How to specify a port to run a create-react-app based project?

If you don't want to set the environment variable, another option is to modify the scripts part of package.json from:

"start": "react-scripts start"

to

Linux (tested on Ubuntu 14.04/16.04) and MacOS (tested by @aswin-s on MacOS Sierra 10.12.4):

"start": "PORT=3006 react-scripts start"

or (may be) more general solution by @IsaacPak

"start": "export PORT=3006 react-scripts start"

Windows @JacobEnsor solution

"start": "set PORT=3006 && react-scripts start"

cross-env lib works everywhere. See Aguinaldo Possatto answer for details

Update due to the popularity of my answer: Currently I prefer to use environment variables saved in .env file(useful to store sets of variables for different deploy configurations in a convenient and readable form). Don't forget to add *.env into .gitignore if you're still storing your secrets in .env files. Here is the explanation of why using environment variables is better in the most cases. Here is the explanation of why storing secrets in environment is bad idea.

Here is another way to accomplish this task.

Create a .env file at your project root and specify port number there. Like:

PORT=3005

Create a file with name .env in the main directory besidespackage.json and set PORT variable to desired port number.

For example:

.env

PORT=4200

You can find the documentation for this action here: https://create-react-app.dev/docs/advanced-configuration

You could use cross-env to set the port, and it will work on Windows, Linux and Mac.

yarn add -D cross-env

then in package.json the start link could be like this:

"start": "cross-env PORT=3006 react-scripts start",