bash Idioms

Chapter 8. Arguing

Some scripts are meant to do a single task; they need no variations. Many others, though, take arguments: one or more filenames, or options to provide variations on their behavior. Once you have more than a single option (or maybe two), you need to parse those arguments in an orderly fashion to be sure that you’ve covered all the possible ways that a user of that script might order those arguments. And come to think of it, even that single task script probably wants -h (or even --help). Let’s take a look at how to parse those arguments and still have a readable, maintainable script.

Your First Argument

If your script just wants a single parameter, you can reference that in your script as $1. You might have statements like echo $1 or cat $1 as part of your script. We don’t recommend using $1 throughout your script as it doesn’t tell the reader anything about this parameter. It’s better, for readability’s sake, if you assign this parameter to a variable with an informative name. If the parameter is meant to be a filename, then choose a variable name like in_file or pathname or similar and assign it right away, early in the script. As we saw in “Default Values”, we can even supply a default value:

filename=${1:-favorite.txt}    # Or maybe use /dev/null as the default?

If the user doesn’t supply any parameter when invoking your script, $1 will be unset. In the preceding example, the shell will assign favorite.txt as the value when parameter one is unset.

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