diff --git a/programs/demo_common.sh b/programs/demo_common.sh
new file mode 100644
index 0000000..91b33b9
--- /dev/null
+++ b/programs/demo_common.sh
@@ -0,0 +1,89 @@
+## Common shell functions used by demo scripts programs/*/*.sh.
+
+## How to write a demo script
+## ==========================
+##
+## Include this file near the top of each demo script:
+##   . "${0%/*}/../demo_common.sh"
+##
+## As the last thing in the script, call the cleanup function.
+##
+## You can use the functions and variables described below.
+
+set -e -u
+
+## $root_dir is the root directory of the Mbed TLS source tree.
+root_dir="${0%/*}"
+n=4 # limit the search depth
+while ! [ -d "$root_dir/programs" ] || ! [ -d "$root_dir/library" ]; do
+  if [ $n -eq 0 ]; then
+    echo >&2 "This doesn't seem to be an Mbed TLS source tree."
+    exit 125
+  fi
+  n=$((n - 1))
+  case $root_dir in
+    .) root_dir="..";;
+    ..|?*/..) root_dir="$root_dir/..";;
+    ?*/*) root_dir="${root_dir%/*}";;
+    /*) root_dir="/";;
+    *) root_dir=".";;
+  esac
+done
+
+## $programs_dir is the directory containing the sample programs.
+programs_dir="$root_dir/programs"
+
+## msg LINE...
+## msg <TEXT_ORIGIN
+## Display an informational message.
+msg () {
+  if [ $# -eq 0 ]; then
+    sed 's/^/# /'
+  else
+    for x in "$@"; do
+      echo "# $x"
+    done
+  fi
+}
+
+## run "Message" COMMAND ARGUMENT...
+## Display the message, then run COMMAND with the specified arguments.
+run () {
+    echo
+    echo "# $1"
+    shift
+    echo "+ $*"
+    "$@"
+}
+
+## Like '!', but stop on failure with 'set -e'
+not () {
+  if "$@"; then false; fi
+}
+
+## run_bad "Message" COMMAND ARGUMENT...
+## Like run, but the command is expected to fail.
+run_bad () {
+  echo
+  echo "$1 This must fail."
+  shift
+  echo "+ ! $*"
+  not "$@"
+}
+
+## Add the names of files to clean up to this whitespace-separated variable.
+## The file names must not contain whitespace characters.
+files_to_clean=
+
+
+
+################################################################
+## End of the public interfaces. Code beyond this point is not
+## meant to be called directly from a demo script.
+
+cleanup () {
+  rm -f -- $files_to_clean
+}
+trap 'cleanup; trap - HUP; kill -HUP $$' HUP
+trap 'cleanup; trap - INT; kill -INT $$' INT
+trap 'cleanup; trap - TERM; kill -TERM $$' TERM
diff --git a/programs/psa/key_ladder_demo.sh b/programs/psa/key_ladder_demo.sh
index e21d1ab..0186183 100755
--- a/programs/psa/key_ladder_demo.sh
+++ b/programs/psa/key_ladder_demo.sh
@@ -15,36 +15,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-set -e -u
+. "${0%/*}/../demo_common.sh"
 
-program_name="key_ladder_demo"
-program="${0%/*}/$program_name"
-files_to_clean=
+msg <<'EOF'
+This script demonstrates the use of the PSA cryptography interface to
+create a master key, derive a key from it and use that key to wrap
+the derived key using an AEAD algorithm.
+EOF
 
-if [ ! -e "$program" ]; then
-    # Look for programs in the current directory and the directories above it
-    for dir in "." ".." "../.."; do
-        program="$dir/programs/psa/$program_name"
-        if [ -e "$program" ]; then
-            break
-        fi
-    done
-    if [ ! -e "$program" ]; then
-        echo "Could not find $program_name executable"
-
-        echo "If building out-of-tree, this script must be run" \
-             "from the project build directory."
-        exit 1
-    fi
-fi
-
-run () {
-    echo
-    echo "# $1"
-    shift
-    echo "+ $*"
-    "$@"
-}
+program="${0%/*}"/key_ladder_demo
 
 if [ -e master.key ]; then
     echo "# Reusing the existing master.key file."
@@ -68,7 +47,7 @@
     cmp input.txt hello_world.txt
 
 files_to_clean="$files_to_clean hellow_orld.txt"
-! run "Derive a different key and attempt to unwrap the data. This must fail." \
+run_bad "Derive a different key and attempt to unwrap the data. This must fail." \
   "$program" unwrap master=master.key input=hello_world.wrap output=hellow_orld.txt label=hellow label=orld
 
 files_to_clean="$files_to_clean hello.key"
@@ -79,5 +58,4 @@
     "$program" unwrap master=hello.key label=world \
                input=hello_world.wrap output=hello_world.txt
 
-# Cleanup
-rm -f $files_to_clean
+cleanup