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Copyright 2001, Lineo Inc. All rights reserved. This is a small version of the cron daemon. CRONTAB ======= The default crontab file is located in /etc/config/crontab. This file must be a regular file (i.e. not a symbolic link) and it must be both owned by root and not be group or world writable. It is formatted as follows. Lines begining with '#' and blank lines are ignored. Lines begining with a digit (or '*') are cron job definitions. The format of these lines is described later. Other lines are treated as environment variable definitions. These take two forms. The first is "NAME=VALUE" which assigns a value to the specified environment variable. The second is "NAME=" which undefines the named variable. Leading and trailing spaces are trimmed from both the name and the value. There is no support for quoted strings in environment variables. The first five space or tab separated fields of the job definition lines represent the time and date on which to execute jobs. They are (in order): * Minute (0..59) * Hour (0..24) * Day of Month (1..31) * Month (1..12) * Day of Week (0..7) As is usual the month and day of week can be repesented either as numbers or as three character abbreviations. Sunday is represented by both 0 and 7. All fields can be expressed as single values; ranges (e.g. "3-5" == 3,4,5); ranges with step values (e.g. "3-10/3" == 3,6,9) or as a comma separated list made from these components. Thus: "2,3-7/2,11,13" is a valid list of prime numbers. The special value '*' means all allowable values and this can be supplemented with the step indication. Thus: "*/2" would represent the legal odd values. A job is executed when *all* of the time and date conditions are met. The sixth field is the user to run the job as. This can be expressed as a number (UID) or as a login name. The final field is the command to execute. In this field spaces are permitted. The special character '%' maps to a new line and all text after the first such character is provided to the command as its standard input. Use the sequence "%%" to get the '%' character. ENVIRONMENT =========== Interesting environment variables include: * PATH This defaults to "/bin:/usr/bin:/etc" but can be overridden. * MAILTO This specifies the user to send notification of job execution to. This defaults to the user specified in the crontab file. Unset this variable to disable email notification. * MAILHOST This specifies the SMTP host to send the email notifications via. It is clear by default and unless set, email notifications will be disabled. * HOME This specifies the initial directory to execute the job in. It defaults to "/tmp" if not set. * HOSTNAME This allows the sending host name to be overridden in the outgoing email notificiations. This allows a non-DNS listed host to send emails. SIGNALS ======= None. The cron daemon doesn't require signals to be sent to it. It automatically detects modifications to its crontab file and reloads it as required. INTERNALS ========= Due to the absence of a real fork() call on our platform, it is necessary to break the cron program into two sections. The first section is cron itself and this program manages the crontab file and job spawning. The second program cron-parent manages the actual cron job from their initial creation through until their death. When a cron job is due to be executed, the cron program creates a cron-parent process which revokes its root permissions; decodes the command line and executes the child. The cron-parent program provides standard input if required (or /dev/null if not). The cron-parent program forks another child process to deal with standard output capture and forwarding (or /dev/null if no capture is required). Because of this structure and the fact that in an embedded environment the shells tend to be fairly crippled, this version of cron does not use any form if shell to execute commands. They are parsed and processed internally.