cron - The system clock daemon
The cron daemon runs shell commands at specified dates and times. Commands that are to run according to a regular or periodic schedule are found within the crontab files. Commands that are to run once only are found within the at files. You submit crontab and at file entries by using the crontab and at commands. Because the cron process exits only when killed or when the system stops, only one cron daemon should exist on the system at any given time. Normally, you start the cron daemon from within a run command file.
During process initialization and when cron detects a change, it examines the crontab and at files. This strategy reduces the overhead of checking for new or changed files at regularly scheduled intervals.
The cron daemon executes a sync system call (approximately once a minute) to ensure that all information in memory that should be on disk (buffered output) is written out. These periodic updates minimize the possibility of file system damage in the event of a crash. The cron command creates a log of its activities. The cron daemon must be started from the system startup scripts because it must begin execution without a login user ID set. The cron daemon starts each job with the following process attributes stored with the job by the invoking process: Login user ID Effective and real user IDs Effective and real group IDs Supplementary groups
It also establishes the following attributes from the authentication profile of the account associated with the login user ID of the invoking process: Audit control and disposition masks Kernel authorizations
The at and batch programs will refuse to accept jobs submitted from processes whose login user ID is different from the real user ID.
Specifies the command path.
Commands: at(1), crontab(1), rc0(8), rc2(8), rc3(8)
Functions: sync(2) delim off