Process scheduling commands in unix.pptx

basic commands

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. b atch 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 c ommand 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.