Cron scheduled tasksλ︎

cron is daemon that automatically runs a script or command at predefined time. cron is started automatically from /etc/init.d on entering multi-user runlevels.

crontab is used to create and update the schedule of tasks.

Linux Daemon

Daemon is a Linux background system process

Scheduled GitHub workflow

Use a schedule: directive to add cron: events that automatically run on the scheduled time

GitHub Docs - Schedule

Crontabλ︎

The crontab is used to automate all types of tasks on Linux systems.

Cron Concepts
Setup Crontab Access for a User Account
Handle errors with cronjobs
Creating cronjobs

Difference between Cron, Crontab and Cron Job

Element Linux Name Meaning - crontab: rows in a table for each cron job, each ‘*’ asterisk represents a segment of time and a corresponding column in each row. - Cron Job: specific task to be performed described in a row, paired with its designated time id

ps aux | grep crond

This command will search current processes for all users and return any instances of ‘crond’.

ps ux | grep crond
practicalli  8942  0.0  0.0  18612   840 pts/0    S+   02:16   0:00 grep --color=auto crond

The daemon is running for the practicalli user account.

Crontab syntaxλ︎

crontab [options]

- - - - updatedb OR

List the crontab entries for the current user account.

crontab -l

Edit entries in the crontab

crontab -e

Select a tool to edit the crontab for the first time

no crontab for practicalli - using an empty one

Select an editor.  To change later, run 'select-editor'.
  1. /bin/nano        <---- easiest
  2. /usr/bin/vim.basic
  3. /usr/bin/kak
  4. /usr/bin/vim.tiny
  5. /usr/bin/code
  6. /bin/ed

Choose 1-6 [1]:

Examplesλ︎

Redirect cron output to a file

* * * * * sh /path/to/script.sh &> log_file.log

log errors

* * * * * sh /path/to/script.sh >> /tmp/cron.txt 2>&1

drop out debug logs

* * * * * /bin/bash -x /path/to/script.sh >> /tmp/cron.txt 2>&1

Tipsλ︎

echo the date and time at start of script