What is a Cron Job and how does it work?
A cron job is a time-based job scheduler in Unix-like computer operating systems. Users can schedule jobs (commands or scripts) to run periodically at fixed times, dates, or intervals.
How do I write a valid cron expression?
A standard cron expression consists of five fields separated by spaces: Minute (0-59), Hour (0-23), Day of Month (1-31), Month (1-12), and Day of Week (0-6).
How can I schedule a cron job to run every minute?
The syntax for every minute is: * * * * *
What is the syntax to run a cron job once every hour?
The syntax for the top of every hour is: 0 * * * *
How do I schedule a task to run daily at midnight?
The syntax is: 0 0 * * *
What does the asterisk (*) symbol signify in a cron schedule?
The asterisk means 'every'. For example, * in the minute field means 'every minute'.
How can I specify multiple time values in a single cron field?
Use a comma. Example: 1,15,30 in the minute field means run at minute 1, 15, and 30.
How do I define a range of values in a cron expression?
Use a hyphen. Example: 1-5 in the day of week field means Monday through Friday.
How do I set up an interval or step value in cron?
Use a slash. Example: */15 in the minute field means 'every 15 minutes'.
What is the difference between cron and crontab?
Cron is the daemon (service) that runs the jobs. Crontab (cron table) is the file where the jobs are listed and the command used to edit that file.
How do I list all currently scheduled cron jobs?
Run the command 'crontab -l' in your terminal.
What command do I use to edit my crontab file?
Run the command 'crontab -e' to open the cron table in your default editor.
Will my cron jobs run if my computer is turned off?
No. Cron jobs only run when the system is running. If the system is down at the scheduled time, the job is skipped.
What is Anacron and when should I use it instead of Cron?
Anacron is a computer program that performs periodic command scheduling, similar to cron, but it does not assume that the machine is running continuously.
How can I log the output of my cron job to a file?
Append '> /path/to/file.log 2>&1' to the end of your command to save standard output and errors to a log file.
What does '2>&1' mean at the end of a cron command?
It redirects 'Standard Error' (2) to the same location as 'Standard Output' (1), useful for capturing error messages in logs.
How do I stop cron from sending me emails with job output?
Add '> /dev/null 2>&1' to the end of the command to discard all output.
How do I run a cron job automatically at system startup using @reboot?
Use the special string '@reboot' in place of the five time fields to run the command once when the system boots.
Is there a shortcut to run a cron job once a day?
Yes, '@daily' is a shortcut for '0 0 * * *' (run once a day at midnight).
How do I quickly schedule a weekly cron job?
Use '@weekly', which is a shortcut for '0 0 * * 0' (run once a week on Sunday).
What is the shortcut for running a task once a month?
Use '@monthly', which is a shortcut for '0 0 1 * *' (run once a month on the 1st).
How do I schedule a yearly cron job?
Use '@yearly', which is a shortcut for '0 0 1 1 *' (run once a year on Jan 1st).
Can I use text names for months in a cron expression?
Yes, many cron implementations allow JAN-DEC instead of 1-12.
Is it possible to use day names like 'Mon' or 'Sun' in cron?
Yes, most implementations allow SUN-SAT instead of 0-6.
Where are system-wide cron jobs stored on Linux?
System-wide cron jobs are often stored in /etc/crontab or the /etc/cron.d/ directory.
How do I execute a PHP script using a cron job?
Use the php command: '/usr/bin/php /path/to/script.php'.
What is the command to run a Python script via cron?
Use the python command: '/usr/bin/python3 /path/to/script.py'.
Which time zone does the cron daemon use for scheduling?
Cron typically uses the system's local time zone. You can check it with the 'date' command.
Where can I find the log files to verify if my cron job ran?
On many systems, cron logs are located at /var/log/cron or /var/log/syslog.
Why is my scheduled cron job failing to execute?
Common reasons include issues with absolute paths, file permissions, differing environment variables, or the cron service not running.
Do I need to include a shebang line in my cron script?
Yes, if you are executing a script directly (e.g., ./script.sh), it needs a shebang (#!/bin/bash) and execute permissions.
Why do some cron expressions have 6 fields instead of 5?
Some implementations (like Quartz or system crontab) include a Year field or a User field, but standard user crontab has 5 fields.
What is the cron expression for running a task every 5 minutes?
*/5 * * * *
How do I schedule a job to run every 30 minutes?
*/30 * * * *
How can I limit a cron job to run only on weekdays?
* * * * 1-5
What is the syntax to run a job only on weekends?
* * * * 6,0
How do I schedule a job to run only during business hours?
Use '0 8-17 * * *' to run at the top of the hour from 8 AM to 5 PM.
How can I automate a daily database backup using cron?
Schedule a command like 'mysqldump -u user -p pass db > backup.sql' to run daily, e.g., '0 3 * * *'.
Does cron work on Windows or is it Linux-only?
Cron is native to Unix/Linux/macOS. Windows uses 'Task Scheduler', though WSL can run cron.
Is there a tool to test and verify my cron schedule?
You can test and visualize your schedule right here on N28.net.