Basic Use of Command-Line Arguments
14 Feb 2016
What are Command-Line Arguments?
Command-line arguments are values that you pass to a program from the command line. They are used in the same way that arguments passed to any function within a program are used. In the C, C++, and Java programming languages specifically, command-line arguments are passed into the program’s main function if it is defined with the appropriate parameters. Once the arguments are passed, they can be found within the indices of an array that is usually called argv by convention. In the case of C and C++, argv [0] holds the program’s name and the following indices hold the parsed arguments, however, in Java the program’s name is not included in argv.
The arguments are usually passed in by listing them after the name of the program, for example, make hw3. The argument passed to the make program is “hw3”, which in this specific case tells the make program to check that all the conditions necessary to create the hw3 file exist and, if they do, to then execute all the instructions necessary to create said file.
So, What are They Good For?
Generally, command-line arguments are useful if the chances are good that a user will already know the values that they would like to pass at the time that the program is invoked. For instance, consider a case in which you might be writing a user interface for a bank database. After the program has been invoked, you could print a list of transaction options and prompt the user to enter their choice of action. However, you could also allow users to enter command-line arguments specifying their choice at the time that the program is invoked, a feature that more familiar, repeat users of the program might like to make use of in order to save time.
The ability to enter command-line arguments often gives the user a little more flexibility in terms of what they want to do with the program. These arguments can allow you to set initial values within a program that differ from the default settings, enable debugging features, or specify certain external files to access. Allowing command-line arguments to be passed can also make it easier for your program to be invoked by other programs. This could be useful in the situation where your program is part of a larger system of programs that work together cohesively, or in the case that your program acts as an extension for an application.
Are They Necessary?
It depends on how the program is designed. Command-line arguments can be used to provide optional functionality, or they can be required for the program to run as intended. I recently made use of two programs for an assignment in ICS 321, Data Storage and Retrieval, one in which command-line arguments were optional, and the other in which they were required.
The first program was called AutoRun, and by default (without command-line arguments) it performed a sequence of hard-coded queries to find matching records in a text file that acted as a database (using comma-separated values). The name of a text file passed as a single optional command-line argument allowed a user to enter a custom sequence of queries to perform, as long as they were specified in the appropriate format within the text file.
The second program was called FileGenerator, and it was designed to generate text file databases of the kind previously mentioned. If it was run without any arguments it would print a message detailing the proper usage of the program, which was to enter the program name followed by command-line arguments specifying the size of file to generate and the name that this file should be given. Upon entering the appropriate arguments the program would generate a text file with the specified name and size.
As you can see, command-line arguments can provide a number of possible benefits. In the end, if you decide to incoporate them into your program, their usefulness is only limited by your imagination.