bug: prevent lightcontrol from crashing without values

[ALX99]

Bug

Running .local/bin/lightcontrol -d or .local/bin/lightcontrol -s without the required value crashes under Bash strict mode instead of reporting a usage error.

Severity and impact

This is a reliability bug in a user-facing brightness helper. Omitting a required value is a realistic command-line mistake. The script should fail before touching backlight state and tell the user how to call it; instead it exposes a shell unbound variable crash.

Evidence

Relevant files and lines on master:

• .local/bin/lightcontrol: line 4 enables Bash strict mode with set -eou pipefail.
• .local/bin/lightcontrol: lines 44-47 handle -d, run shift, then call delta_change "$1" without verifying that a value remains.
• .local/bin/lightcontrol: lines 49-52 handle -s, run shift, then call set_brightness "$1" without verifying that a value remains.

Observed incorrect behavior from the baseline pattern:

/tmp/lightcontrol: line 3: $1: unbound variable
exit:1

That is deterministic for both missing-value option paths: after shift, $1 is unset, and set -u aborts before the script can produce an intentional error.

Reproduce

Setup:

git checkout master

Triggering condition: pass -d or -s without its required value.

Exact commands:

.local/bin/lightcontrol -d
.local/bin/lightcontrol -s

Actual result on master:

.local/bin/lightcontrol: line 46: $1: unbound variable
.local/bin/lightcontrol: line 51: $1: unbound variable

Expected result:

usage lightcontrol [OPTIONS]

where:
    -h, --help   show this help text
    -c, --check  show the current brightness
    -d [DELTA]   change the current brightness by DELTA
    -s [VALUE]   set the brightness to VALUE

The commands should exit non-zero without reaching brightness reads or writes.

Root cause

The option handlers assume shift leaves an operand in $1. Under set -u, dereferencing $1 after a missing operand is a runtime error.

Fix

The fix adds a small require_value guard used by both -d and -s after shift. Missing operands now print the existing usage text and exit before any brightness calculation or write is attempted.

Validation

Commands run and results:

bash /tmp/dotlight/tests/lightcontrol-missing-value.test.sh
# passed

bash -n /tmp/dotlight/.local/bin/lightcontrol /tmp/dotlight/tests/lightcontrol-missing-value.test.sh
# passed

shellcheck /tmp/dotlight/.local/bin/lightcontrol /tmp/dotlight/tests/lightcontrol-missing-value.test.sh
# not run: shellcheck is not installed in this runtime

Regression test added:

• tests/lightcontrol-missing-value.test.sh verifies both -d and -s without values fail, do not print unbound variable, and do print the usage text.

Additional repository-scope check:

GitHub compare API: branch is 2 commits ahead and 0 behind master; only .local/bin/lightcontrol and tests/lightcontrol-missing-value.test.sh changed.

Scope

Intentionally not changed:

• brightness file paths,
• current brightness reporting,
• actual -d VALUE and -s VALUE behavior,
• sudo usage,
• option names or documented help text,
• any unrelated scripts or configs.