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Rust CLI with Clap

by rajman187 on 7/1/25, 1:25 AM with 88 comments

• by mprovost on 7/1/25, 11:24 AM

  I chose clap as the argument parsing crate for my book on Rust CLI programming and it's been my biggest regret. Folk sometimes complain that Rust itself is a moving target but I haven't found that to be the case at all. On the other hand, clap has gone through several, incompatible major releases over the past few years. Unfortunately, in order to keep this forward momentum, it also starts deprecating things which means that I had to pin it to a quickly outdated version. If I ever go back and update the earlier chapters I'd pick a simpler, slower-moving library.

  On the other hand, this situation is a great example of why keeping things like argument parsing out of the Rust standard library is such a good idea. It's much better to let the community gel around some crates which are able to iterate (and innovate) faster than something with strict backwards-compatibility requirements. Looking at the discussion here there's clearly not one way to solve this - there's no way that something as large and complex as clap has evolved into will ever become "standard".

• by ben0x539 on 7/1/25, 2:52 AM

  structopt/Clap's derive magic is one of the first things I miss when I go to write some more-or-less trivial program in a non-Rust language these days. Being able to define all the data for a command line argument in one place (how/where to store it, what the type/valid input is, the association between the name and a variable/field, the documentation for --help...) seems like table stakes but afaict almost every other argument parsing library makes me repeat myself to the point where it takes all the joy out of writing a simple program.

  (Python's docopt is also amazing, fwiw)

• by csnover on 7/1/25, 4:19 AM

  In my opinion, clap is a textbook example of over-engineering for a single metric (UX) at the expense of all other considerations (compilation speed, runtime cost, binary size, auditability, and maintainability). It is an 18kloc command-line parser with an additional 125kloc of dependencies that takes nearly 6 seconds to compile (‘only’ 400ms for an incremental build) and which adds nearly 690KiB to an optimised release binary (‘only’ 430KiB if you strip out most of the sugar that only clap provides).

  There are many other command-line parsers to choose from that do all the key things that clap does, with half or less the build cost, and most of them with 30x less binary overhead[0]. argh is under 4kloc. gumdrop is under 2kloc. pico-args is under 700loc. What is the value of that extra 10kloc? A 10% better parser?

  I am not saying there is no room for a library like clap—it is, at least, a triumphant clown car of features that can handle practically any edge-case anyone ever thought of—but if I got a nickel every time I spent 15 minutes replacing a trivial use of clap with pico-args and thus reduced the binary size and compile time of some project by at least 80%, I would have at least three nickels.

  Just to try to pre-empt arguments like “disk space is cheap”, “compiler time is cheaper than human time”, etc.: there are no golden bullets in engineering, only trade-offs. Why would you default to the biggest, slowest option? This is the “every web site must be able to scale like Facebook” type logic. You don’t even have to do more work to use argh or gumdrop. If clap ends up having some magic feature that no other parser has that you absolutely need, you can switch, but I’ve yet to ever encounter such a thing. Its inertia and popularity carry it forward, but it is perhaps the last choice you should pick for a new project—not the first.

  [0] https://github.com/rosetta-rs/argparse-rosetta-rs

• by hamandcheese on 7/1/25, 3:25 AM

  I like clap a lot, however I find that clap derive is not very easily discoverable. I always have to google for the right macro incantation to get what I want. Whereas editor completions from rust analyzer get me quite far without needing to leave my editor when I'm just using an ordinary library.

  I think this is more a criticism of rust-analyzer than clap itself, any macro-heavy library I have similar issues with.

  (Yes I know clap can be used without derive, but I'm willing to deal with the pain to parse directly into a struct)

• by woodruffw on 7/1/25, 2:35 AM

  Nice write up. “Good” CLI semantics are pretty devilish, and overall I think clap does a pretty great job of picking the right (or at least most intuitive) behavior.

  (One edge case that consistently trips me up, which other argument parsers similarly struggle with: an environment variable fallback has the same “weight” as its option counterpart, so any CLI that makes use of grouping/exclusivity will eventually hit user confusions where the user passes `--exclusive` and gets a failure because of an unrelated environment variable.)

• by fainpul on 7/1/25, 9:26 AM

  PowerShell has everything related to argument parsing, helptext generation, tab completion, type coercion, validation etc. built-in with a declarative DSL. Many things work even directly OOTB, without requiring special annotations.

  It is by far the nicest way to create CLI tools I have ever seen. Every other shell or commandline parsing library I ever tried, feels extremely clunky in comparison.

  https://learn.microsoft.com/en-us/powershell/module/microsof...

• by ameliaquining on 7/1/25, 2:57 AM

  Worth noting that you can do this in languages other than Rust. Python, for example, has https://github.com/fastapi/typer. https://github.com/shadawck/awesome-cli-frameworks lists relevant libraries in many languages, though not all of them support clap-style structured parsing.

• by msgodel on 7/1/25, 3:03 AM

  It really bothers me how much people use crates in Rust. "Minimalist" crates have tens of dependencies.

  It's like the node.js of systems languages. Touching it feels gross.

• by vikrantrathore on 7/1/25, 3:32 AM

  I have used clap to build perspt (https://github.com/eonseed/perspt). The project has extensive documentation on how it was built, as we did it as a learning exercise.

• by porphyra on 7/1/25, 3:06 AM

  The macro magic in rust is amazing. It's so nice to just define a struct, slap a `#[derive(Parser)]` on it, and call it a day.

• by mootoday on 7/1/25, 3:46 AM

  That was a great intro to clap, thanks for writing it up!

  I've been building clap CLIs for a while and started to put together a template: https://github.com/mootoday/cli-template.

  It also includes a crate I developed to reduce the boilerplate code for nested commands: https://crates.io/crates/clap-nested-commands

• by FujiApple on 7/1/25, 4:08 AM

  Something I’ve been working on recently is a command line tool [1] to bring clap declarative command line parsing to shell scripts. Unfinished WIP but largely functional.

  [1] https://github.com/fujiapple852/claptrap

• by gametorch on 7/1/25, 2:16 AM

  The compile time guarantees + declarative nature make Clap so amazing and foolproof. This is like heaven compared to imperative, arcane incantations like getopt.

• by tempodox on 7/1/25, 4:05 AM

  Clap is the way to go. It makes command line argument parsing a breeze.

• by b0a04gl on 7/1/25, 4:50 AM

  10kloc for command line parsing. TEN THOUSAND LINES. pico-args does it in 700 lines and probably handles 99% of real world use cases. compile times go to shit binary size bloats and for some edge case you'll never hit.most CLI tools need what three four flags max, maybe a subcommand or two. you don't need the swiss army knife of argument parsing for that. tried replacing clap with pico-args on three different projects last month. 80% reduction in compile time every single time. binary went from 8mb to 2mb on one of them.the "disk space is cheap" argument's acceptable partially but compile time isn't. developer experience isn't. startup time isn't. memory usage isn't