Working with the Unix OS

Unix Administration
for (i=0; i&i
done
Sync
The superblock exists both in memory and on disk.
The "sync" command flushes memory to disk. This is done by the kernel or /etc/update at regular intervals.
#define TRUE 1
main() /* update.c */
{
while (TRUE) {
sync(); sleep(30);
}
}
Cron
Cron - from the greek "chronos" meaning "time".
/etc/cron executes commands at specified dates and times. Regularly scheduled commands can be specified by
instructions in /etc/crontab. Cron is started by /etc/rc at boot time and from then on wakes up each minute to
determine if any commands are scheduled to be run.
/etc/crontab record fields:
- minute (0-59),
- hour (0-23),
- day of month (1-31),
- month of year (1-12),
- day of week (0-6 with 0=Sunday),
- command-to-execute
Any of the time fields can be a pattern.
Examples:
Automatically shutdown at 8am each Friday
0 8 * * 1 /etc/shutdown "shutting down for backup"
Order some milk at midnight every Monday - Friday
0 0 * * 1-5 echo "I need more milk" | mail milkman
Run suidcheck every 20 minutes Mon-Fri, 9am - 5pm
0, 20, 40 9-16 * * 1-5 suidcheck
0 17 * * 1-5 suidcheck
Printing
$ lpr filename # BSD
$ lp filename # System V
A "spooler" is a method of buffering data on its way to a specific destination. i.e. hold files to be printed until line
printer is ready to process them.
A "daemon" program wakes up to do the task when required, and then goes back to sleep again. These daemons
reside in /usr/lib or /etc called lpd (BSD) or lpsched (SYS V).
! BSD printing
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