show usage() on invalid use

[carenas]

the application is designed to be run as a filter, but if called without data in stdin will seem to "hang".

detect if stdin is attached to a tty and abort instead printing usage() to stdout as is commonly done by other filters (ex: more, less)

Signed-off-by: Carlo Marcelo Arenas Belón carenas@gmail.com

[mookid]

Great! Thanks for the suggestion.

I see one super minor issue: the usage message is

USAGE:

    diffr reads from standard input and write to standard output.
(...)

which does not respect the template (compared to diffr -h:

(...)
USAGE:
    diffr reads from standard input and write to standard output.
(...)

Plus cargo fmt.

so I suggest the following:

diff --git a/src/main.rs b/src/main.rs
index f861c32..9ee480b 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,3 +1,4 @@
+use atty::{is, Stream};
 use clap::{App, Arg};
 use std::collections::hash_map::DefaultHasher;
 use std::convert::TryFrom;
@@ -9,29 +10,18 @@ use termcolor::{
     Color::{Green, Red},
     ColorChoice, ColorSpec, StandardStream, WriteColor,
 };
-use atty::{is, Stream};

 const ABOUT: &str = "
 diffr adds word-level diff on top of unified diffs.
 word-level diff information is displayed using text attributes.";

-const USAGE: &str = "
+const USAGE: &str = "\
     diffr reads from standard input and write to standard output.

     Typical usage is for interactive use of diff:
     diff -u <file1> <file2> | diffr
     git show | diffr";

-const TEMPLATE: &str = "\
-{bin} {version}
-{author}
-{about}
-
-USAGE:{usage}
-
-OPTIONS:
-{unified}";
-
 const FLAG_DEBUG: &str = "debug";

 #[derive(Debug)]
@@ -45,7 +35,6 @@ fn main() {
         .author("Nathan Moreau <nathan.moreau@m4x.org>")
         .about(ABOUT)
         .usage(USAGE)
-        .template(TEMPLATE)
         .arg(Arg::with_name(FLAG_DEBUG).long("debug").hidden(true))
         .get_matches();