Added libensure

This provides the ensure::set function as well as the ERRNO_ENSURE
variable. The function checks if a variable is set, and returns code 1
if unset, or code 0 if set. It also distinguishes between unset and
empty (empty is considered set).

The ERRNO_ENSURE variable is set to 1 when an unset variable is passed
to ensure::set. This variable is useful for checking a list of variables
without the need to catch every return code (just check ERRNO_ENSURE for
code 1 at the end of the variable sequence).

1 files changed, 24 insertions, 0 deletions

diff --git a/libinstall/ensure.sh b/libinstall/ensure.sh
new file mode 100644
index 0000000..f703087
--- /dev/null
+++ b/libinstall/ensure.sh
@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+export ERRNO_ENSURE=0
+
+# ensures::set:
+# @ref Reference to the variable to be checked
+# @msg Message to print if variable is unset
+#
+# Ensures that the specified variable is set. If varible is unset, the
+# specified variable description is printed out.
+#
+# Note: An empty variable will return as 'set'.
+#
+# Returns 0 when set and 1 when unset
+ensure::set() {
+  local _ref="${1:-}"
+  local _msg="${2:-}"
+  if [ -z "${!_ref+x}" ]; then
+    printf -- "Error: Required variable '%s' unset.\n" "${_ref}" >&2
+    printf -- "       %s: %s\n" "${_ref}" "${_msg}" >&2
+    ERRNO_ENSURE=1
+    return 1
+  fi
+  return 0
+}