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Logical Errors

Unlike syntactic errors, logical errors do not prevent a script from running. The script will run, but it will not produce the desired result, due to a problem with its logic. There are countless numbers of possible logical errors, but here are a few of the most common kinds found in scripts:

1. Incorrect conditional expressions. It’s easy to incorrectly code an if/then/else and have the wrong logic carried out. Sometimes the logic will be reversed, or it will be incomplete.

2. “Off by one” errors. When coding loops that employ counters, it is possible to overlook that the loop may require that the counting start with zero, rather than one, for the count to conclude at the correct point. These kinds of errors result in either a loop “going off the end” by counting too far, or else missing the last itera- tion of the loop by terminating one iteration too soon.

3. Unanticipated situations. Most logic errors result from a program encountering data or situations that were unforeseen by the programmer. As we have seen, this can also include unanticipated expansions, such as a filename that contains em- bedded spaces that expands into multiple command arguments rather than a single filename.


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