install.sh: Recommend using sudo on error.

Show error message that the user shall try running this script with sudo
in case write permissions are missing. Implement proposal of comment
https://github.com/ipfs/go-ipfs/pull/3194#issuecomment-245376993

License: MIT
Signed-off-by: Stephan Kulla <git.mail@kulla.me>

cmd/ipfs/dist/install.sh

@@ -3,6 +3,10 @@
 bin=ipfs
 binpaths="/usr/local/bin /usr/bin"
 
+# This variable contains a nonzero length string in case the script fails
+# because of missing write permissions.
+is_write_perm_missing=""
+
 # this script is currently brain dead.
 # it merely tries two locations.
 # in the future maybe use value of $PATH.
@@ -11,8 +15,21 @@ for binpath in $binpaths; do
   if mv -t "$binpath" "$bin" 2> /dev/null; then
     echo "Moved $bin to $binpath"
     exit 0
+  else
+    if [ -d "$binpath" -a ! -w "$binpath" ]; then
+      is_write_perm_missing=1
+    fi
   fi
 done
 
-echo "cannot install $bin in one of the directories $binpaths"
+echo "We cannot install $bin in one of the directories $binpaths"
+
+if [ -n "$is_write_perm_missing" ]; then
+  echo "It seems that we do not have the necessary write permissions."
+  echo "Perhaps try running this script as a privileged user:"
+  echo
+  echo "    sudo $0"
+  echo
+fi
+
 exit 1