The document outlines various Unix process scheduling commands, including 'at' for executing commands at a specified time, 'batch' for executing commands when the system is less loaded, and the 'cron' daemon which checks for scheduled jobs every minute. It explains the specific format required for crontab entries, which include fields for time and command execution. Additionally, it highlights the advantages of crontab over 'at' and 'batch' for repeated job scheduling and the access controls through cron.allow and cron.deny files.

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.