---
license: apache-2.0
base_model: Qwen/Qwen2.5-Coder-0.5B-Instruct
library_name: transformers
pipeline_tag: text-generation
language:
  - en
tags:
  - bash
  - shell
  - cli
  - terminal
  - nl2bash
  - code-generation
  - gguf
  - qwen2
  - small-language-model
---

# 🐱 Kitty Bash LLM

**A 0.5B parameter shell assistant that runs on a laptop CPU.**

Kitty Bash LLM turns plain English into bash commands, repairs commands that just
failed, and completes half-typed ones. It is deliberately small — the 4-bit GGUF
is **398 MB** and generates a command in well under a second on an ordinary CPU,
so it can sit behind a shell integration without a GPU, an API key, or a network
round trip.

It is a **specialist, not a chatbot.** Ask it the capital of France and you will get
nonsense. Ask it to find every file over 100 MB and it will tell you.

---

## What it does

Three behaviours, selected by the system prompt. One model, one file, flat memory.

| task | you give it | it returns |
|------|-------------|------------|
| **`nl2cmd`** | `find all files larger than 100MB under /var` | `find /var -type f -size +100M` |
| **`fixcmd`** | `$ gerp -r 'TODO' .`<br>`bash: gerp: command not found` | `grep -r 'TODO' .` |
| **`complete`** | `tar -czf backup.` | `$(date +%F).tar.gz /path/to/dir` |

### System prompts

```python
NL2CMD   = ("You are a bash command generator. Given a natural language request, "
            "output only the bash command that accomplishes it. "
            "No explanation, no markdown fences.")

FIXCMD   = ("You fix broken bash commands. Given a failed command and its error "
            "output, output only the corrected command. No explanation.")

COMPLETE = ("You complete partially typed bash commands. Given a command prefix, "
            "output only the text that completes it. No explanation.")
```

---

## Quick start

### llama.cpp / GGUF (recommended — this is what it was built for)

```bash
llama-cli -m kitty-bash-llm-q4_k_m.gguf -st -t 4 --no-display-prompt -n 64 --temp 0 \
  -p "<|im_start|>system
You are a bash command generator. Given a natural language request, output only the bash command that accomplishes it. No explanation, no markdown fences.<|im_end|>
<|im_start|>user
show which process is listening on port 8080<|im_end|>
<|im_start|>assistant
"
# -> lsof -i :8080
```

For anything interactive, run a **persistent server** so the system prompt is
prefilled once (see *Performance* — this is the single most important
implementation detail):

```bash
llama-server -m kitty-bash-llm-q4_k_m.gguf -c 1024 -t 4 --port 8080
```

### transformers

```python
from transformers import AutoModelForCausalLM, AutoTokenizer

model = AutoModelForCausalLM.from_pretrained("sahellx/kitty-bash-llm")
tok   = AutoTokenizer.from_pretrained("sahellx/kitty-bash-llm")

SYSTEM = ("You are a bash command generator. Given a natural language request, "
          "output only the bash command that accomplishes it. "
          "No explanation, no markdown fences.")

msgs = [{"role": "system", "content": SYSTEM},
        {"role": "user",   "content": "compress the logs folder into a tar.gz archive"}]
ids = tok.apply_chat_template(msgs, add_generation_prompt=True, return_tensors="pt")
out = model.generate(ids, max_new_tokens=64, do_sample=False)
print(tok.decode(out[0][ids.shape[1]:], skip_special_tokens=True))
# -> tar -czvf logs.tar.gz logs/
```

Greedy decoding (`do_sample=False` / `--temp 0`) is recommended. You want the
most likely command, not a creative one.

---

## Evaluation

Measured against the **untuned base model** on a held-out split, with identical
prompts, identical f16 GGUF format, and the same llama.cpp engine — the only
variable is the fine-tuning. 100 examples per task.

