Command-line rust notes (1)

Accessing command line arguments

std::env::args() -> Args returns an iterators on the command line arguments passed into the binary:

impl Iterator for Args {
    type Item = String;
    fn next(&mut self) -> Option<String>
}

Channeling standard out and standard error

When running a command line program, standard output goes to channel 1 while standard error goes to channel 2:

# stdout will be written to ./out and stderr will be written to ./err
some-command ... 1>out 2>err

Parsing command line arguments

clap is a crate that can be used declare and parse command line arguments:

cargo add clap@4.0.32 --features derive

1. To declare a CLI schema, bring in the clap::Parser trait, then define a struct with the #[derive(Parser)] attribute
2. Add attributes like #[command(...)] to the argument struct to define metadata for the program. Common options include authors, about, long_about, version. They will show up when running with the -h/--help and/or -v/--version flags. They default to what is written in Cargo.toml
3. For keyword arguments, some common options:
   • #[arg(short='n', long="number")] (use characters for the “short” flag)
   • #[arg(default_value_t=...)]
4. Positional argument behaviors:
   • file: String: a single argument that is required
   • file: Option<String>: a single, optional argument
   • files Vec<String>: zero or more positional arguments

Dependencies for testing only

Adding the --dev flag in cargo add means that the crate will be compiled for testing only. One useful crate is assert_cmd, which can be used to run both system binaries and cargo binaries:

let assert = Command::cargo_bin("mybin")?  // => Result<Command, ...>
    .args(&["-f", "-l"])
    .write_std("Hello, world")
    .assert()  // => assert_cmd::Assert
    .success()
    .try_stdout("some output")?
    .try_stderr("some errors")?