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- [Scott] When we talk about file metadata,

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we'll usually think first of things like EXIF data

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and authorship data,

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things we're accustomed to seeing in various software tools,

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like photo editors and word processing software.

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And as we've seen,

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this kind of metadata is important to understand.

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There's another kind of metadata we should be aware of

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when working with files, though.

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That's the metadata that's stored in the file system,

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outside of an actual files data.

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Before we explore this,

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let's take a moment to refresh ourselves

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about how files are stored on computers.

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A file is made up of information encoded into binary bits.

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These bits represent whatever the content of the file is,

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whether it's text, an image, a video, or whatever.

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The bits are organized into bytes,

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and the bytes are stored on disk

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in little areas called blocks on Unix and Linux type systems

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and clusters on Windows.

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Every file system, regardless of operating system,

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uses these fundamental little buckets of data

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to store files or groups of them called extends,

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and they can be of different sizes

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on different file systems,

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but these blocks, or clusters, or extends

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each have their own address.

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When we save a file on disk,

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its data takes up one or more little unit,

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and the addresses of those storage units

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are recorded in a data structure

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that associates them with other information about the file.

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Most importantly, this is where we associate a name,

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permissions, and creation and modification times

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with the data of a file.

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Depending on the file system,

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other metadata may be associated with files here,

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including ACLs or access control lists

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and extended attributes.

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On Linux and Unix type file systems,

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these records that contain file metadata

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and point to disk blocks are called inodes.

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On a Windows NTFS drive,

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these records are kept as entries in a structure

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called the MFT or master file table.

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We'll explore both of these approaches

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to storing filesystem metadata later in the course.

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Other file systems may use other structures,

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like a FAT or file allocation table

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to record basic information

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like the name and location of a file,

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but often they don't support richer metadata

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that modern file systems handle.

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Support for certain characters and file names

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and case sensitivity also vary across file systems.

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While all file systems support basic metadata,

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like name, size, and modification date

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and while modern file systems have rich metadata support,

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they don't all work in the same way.

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This means that copying or moving a file

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isn't quite as simple of an operation

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as it might appear to be.

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When a file is copied or moved,

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the copy operation includes the metadata

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from the file system or metadata store,

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along with the actual bytes of the file.

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The name, the modification dates, and so on

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are translated into whatever values can be stored

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in the destination file system.

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Though platform-specific data

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sometimes doesn't have a place to go

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and it temporarily get separated from its file

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or even lost entirely.

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As we'll see throughout the course,

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the loss of this metadata is often not a big problem

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because the bytes of the data itself are preserved

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when the file is moved or copied.

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However, metadata loss or cross-platform handling

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can have some odd effects unless you know what's going on.

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And in some situations,

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metadata is quite important,

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and so we need to consider carefully any action

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that might strip or otherwise separate it from its file.

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Because filesystem metadata

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is stored outside of the files it describes,

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we can change many of these metadata aspects of files

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without modifying the actual file data.

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That's what we do whenever we change permission modes

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or set ACLs.

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Those changes are made in the file system,

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not to the individual files themselves,

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and as a consequence,

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if we move or copy these files to a file system

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that doesn't support permissions, or flags, or whatever,

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that particular metadata can be lost.

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This can have security implications, as we'll discuss later.

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Okay, so it turns out the files

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are a little bit more complicated

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than they might seem at first,

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but the important part to think about here

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is that the files themselves, the bytes on the disk,

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don't include all this other metadata information

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that we, as humans, think about when we think about files.

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They're separate even though they're presented as one thing.

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The data structures, strategies,

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and features of different file systems

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are really interesting to learn about,

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but that's not the focus of this course.

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It's important to recognize

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that there are differences, though,

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as we'll see in the next videos.