just-bash

name	just-bash
description	Sandboxed bash execution using just-bash from Vercel Labs. Use when you need to run shell commands safely without modifying the real filesystem, process data files, or test scripts in an isolated sandbox with read-only FS, no network, and in-memory writes.
allowed-tools	Bash(just-bash:*)

Just Bash Skill

Overview

Use this skill when you need to:

• Run bash commands safely without modifying the real filesystem
• Process and transform data files (CSV, JSON, YAML, text)
• Test shell scripts in an isolated sandbox
• Run exploratory commands where mistakes can't cause damage
• Execute complex pipelines (grep, awk, sed, jq, xan, rg)
• Validate scripts before running them for real

When to use just-bash vs the regular Bash tool:

• just-bash: Exploratory work, data processing, script testing, untrusted input, bulk file operations where you want safety
• Regular Bash: git operations, npm/pip commands, running servers, anything needing network or real writes

Prerequisites

# Install globally
npm install -g just-bash

# Or use via npx (no install needed)
npx just-bash -c 'echo hello'

Core Usage

Run Inline Commands

# Simple command
just-bash -c 'ls -la'

# Multi-line script
just-bash -c '
for f in *.ts; do
  echo "$(wc -l < "$f") $f"
done | sort -rn | head -10
'

Run Script Files

just-bash ./scripts/analyze.sh

Pipe Scripts from Stdin

echo 'find . -name "*.ts" | head -5' | just-bash

Key Options

--root <path>      # Mount a specific directory (default: cwd)
--cwd <path>       # Set working directory inside sandbox
--allow-write      # Enable in-memory writes (writes don't touch real FS)
--python           # Enable python3 commands
--json             # Output as JSON: {"stdout", "stderr", "exitCode"}
-e, --errexit      # Exit on first error

Security Model

• Read-only by default - reads real files via OverlayFS, blocks writes
• No network access - cannot curl, wget, or make connections
• No escape - sandboxed to the root directory
• In-memory writes - with --allow-write, writes go to memory only, not disk
• Real filesystem is mounted at /home/user/project

Available Commands (75+)

Text Processing

awk sed grep egrep fgrep rg cut tr sort uniq wc head tail tac rev nl fold expand unexpand column comm join paste split strings

Data Formats

jq (JSON - older build, lacks -R/-s flags) xan (CSV) html-to-markdown

Broken in v1.0.0

yq (YAML - "Dynamic require of process" error) sqlite3 (SQL - "DataView constructor" error)

File Operations

ls find cat cp mv rm mkdir rmdir ln touch chmod stat file tree du basename dirname readlink

Compression

gzip gunzip zcat tar

Checksums

md5sum sha1sum sha256sum base64

Utilities

date seq expr env printenv whoami hostname sleep timeout time which xargs tee diff

Workflow Patterns

Pattern 1: Data Analysis Pipeline

# Analyze a CSV file with xan
just-bash -c '
xan headers data.csv
xan count data.csv
xan frequency data.csv -s status
' --root /path/to/project

Pattern 2: JSON Processing

# Transform JSON data with jq
just-bash -c '
cat api-response.json | jq ".items[] | {name: .name, count: .total}" | head -20
'

Pattern 3: Codebase Exploration

# Find patterns across a project (safe, read-only)
just-bash -c '
echo "=== File counts by extension ==="
find . -type f | sed "s/.*\.//" | sort | uniq -c | sort -rn | head -10

echo "=== TODO/FIXME comments ==="
rg -c "TODO|FIXME" --type ts 2>/dev/null | sort -t: -k2 -rn | head -10

echo "=== Largest files ==="
find . -type f -name "*.ts" | xargs wc -l 2>/dev/null | sort -rn | head -10
'

Pattern 4: Script Testing with In-Memory Writes

# Test a script that writes files - nothing touches disk
just-bash --allow-write -c '
mkdir -p /tmp/output
for f in *.json; do
  jq ".version = \"2.0\"" "$f" > "/tmp/output/$f"
done
ls -la /tmp/output/
cat /tmp/output/package.json
'

Pattern 5: Text Processing Pipeline

# Combine awk, sed, sort for analysis
just-bash -c '
echo "=== Functions per file ==="
rg -c "function " --type ts 2>/dev/null | sort -t: -k2 -rn | head -10

echo "=== Lines by extension ==="
find . -type f -not -path "./.git/*" | sed "s/.*\.//" | sort | uniq -c | sort -rn | head -10
' --root /path/to/project

Pattern 6: JSON Output for Programmatic Use

# Get structured output
just-bash --json -c 'echo "hello world"'
# Returns: {"stdout":"hello world\n","stderr":"","exitCode":0}

Tips for AI Usage

1. Default to just-bash for exploration - when reading/analyzing files, use the sandbox for safety
2. Use --json flag when you need to parse the output programmatically
3. Use --allow-write for temp files - writes stay in memory, safe to experiment
4. Chain with pipes - all standard Unix pipelines work (grep | sort | uniq -c | head)
5. Use --root to scope the sandbox to a specific project directory
6. Combine tools - jq for JSON, xan for CSV, rg for search, awk/sed for transforms
7. Test destructive scripts safely - rm, mv, overwrites all happen in memory with --allow-write

Limitations

• No network access (no curl, wget, npm, git, pip, etc.)
• No persistent writes (in-memory only, lost when command exits)
• No interactive commands (no vim, nano, less, etc.)
• No package managers or language runtimes (except python with --python flag)
• No system administration commands (no sudo, systemctl, etc.)
• No process substitution (<() syntax not supported)
• jq is an older build - -R (raw input) and -s (slurp) flags are missing
• yq is broken in v1.0.0 ("Dynamic require of process" error)
• sqlite3 is broken in v1.0.0 ("DataView constructor" error)
• tree lacks some flags (e.g. --dirsfirst)