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.
Sallar
RedhatEnterpriseLinux Blog
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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