configured, it is told what types of hardware devices to expect. Init will search for and attempt to initialize each physically attached device. Any device that does not initialize  or  that  is  missing  will  be  marked  as nonexistent and the driver disabled. Even if the device is later reconnected, it will be unusable until the system is  rebooted. When all is ready, the kernel verifies the integrity of the root filesystem and then mounts it. Init does the rest of the work that is needed in preparing the system for users. This includes mounting the remaining local disk partitions  (those  found  in  the  file  /etc/checklist); performing some filesystem cleanup operations (fsck); turning  on  the  major  UNIX  subsystems,  such  as accounting and the print service; starting the network; mounting remote file systems; and enabling user logins. SYSTEM MODES.—  There are two primary modes of system operation: single-user and multi-user. Single-user is a system state designed for administrative and  maintenance  activities  which  require  complete  and unshared control of the system. Single-user mode is sometimes  called  the  maintenance  mode.  Single-user mode is entered via manual intervention during the boot process. Sometimes, however, the system will enter single-user mode if there are problems in the boot process that the system cannot handle on its own. Multi-user allows many users to all log onto the same CPU.   Users   can   access   different   applications simultaneously   or   even   the   same   application simultaneously.  The  kernel  manages  the  different  users by scheduling the use of the processing time as well as swapping  programs  and  data  in  and  out  of  memory through virtual memory to disk. The most important fact to remember is that the number of concurrent users depends on the amount of memory installed in the computer. Each user has a certain amount of memory set aside for his or her work, unless everyone is willing to tolerate slow response time from the network. System Shutdown While there are many occasions when shutting down or rebooting the system is appropriate, neither operation  should  be  performed  indiscriminantly.  While it is generally not something to worry about, there is a degree of hardware fatigue associated with turning a computer system off and on again, and it is often better to let it run 24 hours a day than to shut it down at night. REBOOTING.—  There are only four common situations in which rebooting the system is called for: If  you  make  changes  to  any  of  the  system software  or  configuration  files  that  are  examined or executed only when the system is booted, you must reboot for these changes to take effect. Some  devices,  especially  printer  and  modem ports,   can   become   confused   enough   that resetting them is only accomplished by re- initializing  the  system. If the system has been up and running constantly for over a week, it is wise to bring the system down to single-user mode and run fsck. If any fixes are made to the root partition, the system must  be  rebooted. If the system console becomes irretrievably hung, the system must be rebooted. SHUTTING THE SYSTEM DOWN.—  There  are two proper ways to shut down the operating system: shutdown and reboot. As a last resort, the system can be shut down by turning off the power to the CPU. This method  is  recommended  only  under  emergency conditions because of its detrimental impact on system files and certain types of hard disk drives. These disk drives expect their floating heads to be parked prior to shutdown.  Powering  off  the  system  could  cause  the heads to crash and cause irreparable damage to the disk. Shutdown.—  This command is the most often used method of initiating a orderly system shutdown. It is the safest, most considerate, and most thorough to initiate a halt,  reboot,  or  return  to  single-user  mode.  The command  will  send  messages  to  each  user’s  terminal  at progressively  shorter  intervals  as  the  time  for  shutdown approaches.   The   messages   tell   the   time   of   the shutdown. Reboot.— This command terminates all currently executing  processes  except  those  essential  to  the system, then halts or reboots the system. When invoked without  arguments,  reboot  syncs  all  disks  before rebooting the system. The command does not send a message out to the users, unless you use the message option. MONITOR Some people would ask, “Why do I have to expend energy  on  monitoring  the  network  when  I  could  be doing something more productive, like file server or workstation  maintenance?”  There  are  several  reasons why you should monitor your network: 1-3