RCHE Red Hat Certified Engineer Linux Study Guide[c] Exam (Rh302)

Here are some of the key points from the certification objectives in Chapter 5.

Shell Configuration Files

• All system-wide shell configuration files are kept in the /etc directory.

• /etc/profile is the system-wide startup shell script for bash users.

• All users have hidden shell configuration files in their home directories.

Setting Up and Managing Disk Quotas

• Quotas are used to limit a user's or a group of users' ability to consume disk space.

• Quotas are set on specific filesystems mounted to standard Linux formats.

• Quota support must be enabled in the kernel. By default, quotas are enabled in RHEL 3 kernels.

• Quotas have soft limits and hard limits. If both soft and hard limits are set, then a user can exceed his or her soft limit for a modest period of time.

• Users and groups may never exceed their hard limits.

The Basics of the Kernel

• The kernel lies at the heart of the operating system.

• You can recompile Linux kernels to minimize size and maximize efficiency.

• To optimize the Linux kernel, it is a best practice to compile kernels with only needed elements, and configure modules for most hardware.

• Modular kernels, with separate device modules, are more efficient than monolithic kernels, where device drivers are integrated into the kernel.

• If you're going to update your kernel, you should keep a copy of your current working kernel.

• There are a number of RHEL 3 kernels available for different types of CPU and levels of RAM.

• Linux kernels and related files are stored in the /boot directory.

• The /proc directory provides a window to what Linux sees in your computer.

• Changing the values of variables in the /proc directory can change the behavior of your running kernel. For example, setting ip_forward = 1 enables routing.

• Kernel modules are managed by a special kernel thread, kmod. Additional settings can be configured through /etc/modules.conf.

• The lsmod command lists currently loaded modules; important related commands are insmod, rmmod, and modprobe.

• Basic RHEL 3 modules are stored in the /lib/modules/2.4.21-4.EL directory.

New Kernels, the Easy Way

• Kernel version numbers are organized in major.minor.patch format. Red Hat adds a build number to the Linux kernels that it builds from source code.

• It's fairly easy to install a Red Hat kernel from RPM, as long as you remember to install and not upgrade. This allows you to return to the current working kernel if you have a problem.

• Sometimes, what you need is a kernel patch, which supports upgrades of one patch version number. Unfortunately, patches are not always compatible with Red Hat built kernels.

• It's important to create a new boot floppy whenever you install a new kernel.

• When you install a Red Hat kernel from RPM, the process should automatically update your boot loader (GRUB or LILO).

Kernel Sources

• The kernel source tree is accessible through /usr/src/linux-2.4, which is normally linked to the actual directory with kernel source files.

• Kernel sources can be loaded from the kernel-source RPM or from a Linux kernel tarball downloaded from a site such as ftp.kernel.org.

Recompiling a Kernel

• Your current kernel configuration is stored in the config-versionnumber file in the /boot directory.

• You can modify kernel settings from the /usr/src/linux-2.4 directory with one of the following commands: make config, make menuconfig, or make xconfig. The last two lead to a long series of menus.

• Once you've made the proper backups and boot disks, set the EXTRAVERSION variable in your Makefile, run the make mrproper command, and placed current settings in /usr/src/linux-2.4/.config, you're ready to modify your kernel.

• Once you've settled on and saved your changes, run the make dep, make clean, make bzImage, make modules, make modules_install, and make install commands, you've compiled your new kernel, and it should be ready for use from the GRUB boot loader.

The cron and at Systems

• The cron system allows any user to schedule jobs so they run at given intervals.

• The at system allows users to configure jobs to run once at a scheduled time.

• The crontab command is used to work with cron files. Use crontab -e to edit, crontab -l to list, or crontab -d to delete cron files.

• The /etc/cron.allow and /etc/cron.deny files are used to control access to the cron job scheduler.