Implemented whoami on top of system calls.

Details

- The current implementataion the `whoami` utility relies
on a env var to determine the user. With this change we get
the effective user id of the context and lookup that id on
/etc/passwd.
- Added the Redox `libextra` dependency as it has some goodies
for error handling. This will allow to make this crate upto date
with `coreutils` practices.
- Added a new small `arg_parser` crate that was extracted from
`coreutils` as is now commen between `coreutils` and here.

Cargo.toml

@@ -32,6 +32,8 @@ path = "src/bin/whoami.rs"
 
 [dependencies]
 argon2rs = { version = "0.2", default-features = false }
+arg_parser = { path = "../arg_parser" }
+extra = { git = "https://github.com/redox-os/libextra.git" }
 liner = "0.1"
 rand = "0.3"
 redox_syscall = "0.1"

src/bin/whoami.rs

@@ -1,5 +1,74 @@
-use std::env;
+#![deny(warnings)]
 
-pub fn main() {
-    println!("{}", env::var("USER").unwrap_or(String::new()));
+extern crate arg_parser;
+extern crate extra;
+extern crate syscall;
+extern crate userutils;
+
+use std::io::{self, Read, Write};
+use std::process::exit;
+use std::fs::File;
+use std::env;
+use arg_parser::ArgParser;
+use extra::option::OptionalExt;
+use userutils::Passwd;
+
+const MAN_PAGE: &'static str = /* @MANSTART{whoami} */ r#"
+NAME
+     whoami - display effective user id
+
+SYNOPSIS
+     whoami [ -h | --help ]
+
+DESCRIPTION
+     The whoami utility displays your effective user ID as a name.
+
+OPTIONS
+    -h
+    --help
+        Display this help and exit.
+
+EXIT STATUS
+     The whoami utility exits 0 on success, and >0 if an error occurs.
+
+AUTHOR
+    Written by Jose Narvaez.
+"#; /* @MANEND */
+
+const PASSWD_FILE: &'static str = "/etc/passwd";
+
+fn main() {
+   let stdout = io::stdout();
+   let mut stdout = stdout.lock();
+   let mut stderr = io::stderr();
+
+   let mut parser = ArgParser::new(1)
+        .add_flag(&["h", "help"]);
+   parser.parse(env::args());
+
+    if parser.found("help") {
+        stdout.write_all(MAN_PAGE.as_bytes()).try(&mut stderr);
+        stdout.flush().try(&mut stderr);
+        exit(0);
+    }
+
+    match syscall::geteuid() {
+        Ok(euid) => {
+            let mut passwd_string = String::new();
+            if let Ok(mut file) = File::open(PASSWD_FILE) {
+                let _ = file.read_to_string(&mut passwd_string);
+            }
+
+            for line in passwd_string.lines() {
+                if let Ok(passwd) = Passwd::parse(line) {
+                    if euid == passwd.uid as usize {
+                        stdout.write_all(format!("{}\n", passwd.name).as_bytes()).try(&mut stderr);
+                        stdout.flush().try(&mut stderr);
+                        exit(0);
+                    }
+                }
+            }
+        },
+        Err(_) => exit(1)
+    };
 }