المسارات: المطلق والنسبي والرئيسي و. و..

Written Explanation

The folder tree and the current folder

Files live in folders, and folders live inside other folders. The result is a tree. At the very top is the root. On Linux and macOS there is one root, written /. On Windows each drive has its own root, such as C:\.

Whenever you work in a terminal, the shell has a current folder (also called the working directory). It is the folder you are "standing in". Many commands use it silently: if you say "list files" without naming a folder, you get the files in the current folder.

Your home folder is the folder that belongs to your user account. It is where a new terminal usually starts. Typical locations are /home/ana on Linux, /Users/ana on macOS, and C:\Users\Ana on Windows. The exact place depends on your system.

Here is the tree we will use for the examples (Linux style; on Windows, imagine C:\Users\Ana in place of /home/ana):

/home/ana
├── Documents
│   ├── notes.txt
│   └── School Work
│       └── essay.txt
└── projects
    └── shop
        └── README.md

Where am I? pwd

pwd stands for "print working directory". It prints the current folder as a full path.

$ pwd
/home/ana

PowerShell's equivalent is Get-Location. It also has the alias pwd, so pwd works in both shells.

PS> Get-Location

Path
----
C:\Users\Ana

Make pwd a reflex. Before any command that changes or deletes things, check where you are.

What is here? ls, dir, Get-ChildItem

ls lists the contents of a folder in Bash. PowerShell's real command is Get-ChildItem; ls and dir are aliases for it, so all three work in PowerShell on Windows. (On PowerShell for Linux or macOS, which aliases exist can differ, so if ls misbehaves, use Get-ChildItem.)

$ ls
Documents  projects

With no argument, the command lists the current folder. Give it a path and it lists that folder instead: ls Documents. In Bash, ls -l shows a detailed list with sizes and dates; Lesson 3 uses it more.

Moving: cd

cd means "change directory". Give it a path and your current folder becomes that folder.

$ cd Documents
$ pwd
/home/ana/Documents

cd prints nothing on success. If the path does not exist, you get an error and stay where you were.

Absolute paths

An absolute path starts at the root and spells out every folder on the way. It means the same thing no matter where you are.

• Linux/macOS: /home/ana/Documents/notes.txt. It begins with /.
• Windows: C:\Users\Ana\Documents\notes.txt. It begins with a drive letter and \.

Windows separates folders with a backslash \; Linux and macOS use a forward slash /. PowerShell accepts both on Windows, but Bash accepts only /. In Bash, the backslash has a different job (it is an escape character), so do not type Windows-style paths there.

Use an absolute path when you want certainty, for example in a script that must work from any folder.

Relative paths

A relative path does not begin at the root. It starts from the current folder. If you are in /home/ana, then Documents/notes.txt means /home/ana/Documents/notes.txt. The same text typed from somewhere else would mean something different, or nothing at all.

How do you tell them apart? An absolute path starts with / (Linux, macOS) or a drive letter (Windows). Everything else is relative.

Relative paths are short and convenient inside a project, but they depend on where you are. That is why pwd matters.

The shortcuts: ~, ., and ..

Three tiny symbols do a lot of work.

The shell (Bash) expands ~ to your home folder; PowerShell understands ~ as home as well when you use it in a path. If you are unsure in your PowerShell version, type $HOME instead, which holds the home folder.

You can chain .. to climb as many levels as you like. From /home/ana/projects/shop, the folder Documents is at ../../Documents: up to projects, up to ana, then down into Documents.

The . is easy to overlook, but it matters. You will meet it again in Lesson 6, where running a script in the current folder is written ./script.sh.

The same file, two ways

From /home/ana/projects/shop:

• Absolute: /home/ana/Documents/notes.txt.
• Relative: ../../Documents/notes.txt.

Both name the same file. If you move to another folder, the absolute path still works while the relative one must be rewritten.

Paths with spaces

The shell uses spaces to separate words, so this fails:

$ cd School Work

The shell thinks School and Work are two arguments. Wrap the path in quotation marks so it stays one word:

$ cd "School Work"

Single quotes also work in both shells. A second approach, in Bash, is to put a backslash before each space: School\ Work. In PowerShell, the escape character is the backtick, not the backslash. You can also press Tab to autocomplete, which usually adds the quoting or escape for you.

When you are naming your own folders, avoid spaces and use short, lowercase names such as school-work. Programmers do this to avoid the problem.

Reading an error

If the path is wrong you will see a message such as No such file or directory in Bash or Cannot find path ... because it does not exist in PowerShell. Do not guess. Run pwd and ls, and look at what is actually there. The exact wording differs between shells and versions.