Merge pull request #454 from jey/atomic-sbt-download. Closes #454.

Make sbt download an atomic operation

Modifies the `sbt/sbt` script to gracefully recover when a previous invocation died in the middle of downloading the SBT jar.

Author: Jey Kottalam <jey@cs.berkeley.edu>

== Merge branch commits ==

commit 6c600eb434a2f3e7d70b67831aeebde9b5c0f43b
Author: Jey Kottalam <jey@cs.berkeley.edu>
Date:   Fri Jan 17 10:43:54 2014 -0800

    Make sbt download an atomic operation

sbt/sbt

@@ -29,10 +29,11 @@ JAR=sbt/sbt-launch-${SBT_VERSION}.jar
 if [ ! -f ${JAR} ]; then
   # Download
   printf "Attempting to fetch sbt\n"
+  JAR_DL=${JAR}.part
   if hash curl 2>/dev/null; then
-    curl --progress-bar ${URL1} > ${JAR} || curl --progress-bar ${URL2} > ${JAR}
+    (curl --progress-bar ${URL1} > ${JAR_DL} || curl --progress-bar ${URL2} > ${JAR_DL}) && mv ${JAR_DL} ${JAR}
   elif hash wget 2>/dev/null; then
-    wget --progress=bar ${URL1} -O ${JAR} || wget --progress=bar ${URL2} -O ${JAR}
+    (wget --progress=bar ${URL1} -O ${JAR_DL} || wget --progress=bar ${URL2} -O ${JAR_DL}) && mv ${JAR_DL} ${JAR}
   else
     printf "You do not have curl or wget installed, please install sbt manually from http://www.scala-sbt.org/\n"
     exit -1