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Understanding PAM, or Pluggable Authentication Modules

PAM, or Pluggable Authentication Modules, is an authentication layer that allows programs to be written independent of a specific authentication scheme. Applications request authentication via the PAM library, and the PAM library determines whether the user is allowed to proceed. If an administrator wants to implement a different authentication scheme, he just changes the PAM configuration files and the existing programs work seamlessly.

Automation with crontab

Automation with crontab
In any operating system, it is possible to create jobs that you want to reoccur. This
process, known as job scheduling, is usually done based on user-defined jobs. For
Red Hat, this process is handled by the cron service, which can be used to schedule tasks (also called jobs). By default, Red Hat comes with a set of predefined jobs that occur on the system (hourly, daily, weekly, monthly, and with arbitrary periodicity). As an administrator, however, you can define your own jobs and allow your users to create them as well.

Scheduling Tasks with Cron job

Now that you understand the basics of automating tasks with a script or program, the next step is to know how to schedule the tasks so they are executed at a specific time or on a set schedule. Some scripts such as removing users may not need to be scheduled, but others such as performing backups might work better on a schedule so users can anticipate them or so they can be run during a time when they won’t interfere with the daily workload of the system.

Executing Commands in a Bash Script

To execute a series of commands in a Bash script, list each command on a separate line. For example, Listing shows the very basic Bash script that gathers information about system resources.

Writing Scripts with Bash

The GNU Bourne Again Shell (Bash) scripting language is useful when trying to automate a sequence of commands or when you want to execute the same command over and over again until a certain limit is reached.

Emergency Mode & Filesystem Repairing

Emergency Mode
Emergency mode is similar to single-user mode except the root filesystem is mounted read-only and runlevel 1 is not used. Boot into emergency mode using the same method as single-user mode except replace the word single with emergency in the boot method.
Because the filesystem is mounted read-only, files can not be changed or repaired, but files can be retrieved off the system.
Filesystem Repair
If one or more filesystem are corrupt, boot into rescue mode and do not mount the filesystem. Even if you can boot into single-user mode, do not use it because the filesystem cannot be repaired if it is mounted.

Single-User Mode

Single-user mode is equivalent to runlevel 1 on the system. If runlevel 1 is not configured properly, you will not be able to boot into single-user mode. Rescue mode requires a boot media, but single-user mode is specified as a kernel option using the installed boot loader and does not require additional boot media. However, it does require that the boot loader is working properly and that the filesystem be mounted. It does not provide the ability to start a network connection.

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