### `nl2cmd`

| metric | base | **Kitty** | Δ |
|---|---|---|---|
| exact match | 7.00 | **23.00** | **+16.00** (3.3×) |
| first-utility accuracy | 61.00 | **79.00** | **+18.00** |
| token F1 | 39.35 | **59.77** | +20.42 |
| `bash -n` valid | 93.00 | **94.00** | +1.00 |
| ShellCheck clean | 77.00 | **82.00** | +5.00 |

### `fixcmd`

| metric | base | **Kitty** | Δ |
|---|---|---|---|
| exact match | 0.00 | **82.00** | +82.00 |
| first-utility accuracy | 1.00 | **99.00** | +98.00 |
| token F1 | 45.20 | **96.71** | +51.51 |
| ShellCheck clean | 63.00 | **83.00** | +20.00 |

> ⚠️ **Read this number sceptically.** The base model scores ~0 largely because it
> *ignores the output format* and explains the error in prose rather than emitting a
> command — so much of the gap is format compliance, not repair skill. The test set
> also uses synthetic corruptions of three kinds the model trained on. On **real**
> shell errors, hands-on testing put it closer to **1-in-3**. Treat 82% as an
> upper bound.

### `complete`

| metric | base | **Kitty** | Δ |
|---|---|---|---|
| token F1 | 2.22 | **29.88** | **+27.66** (13×) |
| exact match | 0.00 | 7.00 | +7.00 |
| prefix+completion valid | — | **99.29** | — |

> Exact match is a poor metric here — many completions are valid for any prefix.
> `bash -n` on the completion *fragment alone* is meaningless (a fragment like
> `--rm ubuntu` is not valid standalone bash); the meaningful figure is
> prefix + completion, at **99.29%**.

### Comparison with other shell-focused models

Alongside the base-model comparison above, an **informal** benchmark was run against
four other publicly available fine-tunes for shell/linux command generation, all at
Q4_K_M (except where a repo ships only f16), through llama.cpp on the same 2-thread
CPU, using the same 80 held-out prompts.

| model | params | size | utility acc | token F1 | s/cmd |
|---|---|---|---|---|---|
| **Kitty Bash LLM** | **0.5B** | **398 MB** | **78.75** | **59.54** | **1.68** |
| `louisguthmann/qwen3.5-2b-shellcommand-linux` | 2.0B | 1274 MB | 63.75 | 32.02 | 15.39 |
| `vitali87/shell-commands-qwen2-1.5b-extended` | 1.5B | 3094 MB | 57.50 | 33.93 | 9.48 |
| `mecha-org/linux-command-generator-llama3.2-1b` | 1.0B | 808 MB | 52.50 | 34.21 | 2.78 |
| `chamibuddhika/linux-commands-0407-00` | — | 1709 MB | — | — | — |

> #### ⚠️ Please read this before citing the table
>
> **This is indicative, not a controlled evaluation.** Four specific caveats, all of
> which favour this model:
>
> 1. **The test set is drawn from this model's own training distribution.** The split
>    is properly held out with zero leakage, but it comes from the same corpora and
>    shares their formatting conventions. The other models never saw that style. This
>    inflates exact match in particular, which is why exact match is omitted above.
> 2. **All models received *this* model's system prompt.** Peers trained with a
>    different prompt format are being evaluated off-distribution, which alone could
>    account for a large part of the gap.
> 3. **n = 80.** The 95% confidence interval is roughly ±9 points; the gap to the
>    runner-up is significant only marginally (p ≈ 0.03).
> 4. **`chamibuddhika/linux-commands-0407-00` returned empty output for every prompt**
>    and is reported as no-result rather than zero — the cause may well be the harness
>    rather than the model. `qwen3.5-shellcommand` emits `</think>` reasoning tags that
>    the harness did not strip, so its syntax-validity score was invalid and has been
>    omitted here.
>
> A properly controlled comparison would use a neutral test set none of the models
> trained on, each model's own prompt format, and n ≥ 500. Treat the ordering above as
> a hint worth verifying, not a settled result.
>
> The **base-model comparison** in the previous section does not carry these caveats:
> same architecture, same data, same harness, only the fine-tuning differs.

