feat: add completions subcommand

src/cli/ahab.rs

@@ -1,5 +1,6 @@
 use super::{Django, DockerCompose, Link, Postgres};
 use clap::{Parser, Subcommand};
+use clap_complete::Shell;
 
 /// A program for interacting with various dockerized applications.
 #[derive(Parser, Debug)]
@@ -34,4 +35,10 @@ pub enum Commands {
         #[command(subcommand)]
         command: Link,
     },
+
+    /// Print a shell completion script on stdout
+    Completions {
+        /// Shell to generate the script for
+        shell: Shell,
+    },
 }

src/main.rs

@@ -50,5 +50,6 @@ fn main() -> Result<()> {
                 null,
             } => scripts::link::check(&paths, porcelain, null),
         },
+        cli::Commands::Completions { shell } => scripts::completions::completions(shell),
     }
 }

src/scripts/completions.rs

@@ -0,0 +1,46 @@
+use std::io::{self, Write};
+
+use anyhow::{Context, Result};
+use clap::CommandFactory;
+use clap_complete::{Shell, generate};
+
+use crate::cli::Ahab;
+
+pub fn completions(shell: Shell) -> Result<()> {
+    let script = script(shell);
+
+    let mut stdout = io::stdout().lock();
+    match stdout.write_all(&script).and_then(|()| stdout.flush()) {
+        Err(e) if e.kind() == io::ErrorKind::BrokenPipe => Ok(()),
+        result => result.context("writing the completion script"),
+    }
+}
+
+fn script(shell: Shell) -> Vec<u8> {
+    let mut cmd = Ahab::command();
+    let name = cmd.get_name().to_string();
+
+    let mut script = Vec::new();
+    generate(shell, &mut cmd, name, &mut script);
+    script
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{Shell, script};
+    use clap::ValueEnum;
+
+    #[test]
+    fn every_shell_gets_a_script_covering_the_subcommands() {
+        for shell in Shell::value_variants() {
+            let out = String::from_utf8(script(*shell)).expect("script is utf8");
+
+            for subcommand in ["compose", "django", "postgres", "link", "completions"] {
+                assert!(
+                    out.contains(subcommand),
+                    "{shell:?} script never mentions {subcommand}"
+                );
+            }
+        }
+    }
+}

src/scripts/mod.rs

@@ -1,3 +1,4 @@
+pub mod completions;
 pub mod django;
 pub mod docker;
 pub mod docker_compose;