Using Advanced Administration Techniques

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Chapter 11

• Using Advanced Administration Techniques

2
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

3
Writing Shell Scripts
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• 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

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Interpreting and Compiling Programs
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• 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

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Understanding Programming Concepts
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• 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

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A Selection Statement
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A Loop Repeats a Block of Statements
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Components of a Shell Script
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• 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

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Creating a Simple Shell Script
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• 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

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Using Variables in Scripts
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• 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

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Using if/then/else Tests
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• 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

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Structure of an if/then Statement
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File-Testing Operators in the bash Shell
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Structure of an if/then/else Statement
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Adding Loops to a Script
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• 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

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Other Scripting Methods
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• 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

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Popular Scripting Languages
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perl Scripts
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• 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

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Scripts for Graphical Programs
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• 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

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A program Written as a Python Script
21
Automating Tasks with at and crontab
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• 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

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Automating One-Time Tasks
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• You can perform a task automatically at some
  future time
• You can use the at command to schedule the task
  for future execution

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Using the at Command
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• 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

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Example Time Specifications Using the at Command
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Using at Interactively
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• 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 CtrlD
• You can use the tty command to send output from a
  command to the location where you are currently
  logged in

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Using the batch Command
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• 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

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Automating Recurring Tasks
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• 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

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Managing Automated Tasks
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• 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

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Checking the Status of Jobs
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• 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

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Controlling Access to at and crontab
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• 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

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Modifying the Linux Kernel
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• 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

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Installing Kernel Source Code
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• 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

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Configuring Kernel Features
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• 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

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The config Utility for Setting kernel Options
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The menuconfig Utility for Setting kernel Options
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The xconfig Utility for Setting kernel Options
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Compiling a New Kernel
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• 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

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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

39
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