MCP Shell Server

A secure shell command execution server implementing the Model Context Protocol (MCP). This server allows remote execution of whitelisted shell commands with support for stdin input.

Features

• Argv-based Command Execution: Allowed commands run via subprocess argv without shell-string interpretation
• Standard Input Support: Pass input to commands via stdin
• Comprehensive Output: Returns stdout, stderr, exit status, and execution time
• Safe Pipeline Support: Pipelines preserve and validate argv segments instead of invoking a shell
• Execution Limits: Server-side default timeout, maximum timeout, and output byte caps are enforced
• Contained Redirection: <, >, and >> targets must stay inside the requested working directory
• Minimal Child Environment: Child processes receive a small allowlisted environment instead of inheriting all server secrets
• Structured Audit Logging: Success, rejection, timeout, output-cap, and process-error outcomes are logged with redaction

MCP client setting in your Claude.app

Published version

code ~/Library/Application\ Support/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "shell": {
      "command": "uvx",
      "args": [
        "mcp-shell-server"
      ],
      "env": {
        "ALLOW_COMMANDS": "ls,cat,pwd,grep,wc,touch,find"
      }
    },
  }
}

Local version

Configuration

code ~/Library/Application\ Support/Claude/claude_desktop_config.json

{
  "mcpServers": {
    "shell": {
      "command": "uv",
      "args": [
        "--directory",
        ".",
        "run",
        "mcp-shell-server"
      ],
      "env": {
        "ALLOW_COMMANDS": "ls,cat,pwd,grep,wc,touch,find"
      }
    },
  }
}

Installation

Installing via Smithery

To install Shell Server for Claude Desktop automatically via Smithery:

npx -y @smithery/cli install mcp-shell-server --client claude

Manual Installation

pip install mcp-shell-server

Usage

Starting the Server

ALLOW_COMMANDS="ls,cat,echo" uvx mcp-shell-server
# Or using the alias
ALLOWED_COMMANDS="ls,cat,echo" uvx mcp-shell-server

The ALLOW_COMMANDS (or its alias ALLOWED_COMMANDS ) environment variable specifies which commands are allowed to be executed. Commands can be separated by commas with optional spaces around them.

Valid formats for ALLOW_COMMANDS or ALLOWED_COMMANDS:

ALLOW_COMMANDS="ls,cat,echo"          # Basic format
ALLOWED_COMMANDS="ls ,echo, cat"      # With spaces (using alias)
ALLOW_COMMANDS="ls,  cat  , echo"     # Multiple spaces

ALLOW_PATTERNS can be used for comma-separated regular expressions that match command names. Each pattern is applied with full-match semantics, so ALLOW_PATTERNS="ls" allows only the command name ls and does not allow lsof or ls -la. Patterns and command names containing whitespace or shell metacharacters are rejected; do not use ALLOW_PATTERNS to describe shell command strings or argument-level policies.

ALLOW_PATTERNS="python[0-9.]*,node"    # Command-name patterns only

Allowlisting a command name is not a sandbox for that program's own argument-level execution features. The server applies default argument hardening even when the binary is allowed: known exec-capable vectors such as find -exec, shell/interpreter launchers, awk system(), tar --checkpoint-action=exec, env, xargs, and git alias external commands are rejected before subprocess creation. For example, ALLOW_COMMANDS="git" does not permit git -c alias.pwn=!sh -c "touch marker" pwn; the git alias.<name>=!<cmd> exec form is rejected by default.

Child process environment

Commands run with an isolated child environment. The server does not pass the full parent process environment to child commands, so unrelated variables such as API tokens, credentials, and SECRET_TOKEN are absent by default.

By default the child environment contains only the minimal launch keys needed for command execution: PATH on POSIX systems, plus Windows process-launch keys when applicable (COMSPEC, PATHEXT, SYSTEMROOT, and WINDIR).

Use MCP_SHELL_CHILD_ENV_ALLOWLIST to explicitly allow additional environment variable names to be inherited from the parent process or accepted from per-command environment overrides. The allowlist is comma-separated and uses exact environment variable names:

MCP_SHELL_CHILD_ENV_ALLOWLIST="LANG,LC_ALL,MY_TOOL_HOME" \
ALLOW_COMMANDS="printenv,my-tool" \
uvx mcp-shell-server

Only keys named in MCP_SHELL_CHILD_ENV_ALLOWLIST are forwarded. Secret-like names are treated defensively in logs and should not be allowlisted unless you intentionally want a child command to read that secret.

Structured audit logs

Each command invocation emits one mcp-shell-server.audit log event named shell_execution_audit. Audit records cover successful execution, validation rejection before subprocess creation, timeout, output-cap termination, and process errors including subprocess creation failures.

