Advanced Programming in the UNIX Environment

Week 3: umask(2)

In this video lecture, we'll take a look at what file ownership and permissions are applied when we create a new file. In the process, we'll learn about the concept of a process's "umask", and look at how the 'umask' shell built-in is implemented.

https://youtu.be/bRAR2bv2HSM

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Advanced Programming in the UNIX Environment

An illustration of how permission octal modes and umasks work.

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Advanced Programming in the UNIX Environment

Week 3: Union Mounts and Whiteout Files

One of the file types we've encountered are those of type S_IFWHT, so called "whiteout" files. But what the hell is that?? Let's illustrate! We describe the concept of union mounts and see what happens when a file in the upper layer is removed while the same file still exists in the lower layer: a whiteout file is created to cover up the lower file.

https://youtu.be/MkFExG9jhEE

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Advanced Programming in the UNIX Environment

Week 4: The Unix Filesystem

In this video lecture, we begin a closer look at the Unix Filesystem (UFS). We visualize how the filesystem structures the disk and organizes files and directories. We illustrate how directory entries are created and moved around.

https://youtu.be/kY4JAXYyByQ

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Advanced Programming in the UNIX Environment

Week 4: Links

In this video lecture, we'll take a look at the system calls used to create, remove, and rename both hard- and symbolic links. We'll compare expected output against trivial implementations of the ln(1) and mv(1) commands.

https://youtu.be/0l8o2goJ1kc

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Advanced Programming in the UNIX Environment

Week 4: Directories

In this video lecture, we take a look at how directories are created and removed, as well as how to move around the filesystem hierarchy. We also learn why the 'cd' command must be a shell builtin in order to work.

https://youtu.be/xZ7dNXZ58G8

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Advanced Programming in the UNIX Environment

Week 4: Directory Size

In this video lecture, we dive deep into the structure of the directory on a traditional Unix File System and see how its size is independent of the file sizes of its entries, but dependent on the filename lengths. We'll also use hexdump(1) to cheat a bit and look at the directory structure on disk.

https://youtu.be/gY0SE-71LZA

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Advanced Programming in the UNIX Environment

Week 4: /etc/passwd

In this short video, we try to answer the awkward question "Mommy, where do UIDs come from?". We look at the /etc/passwd user database and identify the various fields before we look at how things can get weird.

https://youtu.be/fv16TWDnLYM

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Advanced Programming in the UNIX Environment

Week 4: getpwuid(2) and /etc/groups

In this video lecture, we look at the library functions used to look up account information. We learn about how primary and supplementary groups are handled as well as how the hashed passwords are stored outside of the world readable /etc/passwd file.

https://youtu.be/aomkx6_aWpc

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Advanced Programming in the UNIX Environment

Week 4: atime, mtime, ctime

In this video lecture, we start looking a little bit closer into what our systems think of "time", something that experts most accurately describe as a big ball of wibbly, wobbly, timey, wimey... stuff. In short, analyze how the atime, mtime, and ctimes of a file connect.

https://youtu.be/ffYEEj5vnlw

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Advanced Programming in the UNIX Environment

Week 4: time(3) is an illusion

Having revisited the atime, mtime, and time in our last video segment, we now have to come to terms with time(3) itself. We'll discuss managing the data structures and handling arbitrary concepts such as leap seconds, timezones, and (ugh) Daylight Savings Time. It gets silly real quick.

https://youtu.be/3N2aH1vUacQ

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