cron - daemon to execute scheduled commands (Vixie Cron)
The utility searches /var/cron/tabs for crontab files which are named after accounts in /etc/passwd crontabs found are loaded into memory. The utility also searches for /etc/crontab which is in a different format (see crontab(5)).
The utility then wakes up every minute, examining all stored crontabs, checking each command to see if it should be run in the current minute. Before running a command from a per-account crontab file, checks the status of the account with pam(3) and skips the command if the account is unavailable, e.g., locked out or expired. Commands from /etc/crontab bypass this check. When executing commands, any output is mailed to the owner of the crontab (or to the user named in the MAILTO environment variable in the crontab, if such exists).
Additionally, checks each minute to see if its spool directory's modification time (or the modification time on /etc/crontab has changed, and if it has, will then examine the modification time on all crontabs and reload those which have changed. Thus need not be restarted whenever a crontab file is modified. Note that the crontab(1) command updates the modification time of the spool directory whenever it changes a crontab.
This option can help to smooth down system load spikes during moments when a lot of jobs are likely to start at once, e.g., at the beginning of the first minute of each hour.
The jobs run during the GMT offset changes time as intuitively expected. If a job falls into a time interval that disappears (for example, during the switch from standard time) to daylight saving time or is duplicated (for example, during the reverse switch), then it is handled in one of two ways:
The first case is for the jobs that run every at hour of a time interval overlapping with the disappearing or duplicated interval. In other words, if the job had run within one hour before the GMT offset change (and cron was not restarted nor the crontab(5) changed after that) or would run after the change at the next hour. They work as always, skip the skipped time or run in the added time as usual.
The second case is for the jobs that run less frequently. They are executed exactly once, they are not skipped nor executed twice (unless cron is restarted or the user's crontab(5) is changed during such a time interval). If an interval disappears due to the GMT offset change, such jobs are executed at the same absolute point of time as they would be in the old time zone. For example, if exactly one hour disappears, this point would be during the next hour at the first minute that is specified for them in crontab(5).
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Created 1996-2023 by Maxim Chirkov
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