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An Introduction to Linux Shell Scripting for DBAs

By: Casimir Saternos
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About seven years ago, Oracle released the first commercial database on Linux. Since then, Oracle, Red Hat, and Novell/SUSE have been steadily collaborating on changes to Linux kernel as they relate to database and application performance. For that reason, Oracle Database 10g for Linux includes enhancements that are closely related to the operating system. Now more than ever, DBAs need to have knowledge of and experience on this platform to best administer the systems under their watch.

There is a traditional division of responsibilities between sysadmins and DBAs. However, in practice, the distinction is not always clear. Many IT shops employ individuals who address concerns at the database as well as the operating system levels. And of course the Oracle Database itself uses operating system resources and is designed to interact closely with its environment.

Furthermore, many sysadmins and DBAs find it necessary or convenient to automate tasks related to their work. The installation of software, monitoring of system resources, and management of systems involve repetitive and error-prone tasks are better addressed through automated processes than manual procedures.

One method that is used to automate such tasks is shell scripting. Shell scripts play a significant role in the Linux System from the time it is installed. Various scripts are called when the system is started up and shut down. Utilities by Oracle and other third-party vendors are invoked through shell scripts. Because they can be developed quickly, they have historically been used for prototyping applications. System Administrators have taken advantage of the functionality available through shell scripting to provide solutions that are tailored for the particular requirements and idiosyncrasies of the systems under their watch.

In this article, I will introduce functionality available through "bash" shell scripting that is relevant to installing, running, and maintaining Oracle databases on a Linux platform. Note that this article is designed for Linux scripting beginners or DBAs who are relatively new to Linux; most experienced Linux sysadmins will not find it helpful.

What Is a Shell Script?
A shell script is simply a text file containing a sequence of commands. When you run the file—or script—it executes the commands contained in the file. The term shell simply refers to the particular command-line user interface you use to communicate with the Linux kernel. Several different shells are available, including the C shell (csh), Korn shell (ksh), Bourne shell (sh), and Bourne-Again shell (bash). The shell itself is a command that reads lines from either a file or the terminal, interprets them, and generally executes other commands. The Bourne-Again shell incorporates features of the other shells mentioned and is the one that was used for the scripts this article demonstrates.

The first line in the script file can be used to dictate which shell will be used to run the script. This is the meaning of the first line found in all of the script examples:


Why Use Shell Scripts?
Depending on your background, you may not see any immediate value to shell scripting as it relates to the DBA’s work. If you do not have experience with UNIX or UNIX-like systems, the myriad of cryptic commands might be cause for concern. Besides, in addition to being a relational database, Oracle 10g provides a robust platform for processing data within the database as well as several methods of interacting with the OS outside of the database.

However, there are several reasons you might find yourself delving into the world of shell scripting, including the following:
* You are in a situation in which you must support already existing scripts.
* You want to automate system setup that occurs prior to the installation of the Oracle software. For instance, you could write a script to check the initial state of the OS and report any prerequisites that must be met before installation of the software. The script might also create relevant OS users and groups and set environmental variables for the users.
* A running Oracle database can be used to execute manual or scheduled tasks. However, some tasks need to be run when the database is not running. You can use a script to stop or start a database (as well as a listener or a related database process). Such an action cannot be initiated from within the database itself.
* You need a mechanism for monitoring the state of a database (i.e. if it is running and available to process queries). Such a script could also monitor other processes and resources that are not Oracle-specific to provide a fuller picture of what is occurring in the system.
* You need to automate backups. Oracle Recovery Manager (RMAN) is a utility that allows you to develop backup scripts that can run on any platform. You can call Oracle Recovery Manager from a shell script and use it to perform a wide range of backup and recovery activities.
* You might have a requirement that is not specific to a single database. Perhaps you have several databases on a single machine. It may not be advisable to fulfill the requirement by using a single database, and it introduces potential security issues as well. Under such circumstances, shell scripting provides a means for fulfilling the requirement in a manner that does not associate the process with a single database.

When Not to Use Shell Scripts
Oracle Database includes functionality that extends beyond the traditional definition of an RDBMS. Like any other piece of software, it uses resources that are provided by the operating system, but it can “see” and “change” its environment to a much greater degree than other software. SQL and Oracle’s fixed views provide a picture of the system from inside the database, whereas shell scripting provides a view of the system from outside of the database. Shell scripting is not the solution for every problem.

It important to recognize that many aspects of the operating system can be monitored and modified from within the database. Oracle’s fixed views (the views with a v$ prefix) can be used to determine the host name of the machine (v$instance) or the platform name on which the database is running (v$database). The location and other attributes of the files associated with the database can be determined in this manner as well. The location and other attributes of datafiles (v$datafile, dba_data_files), temp files (v$tempfile, dba_temp_files), redo logs (v$logfile), archive logs (v$archived_log), and control files (v$controlfile) can be queried directly from the database. You can determine information about the flash recovery area ($recovery_file_dest) from this view, as well as by looking at some init.ora parameters (db_recovery_file_dest, db_recovery_file_dest_size). The status of processes (v$process) and memory (v$sga, v$sgastat, and so on) can be queried as well. There are various built-in PL/SQL packages as well as the ability to create Java and C database objects that allow for additional access to the underlying OS.

If you are considering scripting for a task that requires a good deal of database access, scripting is probably not your best option. Later in this article, there is a description of how to access the database by using SQL*Plus but in many cases, you are better off approaching the problem by using another language.

The charts below summarize information accessible from within the database itself:

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