nushell/crates/nu-cmd-lang/src/core_commands/echo.rs

use nu_engine::command_prelude::*;

#[derive(Clone)]
pub struct Echo;

impl Command for Echo {
    fn name(&self) -> &str {
        "echo"
    }

    fn description(&self) -> &str {
        "Returns its arguments, ignoring the piped-in value."
    }

    fn signature(&self) -> Signature {
        Signature::build("echo")
            .input_output_types(vec![(Type::Nothing, Type::Any)])
            .rest("rest", SyntaxShape::Any, "The values to echo.")
            .category(Category::Core)
    }

    fn extra_description(&self) -> &str {
        r#"Unlike `print`, which prints unstructured text to stdout, `echo` is like an
identity function and simply returns its arguments. When given no arguments,
it returns an empty string. When given one argument, it returns it as a
nushell value. Otherwise, it returns a list of the arguments. There is usually
little reason to use this over just writing the values as-is."#
    }

    fn run(
        &self,
        engine_state: &EngineState,
        stack: &mut Stack,
        call: &Call,
        _input: PipelineData,
    ) -> Result<PipelineData, ShellError> {
        let args = call.rest(engine_state, stack, 0)?;
        echo_impl(args, call.head)
    }

    fn run_const(
        &self,
        working_set: &StateWorkingSet,
        call: &Call,
        _input: PipelineData,
    ) -> Result<PipelineData, ShellError> {
        let args = call.rest_const(working_set, 0)?;
        echo_impl(args, call.head)
    }

    fn is_const(&self) -> bool {
        true
    }

    fn examples(&self) -> Vec<Example> {
        vec![
            Example {
                description: "Put a list of numbers in the pipeline. This is the same as [1 2 3].",
                example: "echo 1 2 3",
                result: Some(Value::list(
                    vec![Value::test_int(1), Value::test_int(2), Value::test_int(3)],
                    Span::test_data(),
                )),
            },
            Example {
                description:
                    "Returns the piped-in value, by using the special $in variable to obtain it.",
                example: "echo $in",
                result: None,
            },
        ]
    }
}

fn echo_impl(mut args: Vec<Value>, head: Span) -> Result<PipelineData, ShellError> {
    let value = match args.len() {
        0 => Value::string("", head),
        1 => args.pop().expect("one element"),
        _ => Value::list(args, head),
    };
    Ok(value.into_pipeline_data())
}

#[cfg(test)]
mod test {
    #[test]
    fn test_examples() {
        use super::Echo;
        use crate::test_examples;
        test_examples(Echo {})
    }
}