add in a new command called columns (#519)

crates/nu-command/src/default_context.rs

@@ -49,6 +49,7 @@ pub fn create_default_context() -> EngineState {
             Any,
             Append,
             Collect,
+            Columns,
             Drop,
             DropColumn,
             DropNth,

crates/nu-command/src/filters/columns.rs

@@ -0,0 +1,108 @@
+use nu_protocol::ast::Call;
+use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::{
+    Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
+    Signature, Span, Value,
+};
+
+#[derive(Clone)]
+pub struct Columns;
+
+impl Command for Columns {
+    fn name(&self) -> &str {
+        "columns"
+    }
+
+    fn signature(&self) -> Signature {
+        Signature::build(self.name()).category(Category::Filters)
+    }
+
+    fn usage(&self) -> &str {
+        "Show the columns in the input."
+    }
+
+    fn examples(&self) -> Vec<Example> {
+        vec![
+            Example {
+                example: "[[name,age,grade]; [bill,20,a]] | columns",
+                description: "Get the columns from the table",
+                result: None,
+            },
+            Example {
+                example: "[[name,age,grade]; [bill,20,a]] | columns | first",
+                description: "Get the first column from the table",
+                result: None,
+            },
+            Example {
+                example: "[[name,age,grade]; [bill,20,a]] | columns | nth 1",
+                description: "Get the second column from the table",
+                result: None,
+            },
+        ]
+    }
+
+    fn run(
+        &self,
+        engine_state: &EngineState,
+        _stack: &mut Stack,
+        call: &Call,
+        input: PipelineData,
+    ) -> Result<PipelineData, ShellError> {
+        let span = call.head;
+        getcol(engine_state, span, input)
+    }
+}
+
+fn getcol(
+    engine_state: &EngineState,
+    span: Span,
+    input: PipelineData,
+) -> Result<PipelineData, ShellError> {
+    match input {
+        PipelineData::Value(
+            Value::List {
+                vals: input_vals,
+                span: _,
+            },
+            ..,
+        ) => {
+            let input_cols = get_input_cols(input_vals);
+            Ok(input_cols
+                .into_iter()
+                .into_pipeline_data(engine_state.ctrlc.clone()))
+        }
+        PipelineData::Stream(stream, ..) => {
+            let v: Vec<_> = stream.into_iter().collect();
+            let input_cols = get_input_cols(v);
+
+            Ok(input_cols
+                .into_iter()
+                .into_pipeline_data(engine_state.ctrlc.clone()))
+        }
+        PipelineData::Value(_v, ..) => {
+            let cols = vec![];
+            let vals = vec![];
+            Ok(Value::Record { cols, vals, span }.into_pipeline_data())
+        }
+    }
+}
+
+fn get_input_cols(input: Vec<Value>) -> Vec<String> {
+    let rec = input.first();
+    match rec {
+        Some(Value::Record { cols, vals: _, .. }) => cols.to_vec(),
+        _ => vec!["".to_string()],
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(Columns {})
+    }
+}

crates/nu-command/src/filters/mod.rs

@@ -2,6 +2,7 @@ mod all;
 mod any;
 mod append;
 mod collect;
+mod columns;
 mod drop;
 mod each;
 mod first;
@@ -30,6 +31,7 @@ pub use all::All;
 pub use any::Any;
 pub use append::Append;
 pub use collect::Collect;
+pub use columns::Columns;
 pub use drop::*;
 pub use each::Each;
 pub use first::First;

src/tests.rs

@@ -1287,3 +1287,16 @@ fn flatten_table_column_get_last() -> TestResult {
         "0.22",
     )
 }
+
+#[test]
+fn get_table_columns_1() -> TestResult {
+    run_test(
+        "[[name, age, grade]; [paul,21,a]] | columns | first",
+        "name",
+    )
+}
+
+#[test]
+fn get_table_columns_2() -> TestResult {
+    run_test("[[name, age, grade]; [paul,21,a]] | columns | nth 1", "age")
+}