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Start command

The start command lets you specify a process that runs at the end of the template build — not when a sandbox is created. During the build, E2B executes the start command, waits for the ready command to confirm the process is up, and then takes a snapshot of the entire sandbox including the running process. When you later create a sandbox from that template, the snapshotted process is already running — there is no startup wait. This is how you get servers, seeded databases, or any long-running process available instantly when spawning sandboxes with the SDK.
The start command runs once during template build and is captured in a snapshot. It does not re-execute each time you create a sandbox. If you need to run a command every time a sandbox is created, use sandbox.commands.run() after creating the sandbox instead.This also means that environment variables passed to Sandbox.create() are not available to the start command process — it already ran during the build. If your start command needs environment variables, set them in the template definition using setEnvs() / set_envs().
You can see the full build process here.

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Ready command

The ready command allows you to specify a command that will determine template sandbox readiness before a snapshot is created. It is executed in an infinite loop until it returns a successful exit code 0. This way you can control how long should we wait for the start command or any system state.

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Usage

Set the start command (executed during template build) and the ready command (determines when the process is up before snapshotting): 
// Set both start command and ready command
template.setStartCmd('npm start', waitForPort(3000))

// Set custom start and ready command
template.setStartCmd('npm start', 'curl -s -o /dev/null -w "200"')

// Set only ready command
template.setReadyCmd(waitForTimeout(10_000))
The ready command is used to determine when the sandbox is ready to accept connections.

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Ready command helpers

The SDK provides helper functions for common ready command patterns: 
import {
  waitForPort,
  waitForProcess,
  waitForFile,
  waitForTimeout,
} from 'e2b'

// Wait for a port to be available
waitForPort(3000)

// Wait for a process to be running
waitForProcess('node')

// Wait for a file to exist
waitForFile('/tmp/ready')

// Wait for a timeout
waitForTimeout(10_000) // 10 seconds