Audit metadata includes:

• timestamp, duration, and result_type
• command name and redacted argv
• resolved working directory
• redirection flags for stdin/stdout/stdout append
• redacted per-call environment override metadata, when supplied
• effective timeout and output_limit
• stdout_bytes and stderr_bytes
• return_code when available
• rejection_reason or error_type where applicable

Audit logs intentionally do not include raw stdout or stderr bodies. Secret-like argv and environment names or values containing markers such as SECRET, TOKEN, PASSWORD, PASSWD, API_KEY, ACCESS_KEY, PRIVATE_KEY, KEY, CREDENTIAL, or AUTH are replaced with [REDACTED]. Long non-numeric values are represented by a short SHA-256 digest instead of the raw value.

Request Format

The directory argument is optional. If omitted, commands run in the MCP server process current working directory (server process CWD). Relative directory values are resolved from that same server process CWD. This base is not the MCP client CWD; it is the working directory of the process that launched mcp-shell-server.

# Basic command execution in the server process CWD
{
    "command": ["ls", "-l"]
}

# Command with a relative working directory resolved from the server process CWD
{
    "command": ["pwd"],
    "directory": "subproject"
}

# Command with stdin input
{
    "command": ["cat"],
    "stdin": "Hello, World!"
}

# Command with timeout
{
    "command": ["long-running-process"],
    "timeout": 30  # Maximum execution time in seconds
}

# Command with working directory and timeout
{
    "command": ["grep", "-r", "pattern"],
    "directory": "/path/to/search",
    "timeout": 60
}

Response Format

Successful response:

{
    "stdout": "command output",
    "stderr": "",
    "status": 0,
    "execution_time": 0.123
}

Error response:

{
    "error": "Command not allowed: rm",
    "status": 1,
    "stdout": "",
    "stderr": "Command not allowed: rm",
    "execution_time": 0
}

Security

The server implements several security measures, but it is not an OS sandbox. A command-name allowlist reduces accidental exposure, but allowed binaries may still read accessible files, consume CPU, or perform behavior allowed by the operating system. For hostile workloads, run the server inside an external sandbox such as a container, VM, or OS policy boundary.

1. Command Whitelisting: Only explicitly allowed command names or full-matching ALLOW_PATTERNS entries can be executed.
2. Default Argument Hardening: Known exec-capable vectors such as shells/interpreters, env, xargs, find -exec, awk system(), tar --checkpoint-action=exec, and git external aliases are rejected by default even when the command name is allowlisted.
3. No Shell-String Execution: Normal commands and pipelines are executed with asyncio.create_subprocess_exec(*argv); user-controlled strings are not passed to a shell.
4. Contained Redirection: Redirection paths must be relative to directory; absolute paths, .. traversal, and symlink escapes are rejected before files are opened.
5. Environment Isolation: Children receive a minimal environment plus names listed in MCP_SHELL_CHILD_ENV_ALLOWLIST. Parent secrets such as tokens are not inherited by default. Per-call envs values are only accepted for explicitly allowlisted names.
6. Execution Limits: MCP_SHELL_DEFAULT_TIMEOUT_SECONDS defaults to 30 seconds, MCP_SHELL_MAX_TIMEOUT_SECONDS defaults to 300 seconds, and MCP_SHELL_OUTPUT_LIMIT_BYTES defaults to 1 MiB per captured stdout/stderr stream. Client timeouts are clamped to the server maximum; omitted timeouts receive the default. Processes that time out or exceed the output cap are terminated and reaped before an explicit timeout/output-cap error is returned.
7. Audit Logging: Each invocation emits structured audit metadata for success, rejection, timeout, output cap, and process error outcomes. Secret-like argv values are redacted; stdout/stderr content is not logged.

Security-related environment variables

Variable	Default	Description
ALLOW_COMMANDS / ALLOWED_COMMANDS	empty	Comma-separated command names to allow
ALLOW_PATTERNS	empty	Comma-separated regex patterns matched with fullmatch() against command names
MCP_SHELL_DEFAULT_TIMEOUT_SECONDS	30	Timeout used when the client omits timeout
MCP_SHELL_MAX_TIMEOUT_SECONDS	300	Maximum effective timeout accepted from clients
MCP_SHELL_OUTPUT_LIMIT_BYTES	1048576	Maximum captured stdout/stderr bytes per process
MCP_SHELL_CHILD_ENV_ALLOWLIST	empty	Comma-separated parent or per-call environment variables allowed in children
MCP_SHELL_SAFE_PATH	/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin	PATH supplied to children

Development

Setting up Development Environment

1. Clone the repository

git clone https://github.com/yourusername/mcp-shell-server.git
cd mcp-shell-server

2. Install dependencies including test requirements

pip install -e ".[test]"

Running Tests

pytest

API Reference

Request Arguments

Field	Type	Required	Description
command	string[]	Yes	Command and its arguments as array elements
stdin	string	No	Input to be passed to the command
directory	string	No	Working directory; omitted uses the server process CWD, and relative paths resolve from that server process CWD
timeout	integer	No	Maximum execution time in seconds

Response Fields

Field	Type	Description
stdout	string	Standard output from the command
stderr	string	Standard error output from the command
status	integer	Exit status code
execution_time	float	Time taken to execute (in seconds)
error	string	Error message (only present if failed)

Requirements

• Python 3.11 or higher
• mcp>=1.1.0

License

MIT License - See LICENSE file for details