Error checking and README fix

vault-shim-installer.sh

@@ -1,6 +1,16 @@
 #!/bin/sh
 version=$1
 
+if [ -z "$version" ]; then
+  echo "Usage: $0 <version>"
+  exit 1
+fi
+
+# check if the version starts without a 'v', if so, add it
+if [ "${version:0:1}" != "v" ]; then
+  version="v$version"
+fi
+
 # turn to all lowercase and then capitalize the first letter of the OS name
 os="$(uname -s | tr '[:upper:]' '[:lower:]' | awk '{ print toupper(substr($0, 1, 1)) substr($0, 2) }')"
 arch="$(uname -m | tr '[:upper:]' '[:lower:]')"
@@ -14,5 +24,5 @@ echo "Installing vault-shim version $version"
 echo "os: $os"
 echo "arch: $arch"
 
-curl -L "https://github.com/Bandwidth/vault-shim/releases/download/v${version}/vault-shim_${os}_${arch}.tar.gz" --output "/usr/local/bin/vault-shim_${os}_${arch}.tar.gz"
+curl -L "https://github.com/Bandwidth/vault-shim/releases/download/${version}/vault-shim_${os}_${arch}.tar.gz" --output "/usr/local/bin/vault-shim_${os}_${arch}.tar.gz"
 tar xzf "/usr/local/bin/vault-shim_${os}_${arch}.tar.gz" -C "/usr/local/bin"