Program arguments

Reading in program parameters from the command line at the startup of a program is easy in Rust, just use the method std::env::args(). We can use the function collect() to these parameters into a vector of String, like this:

// code from Chapter 4/code/arguments.rs: 
use std::env; 
 
fn main() { 
   let args: Vec<String> = env::args().collect(); 
   println!("The program's name is: {}", args[0]); 
   for arg in args.iter() { 
         println!("Next argument is: {}", arg) 
   } 
   println!("Total arguments supplied: {}", args.len() - 1); 
   for n in 1..args.len() { 
      println!("The {}th argument is {}", n, args[n]); 
   } 
} 

Call the program like this:

• arguments arg1 arg2 on Windows
• ./arguments arg1 arg2 on Linux and OS X

Here is the output from a real call:

The argument args[0] is the program's name, the next arguments are the command-line parameters. We can iterate through the arguments or access them by index. The argument args.len()- 1 ;gives us the number of parameters.

For more complex parsing with options and flags use the getopts or the docopt crate. To get started there is an example at http://rustbyexample.com/arg/getopts.html.

In the same way env::vars() returns the operating system environment variables:

let osvars = env::vars(); 
for (key, value) in osvars { 
    println!("{}: {}", key, value); 
} 

This starts with printing out on Windows for example:

    HOMEDRIVE: C:
    USERNAME: CVO
    LOGONSERVER: \\MicrosoftAccount