Using clap to generate shell completions.

Added a new subcommand 'completions' that takes an argument (the shell
to generate the completions for) and outputs the shell completion to
the standard output. This is working the same way that rustup does.

Author: erickpires

src/cli.yml

@@ -1,7 +1,10 @@
-name: loop 
+name: loop
 version: "0.1.0"
 author: Rich Jones <miserlou@gmail.com>
-about: UNIX's missing `loop` command 
+about: UNIX's missing `loop` command
+settings:
+    - SubcommandsNegateReqs
+
 args:
     - INPUT:
         help: The command to be looped
@@ -74,4 +77,13 @@ args:
     - stdin:
         short: i
         long: stdin
-        help: Read from standard input
+        help: Read from standard input
+
+subcommands:
+    - completions:
+        about: Generates completion scripts for your shell
+        args:
+            - shell:
+                required: true
+                possible_values: [ bash, fish, zsh, powershell, elvish ]
+                help: The shell to generate the script for

src/main.rs

@@ -11,7 +11,7 @@ use std::io::{self, BufRead};
 use std::process::Command;
 use std::time::{Instant, SystemTime};
 
-use clap::App;
+use clap::{App, Shell};
 use humantime::{parse_duration, parse_rfc3339_weak};
 use isatty::{stdin_isatty};
 use regex::Regex;
@@ -22,6 +22,20 @@ fn main() {
     // Load the CLI
     let yaml = load_yaml!("cli.yml");
     let matches = App::from_yaml(yaml).get_matches();
+    let mut cli = App::from_yaml(yaml);
+
+    match matches.subcommand() {
+        ("completions", Some(sub_matches)) => {
+            let shell = sub_matches.value_of("shell").unwrap();
+            cli.gen_completions_to(
+                "loop",
+                shell.parse::<Shell>().unwrap(),
+                &mut io::stdout()
+            );
+            std::process::exit(0);
+        },
+        _ => (),
+    }
 
     // Main input command
     let input = matches.value_of("INPUT").unwrap();