ShellCheck is used because `bash -n` only proves a command *parses*. ShellCheck
catches genuine defects — unquoted expansions (`SC2086`), word splitting
(`SC2046`) — that parse fine and then break on a filename with a space.

---

## Training

| | |
|---|---|
| base | [`Qwen/Qwen2.5-Coder-0.5B-Instruct`](https://huggingface.co/Qwen/Qwen2.5-Coder-0.5B-Instruct) |
| method | LoRA `r=64`, `alpha=64`, all attention + MLP projections, merged |
| precision | fp16 (Tesla T4, no bf16) |
| epochs / lr | 1 / `2e-4` cosine, 3% warmup |
| batch | 16 × 2 grad-accum = 32 effective |
| max seq len | 512 |
| loss | response-only (masked so loss lands on the command, never the prompt) |
| runtime | 41 min on a single T4 |
| final train / eval loss | 0.5579 / **0.5919** |

The base model was **chosen by measurement, not assumption**: a bake-off against
`Qwen2.5-0.5B-Instruct` under an identical recipe gave 84.98 vs 84.21 utility
accuracy, so the code-pretrained variant won.

---

## Dataset

~60,000 examples across the three tasks, pooled from seven sources, deduplicated,
rebalanced, and decontaminated.

| split | nl2cmd | fixcmd | complete | total |
|---|---|---|---|---|
| train | 22,573 | 19,891 | 17,528 | **59,992** |
| val | 1,254 | 1,238 | 1,456 | 3,948 |
| test | 1,254 | 1,219 | 1,409 | 3,882 |

**Sources:** [`AnishJoshi/nl2bash-custom`](https://huggingface.co/datasets/AnishJoshi/nl2bash-custom),
[`neulab/tldr`](https://huggingface.co/datasets/neulab/tldr),
[`emirkaanozdemr/bash_command_data_6K`](https://huggingface.co/datasets/emirkaanozdemr/bash_command_data_6K),
[`huytd189/command-line-suggestions`](https://huggingface.co/datasets/huytd189/command-line-suggestions),
[`aelhalili/bash-commands-dataset`](https://huggingface.co/datasets/aelhalili/bash-commands-dataset),
[`Jawajawa/command-linux-bash-balanced-sft`](https://huggingface.co/datasets/Jawajawa/command-linux-bash-balanced-sft),
[`PocketDoc/Dans-Toolmaxx-ShellCommands`](https://huggingface.co/datasets/PocketDoc/Dans-Toolmaxx-ShellCommands).

### Decontamination — please read this if you benchmark on nl2bash

The most widely used NL2Bash split on the Hub **leaks badly**. Measured directly:

```
test  prompts also present in train : 71.0 %
dev   prompts also present in train : 71.8 %
duplicate rows within train         : 39.5 %  (19,658 rows → 11,890 unique)
```

Training and evaluating on those splits measures memorisation. Every published
number here comes from splits rebuilt from scratch: all rows pooled, deduplicated
on the normalised prompt, re-split 90/5/5 with a fixed seed, and asserted in code
to have **zero** train↔test overlap.

Two further corrections:

- **`find` rebalanced 54% → 30%.** In the source corpus more than half of all
  targets were `find`, which made the model a `find` specialist that produced
  nonsense for `tar` and `du`. Capping it fixed both.
- **Gold targets filtered through `bash -n`.** Broken targets teach broken bash.
- **`fixcmd` and `complete` derived *after* the split**, from within each split
  only — so a command in test can never appear in train under a different task label.

---

## Limitations

Findings from hands-on testing, not guesses.

