// Patrick Louis

Making the best CLI

THE BEST THE BEST THE BEST (Transcript of the podcast)

Unix is known for its set of small command line utilities. What are your ideas and features for the best CLI programs. What makes a complete utility.

The extravagance is only for the title, there’s no best way.
There’s not one way, there are multiple ways and some are more used than others some less used, every one of them has its advantages, some are more frequently used, more common let’s say.

What Are We Talking About

Let’s remove the fuzziness around the title, and some previous recording.

I missed out a lot of things in my earlier version of this podcast, and there also might have been some confusion - Partially because it was one of the first episode and I was getting used to presenting subjects in the form of podcasts.

What’s a CLI, and what exactly are we going to talk about today? There are many ways of interfacing with a machine the most popular ones are the text interface and the graphical interface. Other types of interaction exist such as an audio interface (like OK Google), tactile, etc.. There’s probably one for every of the five senses.

We’re discussing the command line interface which is a subset of text interfaces. Within this subset there is yet another subset layer containing different types of command line ui.

Text UI

A Text UI or text mode is an interface which only outputs text - not surprising. Text UI are probably the oldest and simplest type of UI that is still alive today. It was first used on teletype printers which are sort of like typewriters but connected to computers. The first versions of Unix had their outputs on those teletype printers, and thus the text ui and more precisely the command line utilities were favored.

Unix is now well known for its set of small command line utilities, they’re inherent to unix. What are your ideas and features for the best CLI programs. What makes a complete utility. What are the intricacies that make the best command line interface. What does “best” mean?
That’s what we’re gonna see, the multiple definitions of best by considering the multiple facets a command line interface should have.

The Prevalence of CLI

Why is it so prevalent, let’s review some history. Let’s visit the reasons why we use CLI and from those premises maybe we can deduce some things that might lead us in the direction of the “best” CLI.

As we’ve said the command-line interface evolved from a need to communicate with a machine remotely via a teleprinter. You issue commands, exchange info, and the machine prints back the associated computed answer. Because of the nature of paper, everything had to be printed as text line by line.

Soon enough, the paper was replaced by glass tty and peanuts peanuts later we have what we have today. CLI still remains a powerful way of communicating with a machine. Why’s that?

The most important is that it goes along with the core of unix minimalism, it works in the most minimal environment. It’s also a transparent view on the operating system because most system calls have symmetrical command line utility associated with it. You enter those commands in a shell or “command line interpreter”.

The unix shell is known for it’s ability to combine, compose, and automate multiple commands together in a script-like fashion. It has its advantages and that’s why it prevails.

Unix is an expert-friendly system where you can easily configure everything for your precise needs, and that’s the attraction.

The Unix Philosophy, KISS

The Unix philosophy percolates into the way of interfacing. Or maybe it’s just that both the way of interfacing and the unix philosophy move along in an arms race influencing each others.

A good command line utility may want to follow the direction of the Unix philosophy. Bluntly put, small programs that are easy to combine with other programs. One that does exactly what it says with no extra whistles and bells.


A user needs to tell the program he’s interfacing with exactly what he needs. One way to do that is through configuration. But what should be configurable and how should the user configure it?

Unix users crave flexibility and power but at the same time want to keep programs simple and minimal. There’s the dilemma between too much choose and configuration and not enough, what should we keep and what should we leave out.

Before dealing with the ways you can get the configurations from let’s first lay down some simple principles that can help choose whether a configuration option is needed or not.
First, don’t make it mandatory to provide configuration for things you could deduce without the user’s interaction. That goes as well for environment and for reasonable default values. Auto-detection is a great way to reduce the overhead and have a clean command line interface.
Second, don’t add an option or feature or configuration that could be done by using another command line tool, that complexity should be deferred to the other specialized program.

Ask yourself the following questions:

  • Can I leave this feature out?
  • Why am I fattening the manual and burdening the user?
  • Could the program’s normal behavior be changed in an innocuous way that would make the option unnecessary?
  • Is this option merely cosmetic? Should I be thinking less about how to make the user interface configurable and more about how to make it right?
  • Should the behavior enabled by this option be a separate program instead?

Those are the kind of things you need to have in mind when choosing the options your command line utility will have. But remember, it’s through a series of iterations and usage that you’ll find what is superfluous and what is not. However, once an option is added it’s harder to remove it, thus start with as few as possible.

Types of Interaction ( input, output, sink, pipe, etc.. )

To understand how the user will provide the orders to a programs we need to go over the ways a command line program can run. There are different patterns or “type” of CLI.

One way of looking at it might be by looking at the temporality, long running programs vs one-shot programs.
In the case of the long running program it may be better to have a configuration file so that the cli can parse them during the run. Also for long running program you may want to have a way to recover in case of failure and even have a flag of verbosity so that you can tell the user that things are currently happening.
On the other hand one-shot programs can have command line options. But that also depends on the frequency of usage, because it maybe be tedious to always type the same arguments for a one shot program, especially if there are many. You may want to provide a way to wrap them, or let the user wrap them himself in a script.

The generic design of a CLI goes as follow:

  • Gather input from somewhere
  • Do something with the data
  • Display result somewhere or not

Depending on where it gathers inputs, what kind of changes it makes, and where it displays results, the name of the command line pattern differs and also the way of using it while jamming it inside a script.
For example: The Sink Pattern takes an input and doesn’t return output, which turns it into a dark hole when put in the middle of a series of commands separated by pipes.

