Flow Control - Part 1

In this lesson we will look at how to add intelligence to your scripts. So far, our script has only consisted of a sequence of commands that starts at the first line and continues line by line until it reaches the end. Most programs do more than this. They make decisions and perform different actions depending on conditions.

The shell provides several commands that we can use to control the flow of execution in our program. These include:

if
exit
for
while
until
case
break
continue

if

The first command we will look at is if. The if command is fairly simple on the surface; it makes a decision based on a condition. The if command has three forms:

# First form

if condition ; then
    commands
fi

# Second form

if condition ; then
    commands
else
    commands
fi

# Third form

if condition ; then
    commands
elif condition
    commands
fi

In the first form, if the condition is true, then commands are performed. If the condition is false, nothing is done.

In the second form, if the condition is true, then the first set of commands is performed. If the condition is false, the second set of commands is performed.

In the third form, if the condition is true, then the first set of commands is performed. If the condition is false, and if the second condition is true, then the second set of commands is performed.

What is a "condition"?

To be honest, it took me a long time to really understand how this worked. To help answer this, there is yet another basic behavior of commands we must discuss.

Exit status

A properly written Unix application will tell the operating system if it was successful or not. It does this by means of an exit status. The exit status is a numeric value in the range of 0 to 255. A "0" indicates success, any other value indicates failure. Exit status provides two important features. First, it can be used to detect and handle errors and second, it can be used to perform true/false tests.

It is easy to see that handling errors would be valuable. For example, in our script we will want to look at what kind of hardware is installed so we can include it in our report. Typically, we will try to query the hardware, and if an error is reported by whatever tool we use to do the query, our script will be able to skip the portion of the script which deals with the missing hardware.

We can also use the exit status to perform simple true/false decisions. We will cover this next.

test

The test command is used most often with the if command to perform true/false decisions. The command is unusual in that it has two different syntactic forms:

# First form

test expression

# Second form

[ expression ]

The test command works simply. If the given expression is true, test exits with a status of zero, otherwise it exits with a status of 1.

The neat feature of test is the variety of expressions you can create. Here is an example:

if [ -f .bash_profile ]; then
    echo "You have a .bash_profile. Things are fine."
else
    echo "Yikes! You have no .bash_profile!"
fi

In this example, we use the expression " -f .bash_profile ". This expression asks, "Is .bash_profile a file?". If the expression is true, then test exits with a zero (indicating true) and the if command executes the command(s) following the word then. If the expression is false, then test exits with a status of one and the if command executes the command(s) following the word else.

Here is a partial list of the conditions that test can evaluate. Since test is a shell builtin, use "help test" to see a complete list.

Expression	Description
-d file	True if file is a directory.
-e file	True if file exists.
-f file	True if file exists and is a regular file.
-L file	True if file is a symbolic link.
-r file	True if file is a file readable by you.
-w file	True if file is a file writable by you.
-x file	True if file is a file executable by you.
file1 -nt file2	True if file1 is newer than (according to modification time) file2
file1 -ot file2	True if file1 is older than file2
-z string	True if string is empty.
-n string	True if string is not empty.
string1 = string2	True if string1 equals string2.
string1 != string2	True if string1 does not equal string2.

Before we go on, I want to explain the rest of the example above since it also reveals more important ideas.

In the first line of the script, we see the if command followed by the test command, followed by a semicolon and finally the word then. I chose to use the [ expression ] form of the test command since most people think it's easier to read. Notice that the spaces between the "[" and the beginning of the expression are required. Likewise, the space between the end of the expression and the trailing "]".

The semicolon is a command separator. Using it allows you to put more than one command on a line. For example:

[me@linuxbox me]$ clear; ls

will clear the screen and execute the ls command.

I use the semicolon as I did to allow me to put the word then on the same line as the if command, because I think it is easier to read that way.

On the second line, there is our old friend echo. The only thing of note on this line is the indentation. Again for the benefit of readability, it is traditional to indent all blocks of conditional code; that is, any code that will only be executed if certain conditions are met. The shell does not require this; it is done to make the code easier to read.

In other words, we could write the following and get the same results:

# Alternate form

if [ -f .bash_profile ]
then
    echo "You have a .bash_profile. Things are fine."
else
    echo "Yikes! You have no .bash_profile!"
fi

# Another alternate form

if [ -f .bash_profile ]
then echo "You have a .bash_profile. Things are fine."
else echo "Yikes! You have no .bash_profile!"
fi

exit

In order to be good script writers, we must set the exit status when our scripts finish. To do this, use the exit command. The exit command causes the script to immediately terminate and set the exit status to whatever value is given as an argument, for example:

exit 0

exits your script and sets the exit status to 0 (success), whereas

exit 1

exits your script and sets the exit status to 1 (failure).

Testing for root

When we last left our script, we required that it be run with superuser privileges. This is because the home_space function needs to examine the size of each user's home directory, and only the superuser is allowed to do that.

But what happens if a regular user runs our script? It produces a lot of ugly error messages. What if we could put something in the script to stop it if a regular user attempts to run it?

The id command can tell us who the current user is. When executed with the "-u" option, it prints the numeric user id of the current user.

[me@linuxbox me]$ id -u
501
[me@linuxbox me]$ su
Password:
[root@linuxbox me]# id -u
0

If the superuser executes id -u, the command will output "0." This fact can be the basis of our test:

if [ $(id -u) = "0" ]; then
    echo "superuser"
fi

In this example, if the output of the command id -u is equal to the string "0", then print the string "superuser."

While this code will detect if the user is the superuser, it does not really solve the problem yet. We want to stop the script if the user is not the superuser, so we will code it like so:

if [ $(id -u) != "0" ]; then
    echo "You must be the superuser to run this script" >&2
    exit 1
fi

With this code, if the output of the id -u command is not equal to "0", then print a descriptive error message, exit the script and set the exit status to 1, indicating to the operating system that the script executed unsuccessfully.

Notice the ">&2" at the end of the echo command. This is another form of I/O direction. You will often notice this in routines that display error messages. If this redirection was not done, the error message would go to standard output. With this redirection, the message is sent to standard error. Since we are executing our script and redirecting its standard output to a file, we want the error messages separated from the normal output.

We could put this routine near the beginning of our script so it has a chance to detect a possible error before things get under way but in order to run this script as an ordinary user, we will use the same idea and modify the home_space function to test for proper privileges instead, like so:

function home_space
{
    # Only the superuser can get this information

    if [ "$(id -u)" = "0" ]; then
        echo "<h2>Home directory space by user</h2>"
        echo "<pre>"
        echo "Bytes Directory"
            du -s /home/* | sort -nr
        echo "</pre>"
    fi

}   # end of home_space

This way, if an ordinary user runs the script, the troublesome code will be passed over, rather than executed and the problem is solved.

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