diff --git a/programs/demo_common.sh b/programs/demo_common.sh
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+## 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"
+##
+## Start with a "msg" call that explains the purpose of the script.
+## Then call the "depends_on" function to ensure that all config
+## dependencies are met.
+##
+## 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%/*}"
+# Find a nice path to the root directory, avoiding unnecessary "../".
+# The code supports demo scripts nested up to 4 levels deep.
+# The code works no matter where the demo script is relative to the current
+# directory, even if it is called with a relative path.
+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
+
+## $programs_dir is the directory containing the sample programs.
+# Assume an in-tree build.
+programs_dir="$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 "+ ! $*"
+  not "$@"
+}
+
+## config_has SYMBOL...
+## Succeeds if the library configuration has all SYMBOLs set.
+config_has () {
+  for x in "$@"; do
+    "$programs_dir/test/query_compile_time_config" "$x"
+  done
+}
+
+## depends_on SYMBOL...
+## Exit if the library configuration does not have all SYMBOLs set.
+depends_on () {
+  m=
+  for x in "$@"; do
+    if ! config_has "$x"; then
+      m="$m $x"
+    fi
+  done
+  if [ -n "$m" ]; then
+    cat >&2 <<EOF
+$0: this demo requires the following
+configuration options to be enabled at compile time:
+ $m
+EOF
+    # Exit with a success status so that this counts as a pass for run_demos.py.
+    exit
+  fi
+}
+
+## Add the names of files to clean up to this whitespace-separated variable.
+## The file names must not contain whitespace characters.
+files_to_clean=
+
+## Call this function at the end of each script.
+## It is called automatically if the script is killed by a signal.
+cleanup () {
+  rm -f -- $files_to_clean
+}
+
+
+
+################################################################
+## End of the public interfaces. Code beyond this point is not
+## meant to be called directly from a demo script.
+
+trap 'cleanup; trap - HUP; kill -HUP $$' HUP
+trap 'cleanup; trap - INT; kill -INT $$' INT
+trap 'cleanup; trap - TERM; kill -TERM $$' TERM
+
+if config_has MBEDTLS_ENTROPY_NV_SEED; then
+  # Create a seedfile that's sufficiently long in all library configurations.
+  # This is necessary for programs that use randomness.
+  # Assume that the name of the seedfile is the default name.
+  files_to_clean="$files_to_clean seedfile"
+  dd if=/dev/urandom of=seedfile ibs=64 obs=64 count=1
+fi