A crontab holds one user’s scheduled jobs, one per line: five time fields plus the command. crontab -e edits your own; root manages any user’s with crontab -u user.
The administration trap: the underlying spool files must not be edited directly — the cron daemon may ignore or overwrite such changes. Proper tooling always goes through the crontab command so changes reliably take effect.
Frequently asked questions
What do the five fields in a crontab line mean?
Minute, hour, day of month, month, day of week — e.g. 17 3 * * * runs daily at 03:17. The rest of the line is the command.
Why didn’t my crontab change take effect?
Most likely the spool file was edited directly instead of via crontab — the daemon did not reload it. Re-apply the change through the command.