Command Execution

Overview

Execute shell commands inside sandboxes using the exec() method. Commands run via /bin/sh -c, supporting pipes, variables, and shell features.

Prerequisites

Before executing commands, ensure you have:

Installed the Sandbox SDK (see Quickstart)
Created a sandbox instance
Basic familiarity with shell commands and async/await in JavaScript/TypeScript

Basic execution

// Simple command
const result = await sandbox.exec("echo 'Hello World'");
console.log(result.stdout);    // "Hello World\n"
console.log(result.exitCode);  // 0
console.log(result.success);   // true

Shell features

Commands support full shell syntax:

// Pipes and redirection
await sandbox.exec("cat file.txt | grep error > errors.log");

// Multiple commands
await sandbox.exec("npm install && npm test");

// Variables and globs
await sandbox.exec("for f in *.txt; do echo $f; done");

Options

Timeout

Set a maximum execution time (max 60 seconds):

const result = await sandbox.exec("npm install", {
  timeoutMs: 30_000  // 30 seconds
});

Max per-command timeout is 60 seconds. For long-running servers, use background: true and redirect output to a log file (for example, >/tmp/server.log 2>&1).

Environment variables

Pass environment variables to individual commands using the env option:

// Single environment variable
const result = await sandbox.exec("echo $MY_VAR", {
  env: { MY_VAR: "hello world" }
});

// Multiple environment variables
await sandbox.exec("node app.js", {
  env: {
    NODE_ENV: "production",
    PORT: "3000",
    API_KEY: "secret-key-12345",
    DEBUG: "true"
  }
});

// Use in shell commands
await sandbox.exec("echo Database: $DB_URL", {
  env: { DB_URL: "postgresql://localhost:5432/mydb" }
});

Environment variables are merged with the sandbox’s existing environment, with your custom values taking precedence:

// Start a Node.js server with environment configuration
await sandbox.exec("node server.js >/tmp/server.log 2>&1", {
  background: true,
  env: {
    NODE_ENV: "production",
    PORT: "8080",
    DATABASE_URL: "postgres://user:pass@db:5432/app"
  }
});

// Run Python script with custom environment
await sandbox.exec("python app.py", {
  env: {
    PYTHONPATH: "/app/lib",
    FLASK_ENV: "development",
    SECRET_KEY: "my-secret-key"
  }
});

Environment variables are only set for the specific command execution. To persist environment variables across multiple commands, either:

Pass the same env object to each exec() call
Use shell export syntax: export VAR=value && command
Write them to a file and source it: source /tmp/env.sh && command

Working directory

Set a persistent working directory for commands:

sandbox.cwd("/app/project");

// Subsequent commands run in /app/project
await sandbox.exec("bun install");
await sandbox.exec("bun test");

// Override for a single command
await sandbox.exec("ls", { cwd: "/tmp" });

// Get current directory
console.log(sandbox.getCwd());  // "/app/project"

Working directory persists across commands and affects exec() and file operations.

Standard input

Pass data to stdin:

await sandbox.exec("cat > /tmp/input.txt", {
  stdin: "line 1\nline 2\nline 3\n"
});

// Or pipe data
await sandbox.exec("python process.py", {
  stdin: JSON.stringify({ data: [1, 2, 3] })
});

Background processes

Run long-lived processes without blocking:

// Start server in background
await sandbox.exec("python server.py >/tmp/server.log 2>&1", {
  background: true
});

// Command returns immediately
// Server continues running

Real-time output streaming

Stream command output with callbacks:

await sandbox.exec("npm install && npm test", {
  onStdout: (line) => console.log(line),
  onStderr: (line) => console.error(line)
});

// Stream logs from background process
await sandbox.exec("python server.py 2>&1", {
  background: true,
  onStdout: (line) => {
    if (line.includes("ERROR")) {
      console.error("Server error:", line);
    }
  }
});

Callbacks receive output line-by-line as commands execute.

