Cron jobs

Cron also known as cron jobs is a built-in software utility that is used to schedule a time-based job in Unix-like computer operating systems. Users setup and maintain software environments to use cron to schedule jobs which could be either commands or shell scripts to run periodic tasks at fixed time, date or interval. It automates system administration, system maintenance tasks or repetitive like sending the register email, taking database backup, sending notifications.

Crontab:

Crontab or cron table file handles actions of cron. The crontab is a configuration file which specifies shell commands to run periodic tasks at a given schedule. Each user can have their own individual crontab files. There is also a system-wide crontab file usually located in /etc.

Prerequisites for cron jobs

1. A linux system
2. command line/terminal
3. A user account with root or sudo privileges

Edit or Update cron jobs

You can edit crontab by running following commands for current user

        crontab -e
    

For sudo users

    sudo crontab -e

You can use -u pre {username} to specify which users crontab to edit

    sudo crontab -u username -e

All users have their own crontab. You can edit the root's crontab by using sudo

list the crontab jobs:

    crontab -l
    sudo crontab -l # for sudo users
    sudo crontab -u username -l     # for username

Basic Crontab Syntax

Each line in crontab expects a cron expression mad of five fields which represent the time to execute the command, followed by a shell command to execute and optional output

    a b c d e /directory/command output

So, the parts of a cron command are:

1. The first five fields a b c d e specify the time/date and recurrence of the job.
2. In the second section, the /directory/command specifies the location and script you want to run.
3. The final segment output is optional. It defines how the system notifies the user of the job completion

Cron Job Time Format

Examples :

RUN EVERY MINUTE

The * * * * * symbol means to run on every occurrence. It means * on every minute * of every hour * on each day of the month * for each month * every day of the week

RUN EVERY 15 seconds

To schedule a task to execute every 30 seconds is not possible by time parameters, But it can be done by scheduling same task as below

    * * * * * /path/to/executable/script.py
    * * * * *  sleep 30; /path/to/executable/script.py

RUN EVERY MINUTES

    * * * * *    /path/to/script

RUN EVERY 15 MINUTES

    */15 * * * *   /path/to/script

RUN EVERY SIX hours

    0 */6 * * * /path/to/executable/script.py

Run at 2am daily:

    0 2 * * * /bin/sh backup.sh

Run twice a day (11 AM and 11 PM daily).

You can specify multiple timestamps by comma separated

    0 11,23 * * * /path/to/executable/script.py

Run at one minute past midnight (00:01) every day:

    1 0 * * * /path/to/script

RUN script at 01:10 (01:10 AM) every Sunday.

    10 1 * * 0 /path/to/script

RUN MONTHLY ON THE 1ST AT 2:15 AM

    15 2 1 * *    /path/to/script

RUN at selected month

    * * * jan,may,aug *  /script/script.sh

RUN at selected days (Sunday and Friday at 5 PM)

    0 17 * * sun,fri  /script/script.sh

RUN at twice on every Sunday and Monday.

    0 4,17 * * sun,mon /path/to/executable/script.py

RUN yearly

@yearly is similar to 0 0 1 1 * as it will run at first minute of every year. It can be used to send the year greetings to the customers or friends.

    @yearly /path/to/executable/script.py

RUN monthly

@monthly timestamp is similar to 0 0 1 * * as it will execute in the first minute of the month. It can be used to automate the tasks like paying bills or send invoice to the customers.

    @monthly /path/to/executable/script.py

RUN weekly

@weekly timestamp is similar to 0 0 * * mon as it will execute a task in the first minute of the week. It may be useful to do weekly tasks like the cleanup of the system etc.

    @weekly /bin/script.sh

RUN daily

@daily timestamp is similar to 0 0 * * * as it will execute a task in the first minute of every day. It may be useful to do daily task.

    @daily /path/to/executable/script.py

RUN hourly

@hourly timestamp is similar to 0 * * * * as it will execute a task in the first minute of every hour, It may be useful to do hourly tasks.

     @hourly /path/to/executable/script.py

RUN tasks on reboot

@reboot is useful for those tasks which you want to run on your system startup to start tasks in the background automatically when system starts.

    @reboot /path/to/executable/script.py

Run a command from a Python virtual environment

We can run the python command using the python virtual environment. There is no need to activate the virtual environment or create a script to activate it. Python interpreter can directly be invoked using the full path from virtual environment It will automatically use the packages associated with that from virtual environment location.

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

pass environment variables

    * * * * *    MY_ENV_VAR="test" MY_ENV_VAR_2="bvariable" /path/to/executable/script.sh

Running multiple commands at once

We can run multiple commands in a row by Concatenation with &&. If any scripts fails(returns non-zero exit code), it will exit and will not run the next command

    * * * * *    /script1.sh && /script2.sh && /script3.sh

or

    * * * * * /script1.sh; /script2.sh; /script3.sh

REDIRECT OUTPUT TO A LOG FILE

    * * * * *     /path/to/executable/script.sh > /path/to/log/file.txt 2>&1

DISCARD OUTPUT

    * * * * *     /path/to/executable/script.sh > /dev/null 2>&1

View cron logs

The location of log file differ by linux distribution. The log file is mostly found at: * The journalctl output systemd's cron.service * Log files in /var/log/ like /var/log/cron

To check log files using journalctl command:

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

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

    tail -f /var/log/cron

Cron Schedules edited with crontab -e are stored in /var/spool/cron and they should never be modified directly

Backup cron to text file:

    crontab -l > cron-backup.txt

Removing current scheduled cron:

    crontab -r

Restore cron from text file:

    crontab cron-backup.txt

Tips

• Always useful path of files and executables
• Scripts must be executable. You can make script executable by running (sudo chmod +x filename)
• If several steps are needed, put all the steps in the script files and call the script file.
• Use quotation marks for anything with spaces.
• Use the lowest privilege user possible. Do not run jobs as root unless absolutely needed.