Process scheduling commands in unix.pptx
- 1. Process scheduling commands The UNIX system provides facilities to schedule jobs to be executed at a specified date and time. Few process scheduling commands are :
- 2. at command • This command is used to execute UNIX commands at a future date and time. • Once the commands are submitted, this command displays the details regarding the job-id, date and time at which the command must be executed. options -f can be used to specify a file as an argument to the at command -l can be used to display different jobs submitted by user -r removes a job from the queue, where job-id must be specified as an argument
- 3. batch command • This command does not need the user to specify the time at which it should be executed. • Therefore jobs submitted are executed when the system load is less and is relatively free. • The extension .b is added to the job id indicating that this job is submitted using the batch command.
- 4. The cron daemon • It is asleep most of the time. • It wakes up once in every minute and checks the crontab file for any job to be executed during that minute. • The crontab containing one or more lines must be created in a specific format. • Each line here contains 6 fields, with each field separated by a blank.
- 5. • The first field specifies minutes from 0-59. • The second field specifies hour in 24 – hour format. • The third field specifies day of month from 1-31. • The fourth field specifies the month from 1-12. • The fifth field specifies the day of week from 0-6, where 0 is Sunday. • The sixth field contains the command line to be executed. day of month month hour minute day of week command to be executed
- 6. • An asterisk in a crontab represents all possible values. • For ex., 40 9 * * * studdet.sh when executed, runs the file studdet.sh at 9:40 every day.
- 7. The crontab command • Though at and batch commands are powerful scheduling command, once submitted, they will execute the jobs only once. • Next the jobs will have to be rescheduled if they have to be executed again. • The crontab command has great advantage over at and batch in this respect. • It can execute a submitted job every day for years together, without user interference.
- 8. • When crontab command is executed, the file contents are automatically transferred to the crontabs directory (/var/spool/cron/crontabs) • -l option is used to display the contents of the crontab file • -r option is used to remove a submitted file. • Every user has only one crontab file. • Two files cron.allow and cron.deny exist, to decide the users to be allowed and to be denied access the crontab command.