Guide to Linux Installation and Administration, 2e1 Chapter 11 Using Advanced Administration...

Guide to Linux Installation and Administration, 2e 1

Chapter 11

Using Advanced Administration

Techniques

Objectives

In this chapter, you will:

• Create shell scripts using basic shell programming features

• Automate one-time and repetitive tasks using at and crontab

• Reconfigure the Linux kernel

Writing Shell Scripts

• You can automate the process of entering frequently used commands by creating a shell script

• A shell script is an executable file containing lines of text as you would enter them at a command line, including special commands to control the order in which lines in the file are executed

Valued Gateway Client:

Interpreting and Compiling Programs

• A compiler converts the source code of a compiled language into a binary file

• An interpreter converts the source code of an interpreted language into numeric codes

• A shell script is an interpreted program

Understanding Programming Concepts

• A selection statement lets a computer programmer determine which parts of a program will be executed according to values that are determined as the program is executed

• A loop statement is used to determine whether part of a computer program should be executed more than once

• The list of statements controlled by a selection or loop statement is often called a block, or a statement block

A Selection Statement

A Loop Repeats a Block of Statements

Components of a Shell Script

• Each script file must start with a line identifying the interpreter to use for that script

• Each script must have the execute permission set• Each script must use only correctly formed

statements for the interpreter

Creating a Simple Shell Script

• Some Linux commands are used mostly within shell scripts

• The read command causes the shell to pause for a user to enter information at the keyboard

• You can create your own variables to store values within a script

• Shell scripts use the Linux redirection operators to change the flow of information to and from commands

Using Variables in Scripts

• A variable used in a shell script in this way is sometimes called a shell variable

• To initialize the variable, assign it a value• A positional variable takes a value based on the

information that the user includes on the command line

• You indicate positional variables using a dollar sign and a number

• Positional variables are a useful way to provide information to the commands in a script

Using if/then/else Tests

• A test is a type of selection statement that lets you determine if a condition is true or false

• An if command must be followed by a then command, which lists the commands to be executed if the test succeeds

• The fi command marks the end of the if statement• The test command evaluates parameters you

provide and returns either true or false• The else command adds a block of commands

that are only executed if a test returns a value of false

Structure of an if/then Statement

File-Testing Operators in the bash Shell

Structure of an if/then/else Statement

Adding Loops to a Script

• A loop statement causes a block of commands to be repeated a certain number of times or until a condition is met

• The for command creates a for loop, which repeats a statement block once for each list item

• The do and done commands are keywords used to begin and end a statement block in a for loop

• Loops using for and while repeatedly execute a statement block based on either a fixed number of iterations or a condition being tested

Other Scripting Methods

• The statements in a shell script must follow specific syntax rules

• The specific syntax rules a script must follow depend on the interpreter that will execute the script

• Different scripting languages are used for different purposes

• Scripts often use standardized file extensions to help users identify them

Popular Scripting Languages

perl Scripts

• Perl– A programming language developed by Larry Wall

– Especially well suited to processing text strings

– Was very popular as a tool for managing data submitted by Web page forms

– perl scripts interact with Web servers using a communication standard called the Common Gateway Interface, or CGI

Scripts for Graphical Programs

• Tcl/Tk (tool control language/toolkit) is a scripting language developed by John Ousterhout

• Scripts written in Tcl/Tk are executed by an interpreter called wish

• The Python scripting language uses the same Tk graphical programming toolkit as Tcl

A program Written as a Python Script

Automating Tasks with at and crontab

• The at command lets you define one or more commands to be executed at some future time

• The crontab command lets you define one or more commands that will be executed repeatedly at intervals that you designate

• The at command relies on a daemon called atd• The crontab command relies on a daemon called

crond• A scheduled task is often called a job

Automating One-Time Tasks

• You can perform a task automatically at some future time

• You can use the at command to schedule the task for future execution

Using the at Command

• To automate a task with the at command, you can either enter commands directly at the command line, or you can list them in a file

