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CHAPTER 19 LINUX TROUBLESHOOTING
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Figure 19-8. The parameter you add to enter single-user mode At this point, the system assigns you the single-user mode shell on runlevel 1.
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In chapter 17, you learned how to download and compile a new Linux kernel from http://www.linux.org. Assuming you tried to do this, you might have experienced the situation where your new kernel didn t continue to load after the initial boot. Specifically, you might have seen an error message complaining about a missing device, such as a console. Or your boot sequence might have hung in a manner similar to what you see in Figure 19-9.
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Figure 19-9. Your new kernel got stuck in the middle of a system boot
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If this happened to you, your system might be using an old version of udev. For example, this can happen if you have an older version of CentOS or Fedora. Linux uses the udev command to manage devices that your system uses, such as hard disks and/or optical drives. udev is responsible for creating special files such as hda to represent the first IDE hard disk inside the /dev directory of your system. The problem lies in the fact that udev can t create the required device entries because it might be an old version that contains problems. Fixing this doesn t require that you re-create each device that the udev command must make during boot up. You need to create only three types inside the /dev directory of your system: the console, the null, and the tty devices. Linux uses the console device to leverage devices assigned as consoles. With console devices, you can enter commands to use your system. Linux uses the null device as a general garbage bin for useless data or unneeded output. The tty devices lets Linux create terminals for you to log into the system.
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Begin by booting into the rescue environment and choosing Skip when prompted to mount your Linux system. This prevents the rescue environment from creating temporary special files in your system s /dev directory. Next, implement the commands shown in Listing 19-1. ls ls ls ls ls ls l l l l l l /dev/console /dev/null /dev/ttyS0 /dev/ttyS1 /dev/ttyS2 /dev/ttyS3
Listing 19-1. Getting the Details of the Rescue Environment s Special Files The list you create by running the code in Listing 19-1 probably looks similar to the list shown in Figure 19-10, but this could look different in future releases of CentOS.
Figure 19-10. The devices of the rescue environment So far you ve listed the rescue environment s device files for console, null, and some tty devices. Next, you need to copy the major and minor numbers of each device onto a piece of paper. For example, the /dev/console special file of the rescue environment has the major number of five and minor number
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CHAPTER 19 LINUX TROUBLESHOOTING
of one. Linux uses these numbers to identify individual devices. Next, you need to use these numbers to create the devices on your Linux system. Make the required special files in the /dev directory of your Linux system using mknod command. This command was built for that purpose, and it uses this syntax, where specialfilename is the name of the special file you want to create: mknod specialfilename specialfiletype givenmajornumber givenminornumber Your file will be given the type of specialfiletype. The givenmajornumber and givenminornumber are the major number and minor number you need to assign to the special file once you create it. Assuming that you mounted your Linux system s root directory in /mnt/temp, you can use the list in Figure 19-9 to run the commands listed in Listing 19-2 in succession. mknod mknod mknod mknod mknod mknod /mnt/temp/dev/console c 5 1 /mnt/temp/dev/null c 1 3 /mnt/temp/dev/ttyS0 c 4 64 /mnt/temp/dev/ttyS1 c 4 65 /mnt/temp/dev/ttyS2 c 4 66 /mnt/temp/dev/ttyS3 c 4 67
Listing 19-2. Making the Special Files on Your Linux System s Dev Directory Each command in Listing 19-2 creates the required special file inside the /dev directory of your system. For example, the first mknod command creates the console special file in the /dev directory of your system as a character device specified by the c character. You copy the major and minor numbers used here from the rescue environment s console special file shown in figure 19-9. If you make a mistake on any of the commands making the special files, such as mixing up the major and minor numbers, you can remove that file with the rm command and re-create it with the mknod command. At this point, you should be able to reboot your system using the new kernel which should load. If you reboot to your new kernel, and it still won t continue to load, you will need to investigate the issue further. For example, you might have selected the kernel to use a Pentium CPU, but your machine is an Athlon CPU. Or you might have forgotten to select the filesystem your Linux system is using, such as xfs. Review your kernel configuration because the problem starts there most of the time.
Summary
In this chapter, you learned how to use the rescue environment provided by the CentOS and Fedora CDs, and a DVD installer. You also learned some basic troubleshooting techniques, including how to reinstall the GRUB bootloader in case it gets overwritten and how to change the root password to avoid a lockout. You also learned how to use the single-user mode in Linux to troubleshoot the system in case the rescue environment is unavailable. Together, these techniques give you a fighting chance to troubleshoot your Linux system successfully. Later, as you gain more experience, your troubleshooting tactics will improve, and you ll become an expert Linux user with CentOS. Good luck!
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