Argument passing strategy - environment variables vs. command line

I think this question has been answered rather well already, but I feel like it deserves a 2018 update. I feel like an unmentioned benefit of environmental variables is that they generally require less boiler plate code to work with. This makes for cleaner more readable code. However a major disadvatnage is that they remove a layers of isolation from different applications running on the same machine. I think this is where Docker really shines. My favorite design pattern is to exclusively use environment variables and run the application inside of a Docker container. This removes the isolation issue.

You should abstract reading parameters using Strategy pattern. Create an abstraction named ConfigurationSource having readConfig(key) -> value method (or returning some Configuration object/structure) with following implementations:

• CommandLineConfigurationSource
• EnvironmentVariableConfigurationSource
• WindowsFileConfigurationSource - loading from a configuration file from C:/Document and settings...
• WindowsRegistryConfigurationSource
• NetworkConfigrationSource
• UnixFileConfigurationSource - - loading from a configuration file from /home/user/...
• DefaultConfigurationSource - defaults
• ...

You can also use Chain of responsibility pattern to chain sources in various configurations like: if command line argument is not supplied, try environment variable and if everything else fails, return defauls.

Ad 1. This approach not only allows you to abstract reading configuration, but you can easily change the underlying mechanism without any affect on client code. Also you can use several sources at once, falling back or gathering configuration from different sources.

Ad 2. Just choose whichever implementation is suitable. Of course some configuration entries won't fit for instance into command line arguments.

Ad 3. If some implementations aren't portable, have two, one silently ignored/skipped when not suitable for a given system.

1) I would recommend avoiding environmental variables as much as possible.

Pros of environmental variables

• easy to use because they're visible from anywhere. If lots of independent programs need a piece of information, this approach is a whole lot more convenient.

Cons of environmental variables

• hard to use correctly because they're visible (delete-able, set-able) from anywhere. If I install a new program that relies on environmental variables, are they going to stomp on my existing ones? Did I inadvertently screw up my environmental variables when I was monkeying around yesterday?

My opinion

• use command-line arguments for those arguments which are most likely to be different for each individual invocation of the program (i.e. n for a program which calculates n!)
• use config files for arguments which a user might reasonably want to change, but not very often (i.e. display size when the window pops up)
• use environmental variables sparingly -- preferably only for arguments which are expected not to change (i.e. the location of the Python interpreter)
• your point They are global and accessible from anywhere, which is less elegant from architectural point of view, but limits the amount of code reminds me of justifications for the use of global variables ;)

My scars from experiencing first-hand the horrors of environmental variable overuse

• two programs we need at work, which can't run on the same computer at the same time due to environmental clashes
• multiple versions of programs with the same name but different bugs -- brought an entire workshop to its knees for hours because the location of the program was pulled from the environment, and was (silently, subtly) wrong.

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2) Limits

If I were pushing the limits of either what the command line can hold, or what the environment can handle, I would refactor immediately.

I've used JSON in the past for a command-line application which needed a lot of parameters. It was very convenient to be able to use dictionaries and lists, along with strings and numbers. The application only took a couple of command line args, one of which was the location of the JSON file.

Advantages of this approach

• didn't have to write a lot of (painful) code to interact with a CLI library -- it can be a pain to get many of the common libraries to enforce complicated constraints (by 'complicated' I mean more complex than checking for a specific key or alternation between a set of keys)
• don't have to worry about the CLI libraries requirements for order of arguments -- just use a JSON object!
• easy to represent complicated data (answering What won't fit into command line parameters?) such as lists
• easy to use the data from other applications -- both to create and to parse programmatically
• easy to accommodate future extensions

Note: I want to distinguish this from the .config-file approach -- this is not for storing user configuration. Maybe I should call this the 'command-line parameter-file' approach, because I use it for a program that needs lots of values that don't fit well on the command line.

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3) Solution portability: I don't know a whole lot about the differences between Mac, PC, and Linux with regard to environmental variables and command line arguments, but I can tell you:

• all three have support for environmental variables
• they all support command line arguments

Yes, I know -- it wasn't very helpful. I'm sorry. But the key point is that you can expect a reasonable solution to be portable, although you would definitely want to verify this for your programs (for example, are command line args case sensitive on any platforms? on all platforms? I don't know).

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One last point:

As Tomasz mentioned, it shouldn't matter to most of the application where the parameters came from.

I generally agree with previous answers, but there is another important aspect: usability.

For example, in git you can create a repository with the .git directory outside of that. To specify that, you can use a command line argument --git-dir or an environmental variable GIT_DIR.

Of course, if you change the current directory to another repository or inherit environmental variables in scripts, you get a mistake. But if you need to type several git commands in a detached repository in one terminal session, this is extremely handy: you don't need to repeat the git-dir argument.

Another example is GIT_AUTHOR_NAME. It seems that it even doesn't have a command line partner (however, git commit has an --author argument). GIT_AUTHOR_NAME overrides the user.name and author.name configuration settings.

In general, usage of command line or environmental arguments is equally simple on UNIX: one can use a command line argument

$ command --arg=myarg

or an environmental variable in one line:

$ ARG=myarg command

It is also easy to capture command line arguments in an alias:

alias cfg='git --git-dir=$HOME/.cfg/ --work-tree=$HOME'  # for dotfiles
alias grep='grep --color=auto'

In general most arguments are passed through the command line. I agree with the previous answers that this is more functional and direct, and that environmental variables in scripts are like global variables in programs.

GNU libc says this:

The argv mechanism is typically used to pass command-line arguments specific to the particular program being invoked. The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently used.

Apart from what was said about dangers of environmental variables, there are good use cases of them. GNU make has a very flexible handling of environmental variables (and thus is very integrated with shell):

Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value. However, an explicit assignment in the makefile, or with a command argument, overrides the environment. (-- and there is an option to change this behaviour) ...

Thus, by setting the variable CFLAGS in your environment, you can cause all C compilations in most makefiles to use the compiler switches you prefer. This is safe for variables with standard or conventional meanings because you know that no makefile will use them for other things.

Finally, I would stress that the most important for a program is not programmer, but user experience. Maybe you included that into the design aspect, but internal and external design are pretty different entities.

And a few words about programming aspects. You didn't write what language you use, but let's imagine your tools allow you the best possible argument parsing. In Python I use argparse, which is very flexible and rich. To get the parsed arguments, one can use a command like

args = parser.parse_args()

args can be further split into parsed arguments (say args.my_option), but I can also pass them as a whole to my function. This solution is absolutely not "hard to maintain for a large number of arguments" (if your language allows that). Indeed, if you have many parameters and they are not used during argument parsing, pass them in a container to their final destination and avoid code duplication (which leads to inflexibility).

And the very final comment is that it's much easier to parse environmental variables than command line arguments. An environmental variable is simply a pair, VARIABLE=value. Command line arguments can be much more complicated: they can be positional or keyword arguments, or subcommands (like git push). They can capture zero or several values (recall the command echo and flags like -vvv). See argparse for more examples.

And one more thing. Your worrying about memory is a bit disturbing. Don't write overgeneral programs. A library should be flexible, but a good program is useful without any arguments. If you need to pass a lot, this is probably data, not arguments. How to read data into a program is a much more general question with no single solution for all cases.