• The atd daemon will check once per minute for any jobs that have been scheduled using the at command

• The time specification in the at command is very flexible

Example Time Specifications Using the at Command

Using at Interactively

• The only disadvantage of entering commands interactively is that you cannot alter a command after it has been edited

• After entering all the commands that you want the atd daemon to execute, you indicate that you have finished by pressing Ctrl+D

• You can use the tty command to send output from a command to the location where you are currently logged in

Using the batch Command

• The batch command runs your commands when the system load average drops below 0.8

• The batch command is similar to the at command and also relies on the atd daemon

• The batch command accepts tasks for future execution without a fixed time

• Commands are executed when the system is not busy

Automating Recurring Tasks

• You can use the crontab command to execute commands automatically at regular intervals

• Many administrators place scripts in the cron-related directories instead of creating their own cron job definitions

• In Red Hat Linux 7.3, the /etc directory contains subdirectories named cron.hourly, cron.daily, cron.weekly, and cron.monthly

Managing Automated Tasks

• Submitted commands for future execution using at or crontab can be viewed and deleted

• The root user can view or modify jobs submitted by any user on the system

Checking the Status of Jobs

• The two daemons atd and crond are started when you boot Linux using standard service scripts in /etc/rc.d/init.d

• You can use those scripts to stop and restart the daemons

• Jobs submitted using at are stored in the /var/spool/at directory; jobs submitted using crontab are stored in the /var/spool/cron directory

• The atq and atrm commands are used to manage at jobs that are awaiting execution

• You should not directly edit a crontab file in the /var/spool/cron directory

Controlling Access to at and crontab

• You can restrict access to at and crontab so that only certain users can use these commands

• /etc/cron.allow: Contains usernames that are allowed to use the crontab command

• /etc/cron.deny: Contains usernames that are not allowed to use the crontab command

• /etc/at.allow: Contains usernames that are allowed to use the at command

• /etc/at.deny: Contains usernames that are not allowed to use the at command

Modifying the Linux Kernel

• You can modify the Linux kernel to activate or deactivate features, including making features built-in instead of loadable modules

• You can recompile the Linux kernel from source code, creating a new kernel

• The file vmlinuz contains the Linux kernel

Installing Kernel Source Code

• To create a new kernel, make certain the kernel source code and supporting packages are installed on your system

• The kernel source code is available as an rpm package or as raw source code from www.linuxhq.com

Configuring Kernel Features

• Three kernel configuration utilities are supported:– config,

– menuconfig

– xconfig

• Kernel configuration utilities are started using the make command

• The make command is a programming utility that uses instructions in a configuration file (called Makefile) to execute a series of instructions

The config Utility for Setting kernel Options

The menuconfig Utility for Setting kernel Options

The xconfig Utility for Setting kernel Options

Compiling a New Kernel

• After configuration is completed the make command is used to start the actual kernel compilation process

• You also compile and install new modules for the new kernel as a separate operation

• The make bzImage command creates a compressed kernel image called bzImage in the directory /usr/src/linux-2.4/arch/i386/boot

• To use that kernel, you should make a backup copy of your current kernel in /boot, then copy the bzImage file to /boot

Summary• Shell scripts are used to execute a complex set of

commands by entering a single script name• Each script file starts with a line identifying the

interpreter, has the execute permission set, and uses only correctly formed statements for the interpreter

• The if/then/else statement tests a condition and executes statements if the condition is present

• Loops using for and while repeatedly execute a statement block based on either a fixed number of iterations or a condition tested

• Many scripting languages are used on Linux systems, including perl, PHP, Python,and Tcl/Tk

Summary

• The at command schedules a task for execution by atd at some future point

• Tasks scheduled using at can be managed using the atq and atrm commands

• The crontab command schedules tasks that are to be executed on a regular basis, such as daily or weekly

• The kernel source code is available as an rpm package or as raw source code

• Three kernel configuration utilities: config, menuconfig, and xconfig are started using the make command

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