tuned -- nicer generated bash source;

--- a/src/Pure/System/bash.scala	Sun Jan 15 16:47:42 2017 +0100
+++ b/src/Pure/System/bash.scala	Mon Jan 16 15:50:54 2017 +0100
@@ -24,7 +24,7 @@
       case '\f' => "$'\\f'"
       case '\r' => "$'\\r'"
       case _ =>
-        if (Symbol.is_ascii_letter(ch) || Symbol.is_ascii_digit(ch) || "-./:_".contains(ch))
+        if (Symbol.is_ascii_letter(ch) || Symbol.is_ascii_digit(ch) || "+-./:_".contains(ch))
           Symbol.ascii(ch)
         else if (c < 0) "$'\\x" + Integer.toHexString(256 + c) + "'"
         else if (c < 16) "$'\\x0" + Integer.toHexString(c) + "'"

--- a/src/Pure/System/bash_syntax.ML	Sun Jan 15 16:47:42 2017 +0100
+++ b/src/Pure/System/bash_syntax.ML	Mon Jan 16 15:50:54 2017 +0100
@@ -24,7 +24,7 @@
           | "\r" => "$'\\r'"
           | _ =>
               if Symbol.is_ascii_letter ch orelse Symbol.is_ascii_digit ch orelse
-                exists_string (fn c => c = ch) "-./:_" then ch
+                exists_string (fn c => c = ch) "+-./:_" then ch
               else if c < 16 then "$'\\x0" ^ Int.fmt StringCvt.HEX c ^ "'"
               else if c < 32 orelse c >= 127 then "$'\\x" ^ Int.fmt StringCvt.HEX c ^ "'"
               else "\\" ^ ch)