TUI: Use rage binary in example commands if installed

Cargo.lock

@@ -64,6 +64,7 @@ dependencies = [
  "rand 0.7.3",
  "secrecy",
  "sha2",
+ "which",
  "x509",
  "x509-parser",
  "yubikey-piv",
@@ -353,6 +354,12 @@ dependencies = [
  "signature",
 ]
 
+[[package]]
+name = "either"
+version = "1.6.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "e78d4f1cc4ae33bbfc157ed5d5a5ef3bc29227303d595861deb238fcec4e9457"
+
 [[package]]
 name = "elliptic-curve"
 version = "0.8.5"
@@ -1212,6 +1219,16 @@ version = "0.10.0+wasi-snapshot-preview1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "1a143597ca7c7793eff794def352d41792a93c481eb1042423ff7ff72ba2c31f"
 
+[[package]]
+name = "which"
+version = "4.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b55551e42cbdf2ce2bedd2203d0cc08dba002c27510f86dab6d0ce304cba3dfe"
+dependencies = [
+ "either",
+ "libc",
+]
+
 [[package]]
 name = "winapi"
 version = "0.3.9"

Cargo.toml

@@ -37,6 +37,7 @@ pcsc = "2.4"
 rand = "0.7"
 secrecy = "0.7"
 sha2 = "0.9"
+which = "4.1"
 x509 = "0.2"
 x509-parser = "0.9"
 yubikey-piv = { version = "0.3", features = ["untested"] }

src/main.rs

@@ -551,6 +551,9 @@ fn main() -> Result<(), Error> {
         writeln!(file, "#    Recipient: {}", recipient)?;
         writeln!(file, "{}", stub.to_string())?;
 
+        // If `rage` binary is installed, use it in examples. Otherwise default to `age`.
+        let age_binary = which::which("rage").map(|_| "rage").unwrap_or("age");
+
         eprintln!();
         eprintln!("✅ Done! This YubiKey identity is ready to go.");
         eprintln!();
@@ -564,10 +567,16 @@ fn main() -> Result<(), Error> {
         eprintln!("Here are some example things you can do with it:");
         eprintln!();
         eprintln!("- Encrypt a file to this identity:");
-        eprintln!("  $ cat foo.txt | age -r {} -o foo.txt.age", recipient);
+        eprintln!(
+            "  $ cat foo.txt | {} -r {} -o foo.txt.age",
+            age_binary, recipient
+        );
         eprintln!();
         eprintln!("- Decrypt a file with this identity:");
-        eprintln!("  $ cat foo.txt.age | age -d -i {} > foo.txt", file_name);
+        eprintln!(
+            "  $ cat foo.txt.age | {} -d -i {} > foo.txt",
+            age_binary, file_name
+        );
         eprintln!();
         eprintln!("- Recreate the identity file:");
         eprintln!(