Software And Hardware Program Management Of Linux
The Linux operating system calls a running program a process. A process can run in the foreground, displaying output on a display, or it can run in the background, behind the scenes. The kernel controls how the Linux system manages all the processes running on the system.
The kernel creates the first process, called the init process, to start all other processes on the system. When the kernel starts, it loads the init process into virtual memory. As the kernel starts each additional process, it gives it a unique area in virtual memory to store the data and code that the process uses.
Some Linux implementations contain a table of processes to start automatically on bootup. On Linux systems, this table is usually located in the special file /etc/inittabs. Other systems (such as the popular Ubuntu Linux distribution) utilize the /etc/init.d folder, which contains scripts for starting and stopping individual applications at boot time. The scripts are started via entries under the /etc/rcX.d folders, where X is a run level.
The Linux operating system uses an init system that utilizes run levels. A run level can be used to direct the init process to run only certain types of processes, as defined in the /etc/inittabs file or the /etc/rcX.d folders. There are five init run levels in the Linux operating system.
At run level 1, only the basic system processes are started, along with one console terminal process. This is called single-user mode. Single-user mode is most often used for emergency filesystem maintenance when something is broken. Obviously, in this mode, only one person (usually the administrator) can log in to the system to manipulate data.
The standard init run level is 3. At this run level, most application software, such as network support software, is started. Another popular run level in Linux is run level 5.
This is the run level where the system starts the graphical X Window software and allows you to log in using a graphical desktop window.
The Linux system can control the overall system functionality by controlling the init run level. By changing the run level from 3 to 5, the system can change from a console-based system to an advanced, graphical X Window system.
In Chapter 4, you’ll see how to use the ps command to view the processes currently running on the Linux system.
Software Program Management Of Linux
The kernel creates the first process, called the init process, to start all other processes on the system. When the kernel starts, it loads the init process into virtual memory. As the kernel starts each additional process, it gives it a unique area in virtual memory to store the data and code that the process uses.
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| Software And Hardware Program Management Of Linux |
Some Linux implementations contain a table of processes to start automatically on bootup. On Linux systems, this table is usually located in the special file /etc/inittabs. Other systems (such as the popular Ubuntu Linux distribution) utilize the /etc/init.d folder, which contains scripts for starting and stopping individual applications at boot time. The scripts are started via entries under the /etc/rcX.d folders, where X is a run level.
The Linux operating system uses an init system that utilizes run levels. A run level can be used to direct the init process to run only certain types of processes, as defined in the /etc/inittabs file or the /etc/rcX.d folders. There are five init run levels in the Linux operating system.
At run level 1, only the basic system processes are started, along with one console terminal process. This is called single-user mode. Single-user mode is most often used for emergency filesystem maintenance when something is broken. Obviously, in this mode, only one person (usually the administrator) can log in to the system to manipulate data.
The standard init run level is 3. At this run level, most application software, such as network support software, is started. Another popular run level in Linux is run level 5.
This is the run level where the system starts the graphical X Window software and allows you to log in using a graphical desktop window.
The Linux system can control the overall system functionality by controlling the init run level. By changing the run level from 3 to 5, the system can change from a console-based system to an advanced, graphical X Window system.
In Chapter 4, you’ll see how to use the ps command to view the processes currently running on the Linux system.
Hardware Management
Still another responsibility for the kernel is hardware management. Any device that the Linux system must communicate with needs driver code inserted inside the kernel code.
The driver code allows the kernel to pass data back and forth to the device, acting as a middle man between applications and the hardware. Two methods are used for inserting device driver code in the Linux kernel:
- Drivers compiled in the kernel
- Driver modules added to the kernel
Previously, the only way to insert device driver code was to recompile the kernel. Each time you added a new device to the system, you had to recompile the kernel code. This process became even more inefficient as Linux kernels supported more hardware.
Fortunately, Linux developers devised a better method to insert driver code into the
running kernel.
Programmers developed the concept of kernel modules to allow you to insert driver code into a running kernel without having to recompile the kernel. Also, a kernel module could be removed from the kernel when the device was finished being used. This greatly simplified and expanded using hardware with Linux.
The Linux system identifies hardware devices as special files, called device files. There are three classifications of device files:
- Character
- Block
- Network
Character device files are for devices that can only handle data one character at a time.
Most types of modems and terminals are created as character files. Block files are for devices that can handle data in large blocks at a time, such as disk drives.
The network file types are used for devices that use packets to send and receive data. This includes network cards and a special loopback device that allows the Linux system to communicate with itself using common network programming protocols.
Linux creates special files, called nodes, for each device on the system. All communication with the device is performed through the device node. Each node has a unique number pair that identifies it to the Linux kernel. The number pair includes a major and a minor device number. Similar devices are grouped into the same major device number. The minor device number is used to identify a specific device within the major device group.
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