Errors and Signals and Traps (Oh My!) - Part 1

In this lesson, we're going to look at handling errors during script execution.

The difference between a poor program and a good one is often measured in terms of the program's robustness. That is, the program's ability to handle situations in which something goes wrong.

Exit Status

As we recall from previous lessons, every well-written program returns an exit status when it finishes. If a program finishes successfully, the exit status will be zero. If the exit status is anything other than zero, then the program failed in some way.

It is very important to check the exit status of programs we call in our scripts. It is also important that our scripts return a meaningful exit status when they finish. There was once a Unix system administrator who wrote a script for a production system containing the following 2 lines of code:

Why is this such a bad way of doing it? It's not, if nothing goes wrong. The two lines change the working directory to the name contained in $some_directory and delete the files in that directory. That's the intended behavior. But what happens if the directory named in $some_directory doesn't exist? In that case, the cd command will fail and the script executes the rm command on the current working directory. Not the intended behavior!

By the way, the hapless system administrator's script suffered this very failure and it destroyed a large portion of an important production system. Don't let this happen to you!

The problem with the script was that it did not check the exit status of the cd command before proceeding with the rm command.

Checking the Exit Status

There are several ways we can get and respond to the exit status of a program. First, we can examine the contents of the $? environment variable. $? will contain the exit status of the last command executed. We can see this work with the following:

The true and false commands are programs that do nothing except return an exit status of zero and one, respectively. Using them, we can see how the $? environment variable contains the exit status of the previous program.

So to check the exit status, we could write the script this way:

In this version, we examine the exit status of the cd command and if it's not zero, we print an error message on standard error and terminate the script with an exit status of 1.

While this is a working solution to the problem, there are more clever methods that will save us some typing. The next approach we can try is to use the if statement directly, since it evaluates the exit status of commands it is given.

Using if, we could write it this way:

Here we check to see if the cd command is successful. Only then does rm get executed; otherwise an error message is output and the program exits with a code of 1, indicating that an error has occurred.

Notice too how we changed the target of the rm command from "*" to "./*". This is a safety precaution. The reason is a little subtle and has to do with the lax way Unix-like systems name files. Since it is possible to include almost any character in a file name, we must card against file names that begin with hyphens as thy might be interpreted as command options after the wildcard is expanded. For example, if there was a file named -rf in the directory, it might cause rm to do unpleasant things. It's a good idea to always include "./" ahead of leading asterisks in scripts.

An Error Exit Function

Since we will be checking for errors often in our programs, it makes sense to write a function that will display error messages. This will save more typing and promote laziness.

AND and OR Lists

Finally, we can further simplify our script by using the AND and OR control operators. To explain how they work, here is a quote from the bash man page:

"The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form

command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form

command2 is executed if, and only if, command1 returns a non-zero exit status. The exit status of AND and OR lists is the exit status of the last command executed in the list."

Again, we can use the true and false commands to see this work:

Using this technique, we can write an even simpler version:

If an exit is not required in case of error, then we can even do this:

We need to point out that even with the defense against errors we have introduced in our example for the use of cd, this code is still vulnerable to a common programming error, namely, what happens if the name of the variable containing the name of the directory is misspelled? In that case, the shell will interpret the variable as empty and the cd succeed, but it will change directories to the user's home directory, so beware!

Improving the Error Exit Function

There are a number of improvements that we can make to the error_exit function. It is useful to include the name of the program in the error message to make clear where the error is coming from. This becomes more important as our programs get more complex and we start having scripts launching other scripts, etc. Also, note the inclusion of the LINENO environment variable which will help identify the exact line within a script where the error occurred.

The use of the curly braces within the error_exit function is an example of parameter expansion. We can surround a variable name with curly braces (as with ${PROGNAME}) if we need to be sure it is separated from surrounding text. Some people just put them around every variable out of habit. That usage is simply a style thing. The second use, ${1:-"Unknown Error"} means that if parameter 1 ($1) is undefined, substitute the string "Unknown Error" in its place. Using parameter expansion, it is possible to perform a number of useful string manipulations. More information about parameter expansion can be found in the bash man page under the topic "EXPANSIONS".