Bash Mode

Reduce MCP tool bloat by exposing your tools as commands in a Bash interpreter.

Instead of registering dozens of MCP tools (each consuming context window space), register a single exec_bash tool that gives the LLM a Bash interpreter with your custom commands built in. The LLM composes commands using pipes, redirects, and standard Unix utilities -- getting the work done in fewer tool calls with a smaller tool definition.

Built on just-bash and the MCP TypeScript SDK v2.

Install

npm install mcp-bash-mode

Quickstart

A product catalog server that lets an AI agent query, filter, and transform data -- all through a single MCP tool:

import { McpServer } from "@modelcontextprotocol/server";
import { bashModeTool } from "mcp-bash-mode";

const products = [
  { id: 1, name: "Flux Capacitor", price: 199.99, category: "time-travel" },
  { id: 2, name: "Hoverboard", price: 499.99, category: "transport" },
  { id: 3, name: "Mr. Fusion", price: 89.99, category: "energy" },
  { id: 4, name: "Time Circuit", price: 299.99, category: "time-travel" },
  { id: 5, name: "DeLorean DMC-12", price: 42000.0, category: "transport" },
];

const server = new McpServer({ name: "catalog", version: "1.0.0" });

server.registerTool(
  "exec_bash",
  ...bashModeTool({
    commands: {
      products: {
        description: "Query the product catalog. Returns JSON array.",
        execute: async (args) => {
          if (args.includes("--help")) {
            return [
              "Usage: products",
              "",
              "Returns the full product catalog as a JSON array.",
              "Each product has the following fields:",
              "  id        number    Unique product ID",
              "  name      string    Product name",
              "  price     number    Price in USD",
              "  category  string    One of: time-travel, transport, energy",
            ].join("\n");
          }
          return JSON.stringify(products, null, 2);
        },
      },
    },
  }),
);

The LLM sees one tool (exec_bash) and can do things like:

# Discover the output format
products --help

# Get all products
products

# Find products under $200
products | jq '.[] | select(.price < 200)'

# List product names in the "time-travel" category, sorted
products | jq -r '.[] | select(.category == "time-travel") | .name' | sort

# Count products per category
products | jq -r '.[].category' | sort | uniq -c | sort -rn

Without mcp-bash-mode, each of these operations would typically require a separate MCP tool with its own schema.

API

bashModeTool(options?)

Returns a [config, callback] tuple designed to be spread into McpServer.registerTool():

server.registerTool("exec_bash", ...bashModeTool(options));

Options

Option	Type	Description
commands	Record<string, CommandConfig>	Custom commands to register. Each key is the command name.
description	string	Override the auto-generated tool description entirely.

CommandConfig

Field	Type	Description
description	string	One-line description shown to the LLM in the tool definition.
execute	(args: string[], ctx: CommandContext) => Promise<string | ExecResult>	Command implementation. Return a string (becomes stdout) or an ExecResult with stdout, stderr, and exitCode. Throwing an Error produces stderr output.

Exports

mcp-bash-mode re-exports CommandContext and ExecResult from just-bash for convenience.

License

MIT