From Unix Commands to APIs: Designing Software Around Contracts, Not Interfaces

Mon, 07 Sep 2026 00:00:00 +0000

From Unix Commands to APIs: Designing Software Around Contracts, Not Interfaces

There is a recurring problem in software engineering that is rarely discussed explicitly: we often design the program around its user interface.

A command-line application becomes a collection of commands. A web application becomes a collection of HTTP endpoints. A desktop application becomes a collection of screens and event handlers.

Then, sooner or later, someone wants an API, a CLI, a GUI, a Python library, or a browser extension.

The result is often duplication, adapters built on top of other adapters, and business logic scattered across presentation layers.

There is another way.

The approach I have been exploring can be summarized as:

Define the operation and its input/output contract first. Make the interface an adapter.

I would call this Contract-First, Interface-Agnostic Design.

It is not a completely new architecture. It is a practical combination of several established ideas: Unix philosophy, separation of concerns, hexagonal architecture (Ports and Adapters), structured data contracts, and file-based persistence.

But putting them together leads to a particularly useful design style for small Unix-oriented tools.

The Interface Should Not Define the Program

Consider a simple operation:

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get secret

The important question is not whether this operation is exposed through HTTP or a shell command.

The important question is:

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What does "get secret" mean?

What does it receive?

What does it return?

What errors can it produce?

For example:

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Input:
 path = "brisecom/ovh"

Output:
 name
 type
 content
 metadata

Errors:
 NOT_FOUND
 ACCESS_DENIED
 DECRYPTION_FAILED

Once this contract exists, many interfaces can implement it.

A command-line interface might expose:

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secrets get brisecom/ovh

An HTTP API might expose:

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GET /v1/secrets/brisecom/ovh

A Python client might expose:

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secrets.get("brisecom/ovh")

A graphical application might present:

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Secrets
+-- brisecom
 +-- ovh

But underneath, they are all invoking the same operation.

The operation is the important abstraction.

The interface is not.

Unix Already Had This Idea

The Unix philosophy has always encouraged small programs that do one thing well and communicate through simple interfaces.

For example:

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cat file | grep something | sort

Each program has a relatively simple contract:

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input -> processing -> output

Standard input, standard output, standard error and exit codes provide a surprisingly powerful protocol.

The problem is that modern application development often abandons this simplicity.

Instead of:

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input -> operation -> output

we get:

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HTTP request
 |
controller
 |
framework
 |
service
 |
ORM
 |
database
 |
model
 |
serializer
 |
HTTP response

Sometimes all of that is justified.

Sometimes it is simply accidental complexity.

Structured Output Changes Everything

A traditional Unix command might print:

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Secret found: brisecom/ovh
Username: paco
Password: ********

That is convenient for a human but difficult for another program to consume reliably.

Instead, the command can provide structured output:

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{
 "ok": true,
 "data": {
 "name": "brisecom/ovh",
 "username": "paco",
 "password": "..."
 },
 "error": null
}

Now the same program becomes useful to both humans and machines.

For example:

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secrets get brisecom/ovh | jq .data.password

The CLI has effectively become a small API over standard output.

This is an important design principle:

Machine-readable output should be a first-class interface, not an afterthought.

For CLI programs, I particularly like the traditional Unix separation:

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stdout -> data
stderr -> human diagnostics
exit code -> success/failure

That gives shell scripts a stable contract without sacrificing usability for humans.

The Core Should Know Nothing About HTTP

Suppose we implement:

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def get_secret(path):
 ...

That function should not know whether the caller is:

a browser,
curl,
a Python program,
a desktop application,
a shell script,
or another server.

It should simply perform the operation and return a result.

Bad design:

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def get_secret(path):
 print("Looking for secret...")
 return jsonify(...)

Now the business operation knows about both the console and HTTP.

A better design is:

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def get_secret(path):
 return store.get(path)

The HTTP layer converts that result into HTTP.

The CLI layer converts it into terminal output.

The Python library exposes it as a Python object.

The core remains independent.

This is essentially the Ports and Adapters idea applied at a very practical scale.

Files Can Be the Authority

This becomes particularly interesting when the persistent data is deliberately kept outside the service.

Imagine a secret store:

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~/.password-store/

+-- github.gpg
+-- google.gpg
+-- amazon.gpg
|
+-- brisecom/
 +-- ovh.gpg
 +-- cloudflare.gpg

The service does not own a database containing these secrets.

The files are the authority.

GPG provides encryption.

The filesystem provides persistence.

Git can provide versioning.

Syncthing can provide synchronization.

A service can provide an API.

A GUI can provide a graphical interface.

A browser extension can provide browser integration.

The architecture becomes:

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 ENCRYPTED FILES
 |
 +--------------+--------------+
 | | |
 GPG Git Syncthing
 |
 v
 SERVICE
 |
 +----+-----+---------+
 | | | |
 CLI API GUI Browser

This is a very different philosophy from:

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Application
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Database
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Everything

The service becomes replaceable.

The data does not depend on the service.

A File Does Not Have to Contain Text

This principle also avoids a common mistake: assuming that everything in a secrets system is a string.

Credentials can be text.

But secrets can also be:

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private keys
TLS certificates
PKCS#12 files
SSH keys
client certificates
configuration files
binary credentials

There is no reason to turn every one of these into a JSON record.

