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| CATEGORIES ABOUT CRONTAB | |
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The crontab command, found in Unix and Unix-like Operating System s, is used to schedule Command s to be executed periodically. It reads a series of commands from Standard Input and collects them into a file also known as a "crontab" which is later read and whose instructions are carried out. The name is derived from Greek Chronos (χρόνος), meaning time. USAGE Generally, the schedules modified by crontab are enacted by 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. These jobs are generally referred to as ''cron jobs''.To edit the crontab, the command crontab -e, opens the default crontab file with a text editor. Once the user has made the desired changes, saving and quitting that file will automatically enable the jobs. Alternatively, one can first generate a file with the desired configuration, and then specify that file to crontab. The latter is achieved by executing crontab filename.CRONTAB SYNTAX 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).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. Operators There are several ways of specifying multiple Date/time values in a field:
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