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crontab command2 Crontab files 3 See also |
The
Generally,
The crontab files are where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a systemwide crontab file (usually in /etc or a subdirectory of /etc) which is also used but can only be edited by the system administrator(s).
Crontab files have a particular format. Each line of a crontab file follows a particular format as a series of fields, separated by spaces and/or tabs. Each field can have a single value or a series of values.The
crontab commandcrontab command, found in Unix and Unix-like operating systems, is used to schedule commands to be executed periodically at particular times. It reads a series of commands from standard input and collects them into a file known also known as a "crontab" which is later read and whose instructions are carried out.crontab uses a daemon, crond, which runs constantly in the background and checks once a minute to see if any of the scheduled jobs need to be executed. If so, it executes them.Crontab files
| Field no. | Description | Permitted values |
|---|---|---|
| 1 | minute | 0-59 |
| 2 | hour | 0-23 |
| 3 | day of the month | 1-31 |
| 4 | month | 1-12 |
| 5 | day of the week | 0-7 |
The sixth and subsequent fields (i.e. the rest of the line) specify the command to be run.