Glossary

anacron — cron's companion for machines that sleep. Instead of clock times it tracks when each job last ran (timestamp files) and runs whatever is due by period — "daily" means once a day, whenever the machine is awake.

CMD line — the log entry cron writes when it starts a job: CRON[pid]: (user) CMD (the command). The primary evidence that a job ran — and a record of the command as cron understood it, percent-split and all.

cron.allow / cron.deny — files in /etc gating who may use the crontab command at all. If the allow file exists, only its users may; otherwise the deny file's users may not. Root is never gated.

crontab — two things sharing a name: a table of jobs (five time fields plus a command per line), and the command that installs, lists, edits, and removes your personal table.

cron.d — /etc/cron.d/: a directory of system crontab files, one per concern. Same format as /etc/crontab, including the extra user field and the final-newline rule.

daemon — a background program that runs whether or not anyone is logged in. Cron's daemon wakes each minute, reads the tables, and starts what's due.

day fields trap — with both day-of-month and day-of-week restricted, a job runs when either matches (OR), not when both do (AND).

fields — the five time columns of a crontab line: minute, hour, day of month, month, day of week.

flock — a small tool that runs a command while holding a lock on a file. flock -n lockfile command runs the command only if no other copy holds the lock, which is how cron jobs keep from overlapping themselves.

job — one line of a crontab: a schedule plus a command, run as the table's owner (or the user field's account, in system tables).

MAILTO — a crontab header variable naming where job output is mailed. Set it empty (MAILTO="") to send output nowhere, on purpose.

MTA — mail transfer agent (Postfix, Exim, …): the mail system cron hands job output to. Without one, cron logs "No MTA installed, discarding output" and the output is simply gone.

percent trap — in a crontab command, the first unescaped % ends the command; the rest becomes the command's standard input. Escape literal percents as \%.

@reboot — the special string meaning "run once at system startup". A daemon restart later in the day does not re-trigger it; cron checks whether this is really system startup and logs the skip when it isn't.

spool — /var/spool/cron/crontabs/: where user crontabs actually live, one file per user, managed only through the crontab command.

step — the / in a field: */10 means every 10th value; 9-17/2 means every second value in the range.

system crontab — /etc/crontab and the files in /etc/cron.d/. Their lines carry a sixth field, the user the job runs as.

timestamp file — anacron's memory: one small file per job recording the date it last ran, compared against the job's period to decide what's owed.