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Using Advanced Administration Techniques

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Title: Using Advanced Administration Techniques


1
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
Valued Gateway Client
  • 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

4
Interpreting and Compiling Programs
Valued Gateway Client
  • 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

5
Understanding Programming Concepts
Valued Gateway Client
  • 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

6
A Selection Statement
7
A Loop Repeats a Block of Statements
8
Components of a Shell Script
Valued Gateway Client
  • 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

9
Creating a Simple Shell Script
Valued Gateway Client
  • 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

10
Using Variables in Scripts
Valued Gateway Client
  • 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

11
Using if/then/else Tests
Valued Gateway Client
  • 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

12
Structure of an if/then Statement
13
File-Testing Operators in the bash Shell
14
Structure of an if/then/else Statement
15
Adding Loops to a Script
Valued Gateway Client
  • 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

16
Other Scripting Methods
Valued Gateway Client
  • 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

17
Popular Scripting Languages
18
perl Scripts
Valued Gateway Client
  • 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

19
Scripts for Graphical Programs
Valued Gateway Client
  • 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

20
A program Written as a Python Script
21
Automating Tasks with at and crontab
Valued Gateway Client
  • 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

22
Automating One-Time Tasks
Valued Gateway Client
  • You can perform a task automatically at some
    future time
  • You can use the at command to schedule the task
    for future execution

23
Using the at Command
Valued Gateway Client
  • 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

24
Example Time Specifications Using the at Command
25
Using at Interactively
Valued Gateway Client
  • 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

26
Using the batch Command
Valued Gateway Client
  • 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

27
Automating Recurring Tasks
Valued Gateway Client
  • 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

28
Managing Automated Tasks
Valued Gateway Client
  • 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

29
Checking the Status of Jobs
Valued Gateway Client
  • 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

30
Controlling Access to at and crontab
Valued Gateway Client
  • 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

31
Modifying the Linux Kernel
Valued Gateway Client
  • 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

32
Installing Kernel Source Code
Valued Gateway Client
  • 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

33
Configuring Kernel Features
Valued Gateway Client
  • 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

34
The config Utility for Setting kernel Options
35
The menuconfig Utility for Setting kernel Options
36
The xconfig Utility for Setting kernel Options
37
Compiling a New Kernel
Valued Gateway Client
  • 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

38
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
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