travis-build.sh: validate Jupyter notebooks

This is a first attempt at Travis executing Jupyter notebooks,
when the jupyter tool is present in the environment.

If any notebooks have cell execution errors, the build should fail.

This will be useful e.g. for imagej/tutorials#53.

travis-build.sh

@@ -122,4 +122,29 @@ EOL
 	echo travis_fold:end:travis-build.sh-maven
 fi
 
+# Execute Jupyter notebooks.
+if which jupyter >/dev/null 2>/dev/null
+then
+	echo travis_fold:start:travis-build.sh-jupyter
+	echo "= Jupyter notebooks ="
+	# NB: This part is fiddly. We want to loop over files even with spaces,
+	# so we use the "find ... -print0 | while read $'\0' ..." idiom.
+	# However, that runs the piped expression in a subshell, which means
+	# that any updates to the success variable will not persist outside
+	# the loop. So we suppress all stdout inside the loop, echoing only
+	# the final value of success upon completion, and then capture the
+	# echoed value back into the parent shell's success variable.
+	success=$(find . -name '*.ipynb' -print0 | {
+		while read -d $'\0' nbf
+		do
+			echo 1>&2
+			echo "== $nbf ==" 1>&2
+			jupyter nbconvert --execute --stdout "$nbf" >/dev/null
+			checkSuccess $?
+		done
+		echo $success
+	})
+	echo travis_fold:end:travis-build.sh-jupyter
+fi
+
 exit $success