vb.net qr code reader Figure 2.1: UNIX Command, shell, kernel structure in Software

Creator Code39 in Software Figure 2.1: UNIX Command, shell, kernel structure

Figure 2.1: UNIX Command, shell, kernel structure
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Services and Daemons
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Each running task, along with its associated address space, is called a process. The group of active processes running under UNIX is analogous to the generations of a family tree. Child processes are begotten by parent processes. Processes are born, live out their lives, and then pass away. Occasionally they may even run away. The first process to run on every UNIX system is called init. Init is the great-grandparent from which all process generations owe their existence. Like any loving grandparent, init even takes in orphan processes that have lost their immediate parents. During the course of its life, each process wants to take a turn on the CPU. Just like children, processes the guidance of a scheduler to ensure that every process receives its fair share of CPU time. The system administrator acts as the grand overseer of the process universe, wielding ultimate control over the lives of all processes. Every process in the UNIX universe is christened with a positive integer number called the Process Identifier (PID). The PID is a vector index in the process table maintained by the kernel. PIDs are unique and are allocated in a somewhat random fashion. Process table entries point to per-process kernel data structures which define the attributes and values associated with the given process (Table 2.1). The attributes of active processes can be interrogated from the command line by using the ps command (Example 2.1). Table 2.1: Sampling of Process Attributes
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Process Identifier Process Group Identifier Process Parent Identifier Process Owner Effective and Real User and Group Identifiers Priority Controlling Terminal Address Space Size in Pages Paging Statistics Resource Utilization Process State
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Example 2.1 Displaying Process Attributes # ps -elk F S 303 A 200003 A 40401 A 40001 A # ps auxw USER PID root 1 root 0 root 8626 nmbd.pid root 8664 smbd.pid %CPU 0.1 0.1 0.0 0.0 %MEM 0.0 0.0 1.0 1.0 SYSV Process Display Format PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 0 120 16 707 4 0:05 swapper 0 0 60 20 505 220 0:07 init 1 0 60 20 69fa 1060 0:09 nmbd 1 0 60 20 49f2 1440 0:06 smbd BSD Process Display Format SZ 220 4 1060 RSS 180 8 852 TTY STAT A A A A STIME 00:19:58 00:19:28 00:20:17 TIME 0:07 0:05 0:09 COMMAND /etc/init swapper nmbd D f
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PID 0 1 8626 8664
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It will be helpful to understand a few of the columns displayed in the list of running processes when managing execution resources in a running system. The COMMAND and CMD columns represent the program being run in the process address space. Along with its PID, each process records the integer ID of its Parent Process Identifier (PPID). Process short-term CPU usage, execution priority, and nice value are displayed in the PRI, C, and NI fields in the SYSV display. The BSD #CPU field represents the percentage of CPU resources that a process has used in its life time. In every process universe there is a special set of processes called daemons. A daemon is not some miscreant process spawned by some evil hacker to wreck havoc on a system, but a collection of one or more processes that provide some well-defined service. For example, the sendmail daemon manages and distributes electronic mail on behalf of the UNIX user community. In the case of Samba, the smbd daemon manages access to shared resources. The Samba nmbd daemon provides name service to assist clients in locating resource shares.
Init
I've already mentioned the special role that init plays as the ancestor of all processes on the running system. Init is responsible for spawning some of the important system daemons at system startup and "respawning" processes that may have exited due to some unforeseen event. These processes are identified in a configuration file called /etc/inittab. Entries in the inittab configuration file take the form: <identifier>:<runlevel>:<action>:<command> The identifier field uniquely identifies the service. The runlevel corresponds to the running system state which will cause the action to be invoked on the command. Runlevels are represented as numbers 0 through 9 and indicate various system states. An example inittab entry for the cron daemon which time schedules batch jobs on UNIX would look like: cron:2:respawn:/usr/sbin/cron Inittab cron process entry
As we shall see in a following section, there are other mechanisms available for spawning service daemons. In the case of network-based services like Samba, it is often desirable to initiate an instance of a service daemon only at the actual time when its services are requested by a client.
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