fix(interpreter): arithmetic errors report and fail instead of giving 0

[chaliy]

Requested by Михайло · project thread

What changed

Arithmetic evaluation now has an error channel instead of silently producing 0.

• $((1/0)), $((1%0)), $((08)) and $((2#12)) print bash's message and abort the command with status 1.
• (( )) and let print the same message with their own ((: / let: prefix, fail with status 1, and let the script continue, as bash does.
• Valid literals are unchanged: 0x1f, 010, 16#ff, 10#08, 2#101, 64#@.

Why

A script computing $((total/count)) with count=0 got 0 and carried on with a wrong number and a zero exit status. Debian-oracle cases arith-errors and integer-overflow-and-base.

Before / After

Script	Before	After (matches bash)
echo $((1/0)); echo after	0, then after, status 0	bash: line 1: 1/0: division by 0 (error token is "0"), status 1, after not run
echo $((08))	0	bash: line 1: 08: value too great for base (error token is "08")
((1/0)); echo $?	1 with no message	bash: line 1: ((: 1/0: division by 0 ... then 1
let "y=1/0"; echo $?	1 with no message	bash: line 1: let: y=1/0: division by 0 ... then 1

New spec file arith-errors.test.sh (4 cases), verified against bash 5.2.

Risk

Medium-low. Scripts that relied on a silent 0 from a division by zero now fail, which is the point. The error carries over &self in a Mutex<ArithError> on the interpreter (it must stay Sync), locked only on the arithmetic path. An empty right operand, which the operator scan can produce inside a signed literal like -9223372036854775808 / -1, is not treated as a zero divisor, so those paths keep their old values. Error lines read bash: line N: rather than naming the script, since the interpreter does not track the running script's path.

Checklist

• Failing spec first, verified against real bash
• cargo test -p bashkit --lib, --test integration, bash_comparison
• cargo clippy --all-targets -D warnings, cargo fmt --check
• Recursion stack probe: 2020 KB (main 2028-2036 KB)
• knowledge/log.md updated

https://claude.ai/code/session_019aFikmptPc91Fj4N2iDXQA

---

Generated by Claude Code

[cloudflare-workers-and-pages]

Deploying with    Cloudflare Workers

The latest updates on your project. Learn more about integrating Git with Workers.

Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	bashkit	a32e2de	Commit Preview URL Branch Preview URL	Oct 07 2026, 11:23 PM