A file-based service can preserve the original object:

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certificate.p12.gpg
private-key.pem.gpg
client.key.gpg

The encrypted file remains opaque.

When requested, the service decrypts it and returns the original bytes.

This leads to another useful distinction:

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Structured secret -> JSON
Binary secret -> bytes

The API does not need to understand every type of secret.

It only needs to understand the contract for storing and retrieving them.

The API Becomes an Adapter

Once the core operations are defined, an HTTP API becomes relatively boring.

And that is a good thing.

For example:

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GET /v1/secrets
GET /v1/secrets/{path}
PUT /v1/secrets/{path}
DELETE /v1/secrets/{path}

The HTTP layer does four basic things:

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HTTP request
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validate input
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invoke operation
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serialize result

It should not contain the actual storage logic.

The CLI does essentially the same:

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CLI arguments
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validate input
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invoke operation
 |
serialize result

The difference is only the transport.

This is why designing the contract before designing the interface is so powerful.

One Core, Many Interfaces

The resulting architecture is surprisingly simple:

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 +------------+
 | Core |
 | Operations |
 +------+-----+
 |
 +-------------+-------------+
 | | |
 CLI HTTP Library
 | | |
 terminal REST/JSON Python/Go

The interfaces are replaceable.

The core operations are stable.

The persistent data is independent again.

That gives us three particularly useful layers:

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DATA
 |
SERVICE
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INTERFACE

For a Unix-oriented system, this is an extremely natural decomposition.

This Is Not Microservices

There is an important distinction here.

This architecture does not require dozens of services.

In fact, it often works best with one small program.

For example:

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secretsd

could be a single Go binary providing:

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CLI
Unix socket
HTTP API

while the underlying storage remains:

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filesystem + GPG

There is no need for:

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Kubernetes
database cluster
message broker
ORM
service mesh

unless the actual problem requires them.

The objective is not distributed architecture.

The objective is clear boundaries.

A Useful Set of Rules

When designing this kind of application, I find the following rules useful.

1. Define operations before interfaces

Start with:

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get
put
delete
list
generate

not:

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GET /api/...
POST /api/...

2. Define input and output explicitly

Every operation should have a recognizable contract.

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Input -> Operation -> Result

3. Keep presentation out of the core

The core should not know about:

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HTML
HTTP
terminals
GTK
React

4. Make machine-readable output first-class

Use structured output where appropriate.

JSON is useful, but not mandatory for binary data.

5. Use Unix conventions

For CLI applications:

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stdout = data
stderr = diagnostics
exit code = status

6. Keep persistent data independent

If the filesystem is the authority, the service should not secretly create another database containing the same information.

7. Make interfaces replaceable

You should be able to remove the GUI without losing the data.

You should be able to replace the HTTP server without migrating the secrets.

You should be able to access the files directly when necessary.

8. Treat errors as part of the contract

Do not make callers parse human error messages.

Prefer:

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NOT_FOUND
ACCESS_DENIED
INVALID_INPUT
DECRYPTION_FAILED

with human-readable descriptions attached.

The Deeper Principle

The interesting part is not really REST.

It is not JSON.

It is not even Unix.

The deeper principle is this:

Separate what the program does from how somebody talks to it.

Once that separation exists, adding an interface becomes relatively cheap.

Want a CLI?

Add an adapter.

Want REST?

Add an adapter.

Want a Python library?

Add an adapter.

Want a GUI?

Add an adapter.

Want a browser extension?

Add an adapter.

The underlying operation remains the same.

From “Application” to “System”

This way of thinking also changes how I understand small Unix programs.

A program does not necessarily need to be a complete application.

It can be a capability.

For example:

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GPG

provides cryptographic capability.

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filesystem

provides persistent storage.

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pass

provides a password-management interface.

A new service can provide an API over the same underlying capability.

A GUI can provide another interface.

A browser extension can provide another.

The pieces remain useful independently.

This is very close to the original Unix idea of composing simple tools, but applied to modern APIs and software architecture.

Conclusion

I do not think the answer to modern software complexity is always another framework.

Sometimes the better approach is to go back to a simpler question:

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What does this program actually do?

Then define that operation clearly:

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Input
 |
Operation
 |
Output / Error

Everything else can become an interface.

For systems such as a file-based secrets manager, this produces an architecture that is both traditional and modern:

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Encrypted files
 |
 GPG/filesystem
 |
 Service
 |
 +----+-----+-----+
CLI API GUI Browser

The data remains portable.

The service remains replaceable.

The interfaces remain independent.

And the API is no longer something that has to be “added” to the application later.

The API is simply another way of talking to the same program.

unix on Francisco Javier Jiménez Gómez

From Unix Commands to APIs: Designing Software Around Contracts, Not Interfaces

From Unix Commands to APIs: Designing Software Around Contracts, Not Interfaces

The Interface Should Not Define the Program

Unix Already Had This Idea

Structured Output Changes Everything

The Core Should Know Nothing About HTTP

Files Can Be the Authority

A File Does Not Have to Contain Text

The API Becomes an Adapter

One Core, Many Interfaces

This Is Not Microservices

A Useful Set of Rules

1. Define operations before interfaces

2. Define input and output explicitly

3. Keep presentation out of the core

4. Make machine-readable output first-class

5. Use Unix conventions

6. Keep persistent data independent

7. Make interfaces replaceable

8. Treat errors as part of the contract

The Deeper Principle

From “Application” to “System”

Conclusion