Files, folders, and the terminal

A file is one saved thing: a photo, a song, a page of notes. Its name usually ends in a dot and a few letters, like .jpg or .txt. That ending is called the extension, and it tells you what kind of file it is. Java source files end in .java. The ending is part of the name, so Hello.java and Hello.txt are two separate files. Java's tools only accept the one ending in .java.

A folder is a container for files. Folders can hold other folders, like boxes packed inside boxes. Some tools say directory instead; it means the same thing.

A path is a file's address, written as the list of folders you open to reach it. Tomas keeps his Java work on a Mac at /Users/tomas/java-course/ch01/Hello.java. Read it left to right: the Users folder, then Tomas's folder, then java-course, then ch01, and there is the file. Macs and Linux put a forward slash (/) between the names. Windows uses a backslash (\) and starts with a drive letter, as in C:\Users\tomas\....

A full address like that is absolute. It works from anywhere, like a complete postal address. A short address, like ch01/Hello.java, is relative. It's more like "second room on the left": it only makes sense once you know where you're standing.

The folder you're standing in is called the working directory. Two dots (..) mean "the folder one level up." A wavy line (~) means "my home folder," the one with your user name on it.

A file is a named block of data stored on a disk. Most file names end in an extension: a dot followed by a few letters, such as .java, .txt, or .class, which tells programs and people what kind of content the file holds. The extension is part of the name, so Hello.java and Hello.txt are two different files that can sit side by side. Java tools rely on it: the Java compiler accepts Hello.java and rejects the same text saved as Hello.txt.

A folder (also called a directory) holds files and other folders. Because folders nest, everything on a disk forms one tree. A path is the address of one file or folder in that tree: the folder names you pass through to reach it, joined by a separator. macOS writes /Users/tomas/java-course/ch01/Hello.java, and Linux writes /home/tomas/java-course/ch01/Hello.java. Windows uses backslashes and a drive letter: C:\Users\tomas\java-course\ch01\Hello.java. Those are absolute paths: they start at the top of the tree (or at a drive), so they name the same place no matter where you are.

A relative path, such as ch01/Hello.java, has no starting point of its own. It is read from the working directory, the folder a program or a terminal is "in" at that moment. Three short names help: . is the working directory itself, .. is the folder that contains it, and ~ is your home folder, the personal folder every account has.

The terminal is a window where you give the computer instructions by typing them. Each instruction is one line, called a command. You type it, press Enter, and the computer does it. The program inside the window that reads your typing is called the shell.

Every system has one. Windows has an app called Windows Terminal, and the shell inside it is PowerShell. A Mac has an app called Terminal. Linux desktops have one too, usually with "Terminal" in its name. They look a little different, but they work the same way.

A command starts with a verb. Words after it say what to act on; those extra words are called arguments. In mkdir ch01, the verb is mkdir, short for "make directory." The argument ch01 is the name of the new folder.

The terminal is always standing in one folder, its working directory. A brand-new terminal stands in your home folder. Four commands do almost everything you need at first:

• pwd answers "where am I?" by printing the folder's full address. The letters stand for "print working directory."
• ls answers "what's in here?" by listing files and folders. On Windows, dir does the same job.
• mkdir makes a new folder.
• cd means "change directory." It moves you into another folder. cd .. moves up one level, and cd ~ takes you home.

One surprise: when a command works, it often prints nothing at all. Silence means success. When something goes wrong, you get a message.

A terminal is a window where you work with the computer by typing instead of clicking. Inside it runs a shell, a program that reads each line you type, carries it out, and prints any result. Each line is a command: a command name, then (separated by spaces) any arguments, the extra words that tell the command what to act on. In mkdir ch01, mkdir is the command and ch01 is its argument. The default shells are PowerShell on Windows (inside the Windows Terminal app), zsh on macOS, and usually bash on Linux.

Four commands cover what this course needs at first, and all four work in all three shells:

• pwd prints the working directory as an absolute path.
• ls lists what the working directory contains (PowerShell also accepts dir).
• mkdir name makes a new folder inside the working directory.
• cd path changes the working directory to path, absolute or relative; cd .. moves up one level and cd ~ goes home.

A new terminal starts in your home folder. Commands that succeed often print nothing; a command that fails prints an error message. Nothing a terminal does is hidden: it acts on the same files and folders your file browser shows.