cron is the daemon responsible for executing scheduled and recurring commands (every hour, every day, every week, etc.). atd deals with commands to be executed a single time, but at a specific moment in the future.
locate-programmet;
crontab -e (innholdet er lagret i /var/spool/cron/crontabs/bruker-filen).
/etc/crontab-filen, eller skrive i tillegg crontab-filer i /etc/cron.d-mappen. Disse to siste løsningene har fordelen av å kunne spesifisere brukerens identitet når kommandoen utføres.
crontab entry describes a scheduled command with the six (or seven) following fields:
Sun, Mon, etc.);
/etc/crontab-filen, og i fragmentene som ligger i /etc/cron.d/, men ikke i brukerens egne crontab-filer);
a-b describes the interval of all the values between a and b. The syntax a-b/c describes the interval with an increment of c (example: 0-10/2 means 0,2,4,6,8,10). An asterisk * is a wildcard, representing all possible values.
Eksempel 9.2. Sample user crontab file
#Format #min hour day mon dow command # Download data every night at 7:25 pm 25 19 * * * $HOME/bin/get.pl # 8:00 am, on weekdays (Monday through Friday) 00 08 * * 1-5 $HOME/bin/dosomething # every two hours * */2 * * * $HOME/bin/dosomethingelse # Restart the IRC proxy after each reboot @reboot /usr/bin/dircproxy
at executes a command at a specified moment in the future. It takes the desired time and date as command-line parameters, and the command to be executed in its standard input. The command will be executed as if it had been entered in the current shell. at even takes care to retain the current environment, in order to reproduce the same conditions when it executes the command. The time is indicated by following the usual conventions: 16:12 or 4:12pm represents 4:12 pm. The date can be specified in several European and Western formats, including DD.MM.YY (27.07.22 thus representing 27 July 2022), YYYY-MM-DD (this same date being expressed as 2022-07-27), MM/DD/[CC]YY (i.e., 12/25/22 or 12/25/2022 will be December 25, 2022), or simple MMDD[CC]YY (so that 122522 or 12252022 will, likewise, represent December 25, 2022). Without it, the command will be executed as soon as the clock reaches the time indicated (the same day, or tomorrow if that time has already passed on the same day). You can also simply write “today” or “tomorrow”, which is self-explanatory.
$at 09:00 27.07.22 <<END>echo "Don't forget to wish a Happy Birthday to Raphaël!" \>| mail lolando@debian.org>ENDwarning: commands will be executed using /bin/sh job 1 at Wed Jul 27 09:00:00 2022
at now + nummer varighet. varighet kan være minutes, hours, days, eller weeks. nummer indikerer rett og slett antallet av de nevnte enheter som må ha passert før kommandoen utføres.
cron, kjør ganske enkelt crontab -e, og slett den tilsvarende linjen i crontab-filen. For at-oppgaver, er det nesten like lett; kjør atrm oppgavenummer. Oppgavenummeret er indikert av at-kommandoen når du har tidfestet den, men du kan finne det igjen med atq-kommandoen, som gir den gjeldende listen over tidfestede oppgaver.