Bootstrap with -a shell crashes with "component: unbound variable" under set -u

[Zordrak]

Summary

Running --bootstrap with -a shell aborts with component: unbound variable
under set -u, because component is only declared in the non-bootstrap branch
but the shell action references it bare at
bin/terraform.sh:936.

Root Cause

set -uo pipefail is enabled at bin/terraform.sh:8.

component is declared readonly only in the non-bootstrap branch:

bin/terraform.sh:316-326

if [ "${bootstrap}" == 'true' ]; then
  [ -n "${component_arg}" ] \
    && error_and_die 'The --bootstrap parameter and the -c/--component parameter are mutually exclusive';
  ...
else
  [ -n "${component_arg}" ] \
    || error_and_die 'Required argument missing: -c/--component';
  readonly component="${component_arg}";   # only set here
  ...
fi;

In bootstrap mode, -c is explicitly rejected, so component is intentionally
never set. But the shell action prints it bare:

bin/terraform.sh:936

echo -e "Here's a shell for the ${component} component. ...";

Under set -u, this aborts before the shell is ever launched.

This is the same root-cause class as #50 (bootstrap-mode reference to a variable
that is only declared in the non-bootstrap branch) — there it was
${environment}, here it is ${component}.

Evidence

Minimal reproduction of the mechanism (mirrors the conditional declaration then
a bootstrap-mode bare reference):

$ bash -c 'set -uo pipefail
bootstrap="true"; component_arg=""
[ "${bootstrap}" == "true" ] || readonly component="${component_arg}"
echo "before component use"
echo "component is ${component}"'
before component use
bash: line 1: component: unbound variable
# exit code 127

Expected: either a working shell for the bootstrap component, or a clear
error_and_die explaining that shell is not supported with --bootstrap.

Actual: aborts with component: unbound variable.

Reproduction Steps

bin/terraform.sh \
  -p myproject \
  -r eu-west-2 \
  --bootstrap \
  -a shell \
  -- -input=false

(The -- -input=false is required only to work around #49.) With valid AWS
credentials the run proceeds to the shell case and aborts at
bin/terraform.sh:936 with
line 936: component: unbound variable.

Suggested Fix

This is a niche path, so the fixer should decide the intended semantics:

1. Preferred — make the message tolerant of bootstrap mode. Use
   ${component:-bootstrap} (or ${component_name}, which is set by
   bin/terraform.sh:443 to basename of the component
   path and equals bootstrap in bootstrap mode) in the line-936 message, so a
   bootstrap shell works.
2. Or explicitly reject it. If a shell into the bootstrap component is not a
   supported workflow, add an early error_and_die when
   bootstrap == 'true' && action == 'shell'.

Note that ${component_name} is already the safe, always-set equivalent used
elsewhere (e.g. the backend filename at
bin/terraform.sh:680), so switching the message to it
is low-risk.

Affected Platforms

All platforms and all bash versions (verified on bash 5.1.16).

Related

• Bootstrap without -e crashes with "environment: unbound variable" under set -u #50 — identical root-cause class (${environment} unset in bootstrap mode).
• Unbound variable access breaks deployment #49, Uninitialised tf_var_file_paths array crashes with "unbound variable" when no tfvars files exist #51 — sibling set -u regressions from the v2.3.0 hardening. All four
  are best addressed together as a single "complete the set -u hardening"
  pass.

[Zordrak]

Tracked under #53 — [Tracking] Complete the set -u hardening. See the tracker for the recommended unified fix ordering across all four set -u regressions.