chroot: The Jail That Isn’t Really a Jail
chroot — available on Unix since Version 7 in 1979 — changes a process's idea of where the root directory is, trapping it in a subtree of the filesystem. It was designed for building and testing software in clean environments, not security. Security is the use case everyone immediately reached...
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sed: The Stream Editor That Operates Entirely Without Seeing the Whole Stream
sed — the Unix stream editor, present since Version 7 in 1979 — processes text one line at a time, applying editing commands as each line passes through, never holding the full file in memory. This makes it extraordinarily fast on large files and completely bewildering to anyone who approaches...
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inode: The Invisible Bureaucrat Running Every Unix Filesystem
Every file on a Unix filesystem has an inode — a small data structure storing the file's metadata: permissions, ownership, timestamps, size, and pointers to the actual data blocks on disk. Everything except the filename, which lives in the directory instead. This separation means a single file can have multiple...
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UUCP and Bang Paths: Routing Email Like a Treasure Map
Before the internet became universal, Unix machines exchanged mail via UUCP — Unix-to-Unix Copy — a store-and-forward system that dialled other machines over phone lines and exchanged files in batches. Email addresses were written as bang paths: ihnp4!ucbvax!decwrl!user, a literal sequence of machines the message should hop through, separated by...
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The /proc Filesystem: Your Kernel’s Diary, Readable as Text Files
Linux's /proc filesystem is one of the stranger ideas in Unix — a directory that doesn't exist on disk, populated entirely by the kernel at runtime with files that represent live system state. Want the memory map of a running process? It's a file. CPU info? A file. Currently mounted...
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POSIX: The Standard That Tries to Herd Unix Cats
By the mid-1980s, Unix had fragmented into a sprawling ecosystem of incompatible variants — BSD, System V, SunOS, AIX, HP-UX — each subtly different in ways calculated to ruin portability. The IEEE responded with POSIX (Portable Operating System Interface), a family of standards specifying how a conforming Unix-like system should...
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