Operating Systems & Distros

Debian uploads always land in unstable (sid) and migrate automatically into testing after a few days without new release-critical bugs and with builds on every release architecture. Testing is periodically frozen and published as the next stable.

Configuration lives under /etc, persistent state under /var/lib and logs under /var/log. The split is by mutability and ownership: /usr holds static package-owned code, /etc holds locally editable configuration, /var holds data the machine writes while it runs.

A hard link is an extra name for the same inode, so every name is equal and the data survives until the last one is removed. A symbolic link is a separate small file holding a path string, resolved at every access.

Kernel names like /dev/sdb1 depend on device discovery order, so a new disk or a slow controller can renumber them and mount the wrong filesystem. Prefer UUID= from the filesystem superblock, or PARTUUID= from the partition table.

A loadable kernel module is kernel code — typically a driver or filesystem — shipped as a separate .ko file and inserted into the running kernel on demand. Code built in with =y is part of the kernel image itself and can never be unloaded.

Unix builds small single-purpose programs that read bytes from standard input and write bytes to standard output, so a shell can chain them with pipes. Composition becomes nearly free; structured data and precise error reporting do not.

FreeBSD's kernel and core userland are built from one source tree and released, patched and upgraded as a single versioned unit, while every third-party program installs separately under /usr/local and is managed by pkg.

Most kernel objects — regular files, devices, pipes and sockets — are reached through a file descriptor and the same read, write and close calls. The abstraction leaks: sockets and terminals need their own calls, and not every object has a filesystem name.

A jail is one kernel object that confines a process tree to a directory root, a hostname, an address set and a restricted root user in a single step. Linux has no equivalent single object; containers there are composed from several independent kernel facilities.

The ports tree is a collection of build recipes you compile from source with your own options; pkg installs prebuilt binary packages that the project produced from those same recipes using default options. Most installs should use pkg.

A Launch Daemon runs in the system domain from boot, normally as root, with no login session and no access to a user's screen. A Launch Agent runs inside a logged-in user's session, as that user, and can reach the GUI.

The browser tags its download with the com.apple.quarantine extended attribute; curl does not. Gatekeeper only evaluates quarantined files, so the tagged copy is checked for a valid signature and notarization on first launch while the untagged copy simply runs.

Raspberry Pi GPIO pins run 3.3 V logic and are not 5 V tolerant, so 5 V on an input can damage the pin or the SoC. Put a level shifter or a resistor divider in between.

NTFS attaches a security descriptor with an ordered list of ACEs to every file, so any number of users and groups can each be allowed or denied specific rights. Unix mode bits offer only three fixed slots of read/write/execute.

A Windows HANDLE is an opaque, per-process entry in that process's handle table referring to an object created by the NT object manager — a file, event, mutex, process, thread or registry key. A Unix file descriptor is a small integer naming a much narrower set of objects.