Installer: Improve handling of su/sudo

A couple of changes here:

* Check for sudo necessity and availability before doing any real work.
  Better to warn and exit quick and early!
* Remove the support for using `su`. It was broken for two reasons.
  First, unlike `sudo`, `su -c` expects the command as a single
  argument. Second, `su`, unlike `sudo`, reads the password from stdin
  which in this case "contains" the tarball being downloaded. The second
  point is particularly tricky to solve without saving the tarball to
  storage (and we want to avoid extra storage space and wear). And,
  anyway, users that can `su -c` can also `su` manually before
  installing.

Signed-off-by: Leandro Motta Barros <[email protected]>
Change-type: patch

contrib/install.sh

@@ -17,6 +17,19 @@ for cmd in curl tar; do
 done
 [ $abort == 1 ] && exit 1
 
+sudo=
+if [ "$(id -u)" -ne 0 ]; then
+	if [ -z "$(command -v sudo)" ]; then
+		cat >&2 <<-EOF
+		Error: this installer needs the ability to run commands as root.
+		You are not running as root and we are unable to find "sudo" available.
+		EOF
+		exit 1
+	fi
+	sudo="sudo -E"
+fi
+
+# Detect the system architecture
 machine=$(uname -m)
 
 case "$machine" in
@@ -43,26 +56,9 @@ case "$machine" in
 		exit 1
 esac
 
+# Download and decompress into the target location
 url="https://github.com/balena-os/balena-engine/releases/download/${tag}/balena-engine-${tag}-${arch}.tar.gz"
 
-sudo=
-if [[ $(id -u) -ne 0 ]]; then
-	if [[ $(command -v sudo) ]]; then
-		sudo='sudo -E'
-	fi
-	if [[ $(command -v su) ]]; then
-		sudo='su -c'
-	fi
-	if [[ -z $sudo ]]; then
-		cat >&2 <<-EOF
-		Error: this installer needs the ability to run commands as root.
-		We are unable to find either "sudo" or "su" available to make this happen.
-		EOF
-		exit 1
-	fi
-fi
-
-
 curl -sL "$url" | $sudo tar xzv -C /usr/local/bin --strip-components=1
 
 cat <<-EOF