Managing background processes

Check if a process is running:

// Find PID
const pid = await sandbox.exec("pgrep -f server.py || echo 'not running'");
console.log("PID:", pid.stdout.trim());

// Check with ps
const ps = await sandbox.exec("ps aux | grep server.py | grep -v grep");
console.log(ps.stdout);

Read logs from background processes:

// Tail recent logs
const tail = await sandbox.exec("tail -n 100 /tmp/server.log");
console.log(tail.stdout);

// Follow logs (with timeout)
const follow = await sandbox.exec("timeout 5 tail -f /tmp/server.log", {
  timeoutMs: 6_000
});

Stop a background process:

// Kill by name
await sandbox.exec("pkill -f server.py");

// Kill by PID
await sandbox.exec("kill 1234");

// Force kill
await sandbox.exec("pkill -9 -f server.py");

Error handling

const result = await sandbox.exec("ls /nonexistent");

if (!result.success) {
  console.error("Command failed:");
  console.error("Exit code:", result.exitCode);
  console.error("Error output:", result.stderr);
}

Common patterns

Install dependencies and run

await sandbox.exec("npm install", { timeoutMs: 60_000 });
await sandbox.exec("npm test", {
  timeoutMs: 30_000,
  env: { NODE_ENV: "test", CI: "true" }
});

Start server and wait for readiness

// Start server
await sandbox.exec("node server.js >/tmp/server.log 2>&1", {
  background: true
});

// Wait for port to be ready
for (let i = 0; i < 10; i++) {
  const check = await sandbox.exec("nc -z localhost 3000 && echo ready || echo waiting");
  if (check.stdout.includes("ready")) {
    break;
  }
  await new Promise(r => setTimeout(r, 1000));
}

const host = sandbox.expose(3000);
console.log(`Server ready at https://${host}`);

Capture output to file

// Run and capture both stdout and stderr
await sandbox.exec("python script.py > /tmp/output.log 2>&1");

// Read the output
const output = await sandbox.files.read("/tmp/output.log");
console.log(output);

Capture stderr for debugging

To capture error messages when commands fail silently, redirect stderr:

// Redirect stderr to stdout and capture to file
await sandbox.exec("python script.py 2>&1 | tee /tmp/output.log");

// Read captured output
const logs = await sandbox.files.read("/tmp/output.log");
console.log(logs);

Conditional execution

// Run only if file exists
await sandbox.exec("test -f /app/config.json && node app.js || echo 'Config missing'");

// Chain with error handling
await sandbox.exec("npm test || (echo 'Tests failed' && exit 1)");

Using environment variables with background processes

Combine env with background for configurable long-running services:

// Start a web server with custom configuration
await sandbox.exec("node server.js >/tmp/server.log 2>&1", {
  background: true,
  env: {
    NODE_ENV: "production",
    PORT: "3000",
    API_KEY: process.env.MY_API_KEY, // Pass through from your environment
    DATABASE_URL: "postgresql://localhost/mydb",
    LOG_LEVEL: "info"
  }
});

// Give the server time to start
await new Promise(r => setTimeout(r, 2000));

// Expose the port
const host = sandbox.expose(3000);
console.log(`Server running at https://${host}`);

// Check server logs if needed
const logs = await sandbox.exec("tail -n 50 /tmp/server.log");
console.log(logs.stdout);

Best practices

Set timeouts: Always specify timeoutMs for potentially long operations
Use env option: Prefer the env option over inline export commands for cleaner, more maintainable code
Redirect output: For background processes, redirect to log files (>/tmp/log 2>&1)
Check success: Always check result.success before assuming command worked
Use background for servers: Don’t block on long-running processes
Clean up processes: Kill background processes before terminating sandbox
Escape carefully: Be mindful of shell escaping when passing user input
Secure secrets: Use the env option to pass sensitive data instead of embedding it in command strings

Limitations

Max timeout per command: 60 seconds
Background processes continue until sandbox stops or you kill them
No interactive TTY support
Commands run as root inside the container

Getting Started

Core Concepts

Reference

Overview

Prerequisites

Basic execution

Shell features

Options

Timeout

Environment variables

Working directory

Standard input

Background processes

Real-time output streaming

Managing background processes

Error handling

Common patterns

Install dependencies and run

Start server and wait for readiness

Capture output to file

Capture stderr for debugging

Conditional execution

Using environment variables with background processes

Best practices

Limitations

Getting Started

Core Concepts

Reference

​Overview

​Prerequisites

​Basic execution

​Shell features

​Options

​Timeout

​Environment variables

​Working directory

​Standard input

​Background processes

​Real-time output streaming

​Managing background processes

​Error handling

​Common patterns

​Install dependencies and run

​Start server and wait for readiness

​Capture output to file

​Capture stderr for debugging

​Conditional execution

​Using environment variables with background processes

​Best practices

​Limitations

Overview

Prerequisites

Basic execution

Shell features

Options

Timeout

Environment variables

Working directory

Standard input

Background processes

Real-time output streaming

Managing background processes

Error handling

Common patterns

Install dependencies and run

Start server and wait for readiness

Capture output to file

Capture stderr for debugging

Conditional execution

Using environment variables with background processes

Best practices

Limitations