common.sh: new function for a selection menu with a default option.

Defined a bash function choose(), which allows to present a menu and
prompting for a selection, with support for default choice. Usage is
commented appropriately.

Originally reviewed in CL I0d2f20dc8d62ce5fa18c10d9f8b51a46b2ddca5d.

BUG=chromium-os:26010
TEST=Tested in conjunction with a use in image_to_usb.sh, works fine.

Change-Id: I53a42a46a3c90fd486fead578bfbae248f64cfc2
Reviewed-on: https://gerrit.chromium.org/gerrit/15586
Reviewed-by: Gilad Arnold <garnold@chromium.org>
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Ready: Gilad Arnold <garnold@chromium.org>

common.sh

@@ -776,3 +776,67 @@ function enable_strict_sudo {
     `type -P sudo` -n "$@"
   }
 }
+
+# Selection menu with a default option: this is similar to bash's select
+# built-in, only that in case of an empty selection it'll return the default
+# choice. Like select, it uses PS3 as the prompt.
+#
+# $1:   name of variable to be assigned the selected value; it better not be of
+#       the form choose_foo to avoid conflict with local variables.
+# $2:   default value to return in case of an empty user entry.
+# $3:   value to return in case of an invalid choice.
+# $...: options for selection.
+#
+# Usage example:
+#
+#  PS3="Select one [1]: "
+#  choose reply "foo" "ERROR" "foo" "bar" "foobar"
+#
+# This will present the following menu and prompt:
+#
+#  1) foo
+#  2) bar
+#  3) foobar
+#  Select one [1]:
+#
+# The return value will be stored in a variable named 'reply'. If the input is
+# 1, 2 or 3, the return value will be "foo", "bar" or "foobar", respectively.
+# If it is empty (i.e. the user clicked Enter) it will be "foo".  Anything else
+# will return "ERROR".
+function choose() {
+  typeset -i choose_i=1
+
+  # Retrieve output variable name and default return value.
+  local choose_reply=$1
+  local choose_default="$2"
+  local choose_invalid="$3"
+  shift 3
+
+  # Select a return value
+  unset REPLY
+  if [ $# -gt 0 ]; then
+    # Actual options provided, present a menu and prompt for a choice.
+    local choose_opt
+    for choose_opt in "$@"; do
+      echo "$choose_i) $choose_opt"
+      choose_i=choose_i+1
+    done
+    read -p "$PS3"
+  fi
+  # Filter out strings containing non-digits.
+  if [ "${REPLY}" != "${REPLY%%[!0-9]*}" ]; then
+    REPLY=0
+  fi
+  choose_i="${REPLY}"
+
+  if [ $choose_i -ge 1 -a $choose_i -le $# ]; then
+    # Valid choice, return the corresponding value.
+    eval ${choose_reply}="${!choose_i}"
+  elif [ -z "${REPLY}" ]; then
+    # Empty choice, return default value.
+    eval ${choose_reply}="${choose_default}"
+  else
+    # Invalid choice, return corresponding value.
+    eval ${choose_reply}="${choose_invalid}"
+  fi
+}