to:
- Specify the directory the script is in
- Make the script executable
- Start cron
- Add the cron job to the
crontabfile
schedule_task.sh
Bash1#!/bin/bash 2 3# Specify script path and give execute permissions 4SCRIPT_PATH="$PWD/print_time.sh" 5chmod +x $SCRIPT_PATH 6 7# Start cron 8sudo service cron start 9# Verify cron has started 10service cron status 11 12# Create the cron command 13command="*/1 * * * * $SCRIPT_PATH" 14 15# Append the cron job; handle the case when there's no existing crontab 16(crontab -l; echo "$command" ) | crontab - 17 18# Confirm cron job was added 19crontab -l
Let's breakdown this script:
SCRIPT_PATHdefines the absolute path to theprint_time.shfilechmod +x $SCRIPT_PATHmakes the script executablesudo service cron startstarts the cron serviceservice cron statusconfirms cron is runningcommanddefines that theprint_time.shscript should run every minute(crontab -l; echo "$command" ) | crontab -adds the job to thecrontabcrontab -lprints the list of all cron jobs
The output of running schedule_task.sh is:
Plain text1 * Starting periodic command scheduler cron 2 ...done. 3 * cron is running 4*/1 * * * * /usercode/FILESYSTEM/print_time.sh
We see the message from running sudo service cron start and confirmation from running service cron status. The crontab -l commands prints the list of cron jobs, which in our case is just the print_time.sh job.
After creating this cron job, the print_time.sh script will run every minute. The script writes $(date) to time.txt whenever it is run. Since the job runs once every minute, an example output after 3 minutes is:
Plain text1Mon Jul 29 00:06:01 UTC 2024 2Mon Jul 29 00:07:01 UTC 2024 3Mon Jul 29 00:08:01 UTC 2024
Great job on understanding how to use cron to schedule tasks automatically. In this lesson, you learned how to:
- The basics of cron and its syntax.
- How to write cron commands to schedule events
- How to start cron
- How to schedule cron jobs in the
crontab
With this knowledge, you can automate repetitive tasks, increasing your efficiency and ensuring consistency. Now it's time to practice! Head over to the practice section and apply what you've learned to solidify your understanding. Happy coding!