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