Fedora™ Unleashed, 2008 edition

You need to end the statements under each condition with a double semicolon (;;). If you do not, the statements under the next condition will also be executed.

The last condition should be default and is executed if none of the other conditions is met. For each of the specified conditions, all the associated statements until breaksw are executed.

The break and exit Statements

You should be aware of two other statements: the break statement and the exit statement.

The break statement can be used to terminate an iteration loop, such as a for, until, or repeat command.

exit statements can be used to exit a shell program. You can optionally use a number after exit. If the current shell program has been called by another shell program, the calling program can check for the code (the $? or $status variable, depending on the shell) and make a decision accordingly.

Using Functions in Shell Scripts

As with other programming languages, shell programs also support functions. A function is a piece of a shell program that performs a particular process; you can reuse the same function multiple times within the shell program. Functions help eliminate the need for duplicating code as you write shell programs.

The following is the format of a function in bash:

func() {
 Statements
}

You can call a function as follows:

func param1 param2 param3

The parameters param1, param2, and so on are optional. You can also pass the parameters as a single string — for example, $@. A function can parse the parameters as if they were positional parameters passed to a shell program from the command line as command-line arguments, but instead use values passed inside the script. For example, the following script uses a function named Displaymonth() that displays the name of the month or an error message if you pass a month number out of the range 1 to 12. This example works with bash:

#!/bin/sh
Displaymonth() {
 case $1 in
  01 | 1) echo 'Month is January';;
  02 | 2) echo 'Month is February';;
  03 | 3) echo 'Month is March';;
  04 | 4) echo 'Month is April';;
  05 | 5) echo 'Month is May';;
  06 | 6) echo 'Month is June';;
  07 | 7) echo 'Month is July';;
  08 | 8) echo 'Month is August';;
  09 | 9) echo 'Month is September';;
  10) echo 'Month is October';;
  11) echo 'Month is November';;
  12) echo 'Month is December';; *)
  *) echo 'Invalid parameter';;
 esac
}
Displaymonth 8

The preceding program displays the following output:

Month is August

Reference

> http://www.gnu.org/software/bash/bash.html — The bash home page at the GNU Software Project.

> http://www.tldp.org/LDP/abs/html/ — Mendel Cooper's 'Advanced Bash-Scripting Guide.'

> http://www.linuxnewbie.org/nhf/intel/shells/basic.html — Learn basic shell commands at this site.

> http://www.cs.princeton.edu/~jlk/lj/ — 'The New Korn Shell — ksh93,' an article by David G. Korn, Charles J. Northrup, and Jeffery Korn regarding the korn shell.

> http://lug.umbc.edu/~mabzug1/bash-httpd.html — The Bash-httpd FAQ, with links to the latest version of the bash web server, bash-httpd-0.02.

> http://www.tcsh.org/ — Find out more about tcsh here.

> http://www.zsh.org/ — Examine zsh in more detail here.

CHAPTER 34 Advanced Software Management

Fedora is provided with a large number of packages already supplied and ready to go. However, Fedora also comes with a great package management tool, called yum, which enables you to quickly and easily download and install more packages. The yum tool (Yellowdog Updater, Modified) has been included with Fedora since version 1 and has become the tool of choice for installing and updating applications. This chapter takes a look at how yum works, its basic usage, and more advanced package and repository management, including using some of the GUI tools available with Fedora. But before that, we take a look at RPM, which is the underlying technology that yum utilizes.

Using RPM for Software Management