Fix #532: terraform shim fails with "@: unbound variable" on Bash 3.2 when run with no arguments

.github/instructions/bash.instructions.md

@@ -84,7 +84,10 @@ set -uo pipefail;
    word-splitting
 3. **Double-quoted traps:** `trap "rm ${var}" EXIT` expands at definition time.
    Use functions or single quotes.
-4. **`$@` in for-loops:** Always quote: `for arg in "$@"`
+4. **`$@` in for-loops:** Use `for arg; do`, which iterates `"$@"` without
+   expanding it. Unquoted `$@` word-splits, and quoted `"$@"` is an unbound
+   variable under `set -u` on Bash 3.2 (macOS `/bin/bash`) when there are
+   no arguments.
 5. **Regex anchoring:** `^1.1` matches `1.10.x` because `.` is a regex
    wildcard. Use `^1\.1\.` for exact prefix matching.
 

AGENTS.md

@@ -300,7 +300,10 @@ These are the most frequent sources of bugs. Check for them in every change:
    conditions, and assignments. Unquoted variables cause word-splitting.
 3. **Double-quoted traps:** `trap "rm ${var}" EXIT` expands at definition
    time and is vulnerable to word-splitting. Use functions or single quotes.
-4. **`$@` in for-loops:** Always quote: `for arg in "$@"`.
+4. **`$@` in for-loops:** Use `for arg; do`, which iterates `"$@"` without
+   expanding it. Unquoted `$@` word-splits, and quoted `"$@"` is an unbound
+   variable under `set -u` on Bash 3.2 (macOS `/bin/bash`) when there are
+   no arguments.
 5. **Regex anchoring:** `^1.1` matches `1.10.x` because `.` is a regex
    wildcard. Use `^1\.1\.` for exact prefix matching.
 6. **Cross-platform differences:** macOS uses BSD `sed`/`grep`/`readlink`.

lib/tfenv-exec.sh

@@ -29,7 +29,10 @@ function realpath-relative-to() {
 export -f realpath-relative-to;
 
 function tfenv-exec() {
-  for _arg in "${@}"; do
+  # `for _arg; do` iterates "$@" without expanding it. Bash 3.2 (macOS /bin/bash)
+  # treats "$@" as unbound under `set -u` when there are no arguments, and
+  # zsh's bashcompinit runs `complete -C` commands with no arguments (#532).
+  for _arg; do
     if [[ "${_arg}" == -chdir=* ]]; then
       chdir="${_arg#-chdir=}";
       log 'debug' "Found -chdir arg: ${chdir}";

test/test_exec.sh

@@ -0,0 +1,50 @@
+#!/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 (tfenv-exec)';
+
+cleanup || log 'error' 'Cleanup failed?!';
+tfenv install 1.6.1 || early_death 'Failed to install 1.6.1 for shim tests';
+tfenv use 1.6.1 || early_death 'Failed to use 1.6.1 for shim tests';
+
+# zsh's bashcompinit runs `complete -C` commands with no arguments, and Bash 3.2
+# (macOS /bin/bash) treats "$@" as unbound under `set -u` when it is empty (#532).
+# Run the shim under each available bash so the system bash is exercised on macOS
+# even when a newer bash is first on PATH.
+for shell in bash /bin/bash; do
+  command -v "${shell}" >/dev/null 2>&1 || continue;
+  log 'info' "## terraform (no args) under ${shell}: passes through to terraform usage";
+  (
+    declare output;
+    output="$("${shell}" "${TFENV_ROOT}/bin/terraform" 2>&1 || true)";
+    echo "${output}" | grep -q 'unbound variable' && exit 1;
+    echo "${output}" | grep -q 'Usage: terraform' || exit 1;
+  ) && log 'info' "## terraform (no args) under ${shell}: passed" \
+    || error_and_proceed "terraform with no args under ${shell} did not reach terraform usage";
+done;
+
+log 'info' '## terraform -chdir with a space in the path: argument is not word-split';
+cleanup || log 'error' 'Cleanup failed?!';
+(
+  echo '1.6.1' > .terraform-version;
+  mkdir -p 'chdir-dir/with space';
+  echo '1.6.0' > 'chdir-dir/with space/.terraform-version';
+  declare output;
+  output="$(terraform '-chdir=chdir-dir/with space' version 2>&1)" || exit 1;
+  echo "${output}" | grep -q 'Terraform v1.6.0' || exit 1;
+) && log 'info' '## terraform -chdir with a space in the path: passed' \
+  || error_and_proceed 'terraform -chdir with a space in the path did not resolve the version from that directory';
+
+cleanup || log 'error' 'Cleanup failed?!';
+
+finish_tests 'exec';
+
+exit 0;