makeShell: improve `update-nix-bitcoin`

- Don't overwrite `nix-bitcoin-release.nix` on errors
- Show a message to indicate whether `nix-bitcoin-release.nix` was
  updated
- Don't start a shell when called noninteractively

Also, update `usage.md` and reformat `shell.nix`.

docs/usage.md

@@ -1,9 +1,10 @@
 Updating
 ---
-In your deployment directory, enter the nix shell with `nix-shell` and run
+In your deployment directory, enter the nix shell with `nix-shell` and run the
+following to update `nix-bitcoin-release.nix`:
 
 ```
-fetch-release > nix-bitcoin-release.nix
+update-nix-bitcoin
 ```
 
 Nodeinfo

examples/shell.nix

@@ -4,5 +4,5 @@ in
   import "${nix-bitcoin}/helper/makeShell.nix" {
     configDir = ./.;
     # Set this to modify your shell
-    # extraShellInitCmds = (pkgs: ''<my bash code>'');
+    # extraShellInitCmds = pkgs: ''<my bash code>'';
   }

helper/makeShell.nix

@@ -17,6 +17,11 @@ stdenv.mkDerivation rec {
 
     export NIX_BITCOIN_EXAMPLES_DIR="${cfgDir}"
 
+    # Set isInteractive=1 if
+    # 1. stdout is a TTY, i.e. we're not piping the output
+    # 2. the shell is interactive
+    if [[ -t 1 && $- == *i* ]]; then isInteractive=1; else isInteractive=; fi
+
     help() {
         echo "nix-bitcoin path: ${toString ../.}"
         echo
@@ -47,10 +52,21 @@ stdenv.mkDerivation rec {
       ${toString ./fetch-release}
     }
 
-    update-nix-bitcoin() {
+    update-nix-bitcoin() {(
-      fetch-release > "${cfgDir}/nix-bitcoin-release.nix"
+      set -euo pipefail
-      exec nix-shell
+      releaseFile="${cfgDir}/nix-bitcoin-release.nix"
-    }
+      current=$(cat "$releaseFile" 2>/dev/null || true)
+      new=$(fetch-release)
+      if [[ $new == $current ]]; then
+        echo "nix-bitcoin-release.nix already contains the latest release"
+      else
+        echo "$new" > "$releaseFile"
+        echo "Updated nix-bitcoin-release.nix"
+        if [[ $isInteractive ]]; then
+          exec nix-shell
+        fi
+      fi
+    )}
 
     generate-secrets() {(
       set -euo pipefail
@@ -94,10 +110,7 @@ stdenv.mkDerivation rec {
       done
     }
 
-    # Print welcome message if
+    if [[ $isInteractive ]]; then
-    # 1. stdout is a TTY, i.e. we're not piping the output
-    # 2. the shell is interactive
-    if [[ -t 1 && $- == *i* ]]; then
       ${figlet}/bin/figlet "nix-bitcoin"
       echo 'Enter "h" or "help" for documentation.'
     fi