Fix "unbound variable" error when running terraform with no args

lib/tfenv-exec.sh

@@ -29,7 +29,7 @@ function realpath-relative-to() {
 export -f realpath-relative-to;
 
 function tfenv-exec() {
-  for _arg in "${@}"; do
+  for _arg in ${@+"$@"}; do
     if [[ "${_arg}" == -chdir=* ]]; then
       chdir="${_arg#-chdir=}";
       log 'debug' "Found -chdir arg: ${chdir}";

test/test_shim.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+
+# Source common test setup
+source "$(dirname "${0}")/test_common.sh";
+
+#####################
+# Begin Script Body #
+#####################
+
+declare -a errors=();
+
+log 'info' '### Test Suite: terraform shim';
+
+log 'info' '## terraform shim: passes through to terraform (without args)';
+cleanup || log 'error' 'Cleanup failed?!';
+(
+  tfenv install 1.6.1 || exit 1;
+  tfenv use 1.6.1 || exit 1;
+  declare output;
+  output="$(terraform 2>&1 || true)";
+  # https://github.com/tfutils/tfenv/issues/532
+  echo "${output}" | grep -q 'unbound variable' && exit 1;
+  echo "${output}" | grep -q 'Usage: terraform' || exit 1;
+) && log 'info' '## terraform (no args): passed' \
+  || error_and_proceed 'terraform with no args crashed or did not show usage';
+
+log 'info' '## terraform shim: passes through to terraform (with args)';
+(
+  tfenv install 1.6.1 || exit 1;
+  tfenv use 1.6.1 || exit 1;
+  declare output;
+  output="$(terraform version -json)" || exit 1;
+  echo "${output}" | grep -q 'terraform_version' || exit 1;
+  echo "${output}" | grep -q '1.6.1' || exit 1;
+) && log 'info' '## terraform shim: arguments are passed through: passed' \
+  || error_and_proceed 'terraform arguments were not passed through';
+
+finish_tests 'shim';
+
+exit 0;