Add support for environment variable MNEMONIC

Cargo.toml

@@ -13,7 +13,7 @@ name = "eth_staking_smith"
 path = "src/lib.rs"
 
 [dependencies]
-clap = { version = "^4.5", features = ["derive"] }
+clap = { version = "^4.5", features = ["derive", "env"] }
 ethereum_hashing = "0.6.0"
 eth2_key_derivation = { git = "https://github.com/ChorusOne/lighthouse", rev = "1be5253610dc8fee3bf4b7a8dc1d01254bc5b57d"}
 eth2_keystore = { git = "https://github.com/ChorusOne/lighthouse", rev = "1be5253610dc8fee3bf4b7a8dc1d01254bc5b57d"}

src/cli/existing_mnemonic.rs

@@ -6,11 +6,16 @@ pub struct ExistingMnemonicSubcommandOpts {
     /// The mnemonic that you used to generate your
     /// keys.
     ///
+    /// This can be provided in two ways:
+    ///
+    /// 1. Through the MNEMONIC environment variable (recommended)
+    ///
+    /// 2. Through the --mnemonic argument in plain text.
     /// It is recommended not to use this
     /// argument, and wait for the CLI to ask you
-    ///    for your mnemonic as otherwise it will
-    ///    appear in your shell history.
-    #[arg(long)]
+    /// for your mnemonic as otherwise it will
+    /// appear in your shell history.
+    #[arg(long, env = "MNEMONIC")]
     pub mnemonic: String,
 
     /// The name of Ethereum PoS chain you are targeting.