Different patterns:

  • The Filter Pattern
  • The Cantrip Pattern
  • The Source Pattern
  • The Sink Pattern
  • The Compiler Pattern
  • The ed pattern
  • The Roguelike Pattern
  • The ‘Separated Engine and Interface’ Pattern
  • The CLI Server Pattern
  • Language-Based Interface Patterns

The most important part of this is that everything that is input (stdin) and output (stdout and stderr when there are errors) should be easy parseable text and not necessarily readable output. This goes along with the “composability” of unix programs. Unix has that great inter-process communication.
It can be argued that 3 standard file descriptors are not enough but in most cases they’re enough.

Also, on the topic of format, just like extra configurations, once you’ve chosen an input or output format it’s hard to change later on. In sum, the best CLI is composable, whichever pattern it chooses to implement.

Command Line Options

Argument parsing, input parsing, configuration file parsing. Do not reinvent any of those, don’t surprise your users with your brand new self-righteous wisdom, they don’t need it.

Let’s take the example of argument parsing: There were no standard for command line argument parsing until the 1985 UNIFORUM conference. AT&T released the getopt argument parsing library which then became a standard in POSIX. Now it’s a standard and if softwares choose not to implement it they confuse users. Which doesn’t make a good CLI.
Other than getopt, there are many other libraries that adhere to the standard and make it simple to parse arguments.

Another thing that is frequently mentioned is the number of command line arguments. Just like we’ve said before in an earlier section, you shouldn’t add configurations where no configuration is necessary. If you feel like you’re adding a flag for a feature that could be a program on its own then split it on its own. You should limit the number of flags.

Namely, there are two standard ways of passing arguments to a program on the command line: one-letter flags and long arguments. The original Unix tradition uses the single letter while the GNU project added the long ones. You should support both and adhere to the standard. Provide long, readable option names with short aliases.

Least Surprise (Flags, Name, etc..)

Expectation is everything. Don’t break the expectations of the users, always refer to the least surprise rule. When in doubt, use the most common option.

For example, regarding command line options, they should be intuitive. Everyone expects a -V for version or verbose, a -h for help, etc… Those two are the most prevalent but other characters are too, -f for file, -a for all, -o for output, those are obvious.

Same goes for program names, use mnemonic, names that are obvious and relate to what your program does. You should attempt to make the command name easy to remember.

This whole topic of least surprise has been repeated over and over in the other sections but implicitly. It’s probably one of the most important part to make a great CLI.


Another thing that users expect on Unix is documentation that comes along with a software. Someone might issue a -h flag and expect back a small help message. Or sometimes just running a program without any flag.

We also expect a man page that lists everything related to the software. From what the software does, to the ENV it uses, to error codes, to bugs, etc.. Don’t forget to mention if you log errors and where. If you store configuration files, if you have a verbose flag, a human readable flag to add colors to the output, etc..

We expect a README and INSTALL files coming with the software’s source.

Those are all ways to tell your users how to correctly use your software. You could also go as far as creating a webpage, IRC channel, and newsletter. When a command line program is alive is has a community around it.

The key point is that the user should be notified of everything, you should be transparent. This is truly important because CLI are the main way of interfacing with a Unix machine and without documentation the tool is useless.

Argument of Completeness

There are other non-mandatory features that could, maybe, make your CLI better. However, for some they are considered bloat. One thing is related to the documentation. You could add, along with your CLI, during installation, a tab completion for the shell. zsh and fish shell offer those.
Instead of memorizing all the command line options your user can just cycle through them and read what they do. However, that may mean that your CLI options are not straightforward and you may want to revisit your design.

One type of CLI we forgot to mention are the ones included inside other softwares, such as GDB, the gnu debugger. Like the shell, they are interactive prompt, interpreters. They are a step above the ones mentioned above, it lives in it’s own read-interpret-print-loop.

A step even above that, a type of interface that is also based on text, is the curses interface or roguelike ui. It’s a fancy kind of text ui in the console. Those are not fast applications, they can’t be jammed on the command line either. And thus you have to use them only in specific scenarios.

A good case would be an installer, you use this type of UI to ask all the configurations upfront and then, when over, execute whatever needs to be executed. It’s also nice to have multiple kinds of UI for the same program, if they interface the same library, make them work in a hybrid way.

Let’s mention the last category, scripting languages or mini-language. Awk, perl, ruby, sed, etc..

Those are mini-programming-languages that you can use alongside your command line programs. Even the shell is a programming language by itself.

Although most users think of the shell as an interactive command interpreter, it is really a programming language in which each statement runs a command. Because it must satisfy both the interactive and programming aspects of command execution, it is a strange language, shaped as much by history as by design.

— Brian Kernighan & Rob Pike

All in all programs with command-line interfaces are generally easier to automate via scripting.


Let’s conclude on a controversial note. The main things we’ve mentioned in this podcast to make the “best” CLI are related to not going against the tide and not doing the unexpected. This may seem like an argument against innovation and I can understand that point of view. If we simply follow the same old mantra and keep repeating it, it seems like we’re not gonna invent anything.

Well, take the mantra and shape it gently. Don’t underestimate many many years of iterative testing.

This is it for the best CLI, mixed a bit of everything

Show notes/References:

Music: https://dexterbritain.bandcamp.com/album/creative-commons-volume-1

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