2. The Terminal#

Why it matters#

The terminal (also called the command line, shell, or console) is a text window where you type commands instead of clicking buttons. It can look old-fashioned, but FRC programmers use it constantly:

Git is a terminal program. The VS Code buttons just run terminal commands for you.
Building and deploying with Gradle works from the terminal, and the terminal shows errors more clearly than the buttons do.
At competition, when the VS Code buttons misbehave and the queue is calling your team, typing ./gradlew deploy is the fastest way to get code on the robot.

Read this#

Walkthrough#

Opening a terminal#

The easiest way is inside VS Code: Terminal → New Terminal, or Ctrl+` (the key left of 1). It opens at the bottom of the window, already inside your project folder.

Which terminal you get depends on your computer:

Computer	Default terminal	Notes
Windows	PowerShell	Most commands below work. Use `gradlew.bat` instead of `./gradlew` if `./gradlew` fails.
Mac	zsh	Everything below works as written
Linux	bash	Everything below works as written

Reading the prompt#

Before the blinking cursor, the terminal shows a prompt. It tells you where you are:

PS C:\Users\kedrick\robotics\my-first-robot>        ← Windows PowerShell
kedrick@laptop my-first-robot %                      ← Mac

The folder you're "in" is the current directory. Commands act on files there unless you say otherwise.

Moving around#

Command	What it does	Example
`pwd`	Print the folder you're in	`pwd`
`ls`	List files and folders here	`ls`
`cd FOLDER`	Move into a folder	`cd src`
`cd ..`	Go up one level	`cd ..`
`cd ~`	Go to your home folder	`cd ~`
`mkdir NAME`	Make a new folder	`mkdir robotics`
`code .`	Open the current folder in VS Code	`code .`

Two habits that save you time

Tab completion: type the first few letters of a file or folder, then press Tab, and the terminal finishes the name. Press it twice to see options.

History: press Up to bring back the last command. Keep pressing to go further back.

Paths#

A path is the address of a file.

A relative path starts from where you are: src/main/java
An absolute path starts from the top of the drive: C:\Users\kedrick\robotics or /Users/kedrick/robotics
. means "this folder" and .. means "the folder above"

Spaces in names

A folder called My Robot must be typed in quotes: cd "My Robot". Name your project folders without spaces to avoid this.

Gradle: building from the terminal#

Every WPILib project includes a Gradle wrapper, gradlew, in the project's top folder. Run these from that folder:

Command	What it does
`./gradlew build`	Compile the code and run the unit tests (Lesson 16)
`./gradlew deploy`	Build, then copy the code to the roboRIO and restart it
`./gradlew simulateJava`	Build and start the simulator
`./gradlew test`	Run only the unit tests
`./gradlew clean`	Delete old build output. Try this when a build acts strangely.

On Windows, if ./gradlew gives an error, use .\gradlew.bat build or gradlew build.

Deploying needs a connection to the robot

Your laptop must be connected to the robot's network (Wi-Fi or USB/Ethernet) for deploy to find the roboRIO. With team number 4674 set, Gradle finds it automatically.

Reading a build error#

A failed build prints a lot. Here's what to look for:

> Task :compileJava FAILED
/home/kedrick/my-first-robot/src/main/java/frc/robot/subsystems/Intake.java:14: error: ';' expected
        motor.set(0.75)
                       ^
1 error

FAILURE: Build failed with an exception.
...
BUILD FAILED in 3s

Scroll up to the first line that says error:. The bottom is just a summary.
It names the file (Intake.java) and line (14).
It says what's wrong (';' expected).
The ^ points at the exact spot.

Fix the first error and build again. One mistake often causes a pile of later errors that disappear once the first is fixed.

Common errors and what they mean#

Error message	Usual meaning
`';' expected`	Missing semicolon at the end of a line
`cannot find symbol`	A typo in a name, or a missing `import`
`incompatible types`	Passing the wrong type, like a `double` where a `Command` is expected
`class X is public, should be declared in a file named X.java`	The class name and file name don't match
`Could not find roboRIO`	The laptop isn't connected to the robot, or the team number is wrong

Let VS Code help

Red squiggly lines show many errors before you build. Hover over one to see the message, and use the lightbulb icon to add a missing import.

Try it#

Open a terminal in VS Code and run pwd and ls
Use cd to walk into src/main/java/frc/robot, then cd .. back out to the project folder
Run ./gradlew build and get BUILD SUCCESSFUL
Delete a ; in RobotContainer.java, build, and find the file and line number in the error. Put the ; back.
Run ./gradlew simulateJava to start the simulator from the terminal

Check yourself#

How do you get to the folder above the one you're in?

cd ..

What's the difference between a relative and an absolute path?

A relative path starts from your current folder (src/main). An absolute path starts from the top of the drive (C:\Users\... or /Users/...) and works from anywhere.

Where in the output is the real cause of a build failure?

At the first error: line, which names the file and line number. The lines at the bottom are just a summary.

What does ./gradlew deploy do?

It builds the code, copies it to the roboRIO over the robot's network, and restarts the robot program.