Updating the phone book application

The functionality that is added to this new version of the phone book utility is the following:

• The CSV file path can be optionally given as an environment variable named PHONEBOOK
• The list command sorts the output based on the surname field

Although we could have given the path of the CSV file as a command-line argument instead of the value of an environment variable, it would have complicated the code, especially if that argument was made optional. More advanced Go packages such as viper, which is presented in Chapter 6, Telling a UNIX System What to Do, simplify the process of parsing command-line arguments with the use of command-line options such as -f followed by a file path or --filepath.

The current default value of CSVFILE is set to my home directory on a macOS Big Sur machine—you should change that default value to fit your needs or use a proper value for the PHONEBOOK environment variable.

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