Flow Control - Part 1

In this lesson, we will look at how to add intelligence to our scripts. So far, our project 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 much 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. In this lesson, we will look at the following:

• if
• test
• exit

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 the exit status of a command. The if command's syntax looks like this:

where commands is a list of commands. This is a little confusing at first glance. But before we can clear this up, we have to look at how the shell evaluates the success or failure of a command.

Exit Status

Commands (including the scripts and shell functions we write) issue a value to the system when they terminate, called an exit status. This value, which is an integer in the range of 0 to 255, indicates the success or failure of the command’s execution. By convention, a value of zero indicates success and any other value indicates failure. The shell provides a parameter that we can use to examine the exit status. Here we see it in action:

In this example, we execute the ls command twice. The first time, the command executes successfully. If we display the value of the parameter $?, we see that it is zero. We execute the ls command a second time, producing an error and examine the parameter $? again. This time it contains a 2, indicating that the command encountered an error. Some commands use different exit status values to provide diagnostics for errors, while many commands simply exit with a value of one when they fail. Man pages often include a section entitled “Exit Status,” describing what codes are used. However, a zero always indicates success.

The shell provides two extremely simple builtin commands that do nothing except terminate with either a zero or one exit status. The true command always executes successfully and the false command always executes unsuccessfully:

We can use these commands to see how the if statement works. What the if statement really does is evaluate the success or failure of commands:

The command echo "It's true." is executed when the command following if executes successfully, and is not executed when the command following if does not execute successfully.

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:

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 we can create. Here is an example:

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.

Before we go on, We need 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. Most people choose to use the [ expression ] form of the test command since it's easier to read. Notice that the spaces required 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 us to put more than one command on a line. For example:

will clear the screen and execute the ls command.

We use the semicolon as we did to allow us to put the word then on the same line as the if command, because it's 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:

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 terminate immediately and set the exit status to whatever value is given as an argument. For example:

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

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.

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

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:

With this code, if the output of the id -u command is not equal to "0", then the script prints a descriptive error message, exits, and sets 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. We will often see this in routines that display error messages. If this redirection were 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:

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

Further Reading

• Chapter 27 in The Linux Command Line covers this topic in more detail including the [[ ]] construct, the modern replacement for the test command, and the (( )) construct for performing arithmetic truth tests.
• To learn more about stylistic conventions and best coding practices for bash scripts, see the Coding Standards adventure.