Macro exec_command 

macro_rules! exec_command {
    (input, $input:expr) => { ... };
    (raw, $cmd:literal) => { ... };
    (raw, $cmd:expr, $args:expr) => { ... };
    (raw, $input:expr) => { ... };
    (pipe, $cmd:literal) => { ... };
    (pipe, $cmd:expr, $args:expr) => { ... };
    (inherit, $cmd:literal) => { ... };
    (inherit, $cmd:expr, $args:expr) => { ... };
    ($cmd:literal) => { ... };
    ($cmd:expr, [ $($arg:literal),* ]) => { ... };
    ($cmd:expr, $args:expr) => { ... };
}

Calls the exec_command host function to execute a command on the host as a synchronous child process.