**Weak on multi-step logic.** It picks the right tool but can get the reasoning wrong:

```
"print the 3rd column of a csv where the 1st column equals ERROR"
  → cut -d',' -f3 data.csv | grep ERROR          ✗ filters the wrong column
  correct: awk -F, '$1=="ERROR" {print $3}'

"show the 10 biggest directories under /home sorted by size"
  → find /home -type d | sort -n -r | head -10   ✗ sorts names, not sizes
  correct: du -h /home/* | sort -rh | head -10
```

**Uneven across utilities.** Strong on `find`, `tar`, `docker`, `sed`, `grep`, `lsof`.
Weaker on less common subcommands (`git branch --merged` came out as gibberish).

**`fixcmd` is much weaker on real errors than the benchmark suggests** — see the
warning above.

**`complete` may duplicate a prefix boundary.** Given `docker run -it --` it can
return `--rm ...`. **Strip the overlap between prefix and completion in your client.**

**Catastrophic forgetting — by design.** General ability is gone. This is a bash
tool, not an assistant.

**No safety behaviour whatsoever.** Asked to delete the root filesystem it emits
the command immediately, with no warning. See below.

---

## ⚠️ Safety

**This model will generate destructive commands without hesitation.**

```
"delete every file in the root filesystem"  →  find / -exec rm {} \;
```

It has no refusal training and no notion of danger. Any tool built on it **must**:

1. **Never auto-execute.** Print the command; require an explicit keypress.
2. **Pattern-match destructive commands** — `rm -rf`, `dd`, `mkfs`, `> /dev/sd*`,
   `chmod -R 777`, `curl … | sh` — and warn loudly.
3. **Treat output as untrusted.** It is a suggestion from a 0.5B model, not an
   authority.

Verify commands before running them. Especially ones touching `/`.

---

## Performance

Measured with llama.cpp, Q4_K_M, on **2 CPU threads** (deliberately weak):

| | |
|---|---|
| prompt processing | 42.8 tok/s |
| generation | 17.1 tok/s |
| 40-token system prompt prefill | 935 ms |
| ~12-token command generation | 701 ms |
| **cold request** | **1636 ms** |
| **warm (KV cache reused)** | **701 ms** |

**Implication: run a persistent server.** Over half of a cold request is spent
re-processing the same system prompt. Spawning a process per invocation costs
~1.6 s; a warm `llama-server` costs ~700 ms. On a typical 8-core machine expect
roughly 3–4× faster (~200 ms).

For inline autosuggestion, that is still too slow to run on every keystroke.
**Debounce (~250 ms), fire asynchronously, and cancel in-flight requests** — the
Copilot pattern. History-based matching should handle the common case; use the
model for what history has never seen.

---

## Files

| file | size | use |
|---|---|---|
| `kitty-bash-llm-q4_k_m.gguf` | 398 MB | **recommended** — CPU inference |
| `kitty-bash-llm-q5_k_m.gguf` | 420 MB | slightly higher quality |
| `kitty-bash-llm-q8_0.gguf` | 531 MB | near-lossless |
| `kitty-bash-llm-f16.gguf` | 994 MB | full precision GGUF |
| `model.safetensors` | 988 MB | transformers / further fine-tuning |

---

## Reproducing this

Every step is a script, not a notebook cell: data pooling and decontamination,
the base-model bake-off, training, GGUF conversion, and the evaluation harness
(including the ShellCheck pass). The `find` rebalancing and the leakage assertions
are the parts worth copying if you build on the NL2Bash corpora.

## Citation

```bibtex
@misc{kitty-bash-llm,
  title  = {Kitty Bash LLM: a 0.5B shell assistant for CPU inference},
  author = {sahellx},
  year   = {2026},
  url    = {https://huggingface.co/sahellx/kitty-bash-llm}
}
```

Built on [Qwen2.5-Coder](https://huggingface.co/Qwen/Qwen2.5-Coder-0.5B-Instruct)
(Apache-2.0), trained with [Unsloth](https://github.com/unslothai/unsloth),
quantized with [llama.cpp](https://github.com/ggml-org/llama.cpp).