Unix Fundamentals
Unix is a family of operating systems built around a small set of principles: everything is a file, programs do one thing well and compose through text streams, and the shell is the user interface to a tree of processes. Linux, macOS, and BSDs all descend from this lineage.
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Intro
Unix Fundamentals
Unix is a family of operating systems and interfaces built around a small set of durable ideas: files, processes, permissions, text streams, and composable utilities. Linux, the BSD systems, and macOS inherit much of this model. They are not identical products, so treat a Unix command as a contract to check, not a promise that every option exists everywhere.
POSIX is the shared standard that makes much of this environment portable. It specifies a shell command language, utility behavior, pathname rules, and system interfaces. The standard does not make every Unix-like system the same. It gives you a useful common floor.
The model: a process connected to files
A process is a running program. It runs with an identity, a current working directory, an environment, and open file descriptors. In Unix, a file descriptor is a small number that identifies an open input or output channel for that process.
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Sources
- https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap01.html
Supports
- POSIX volume scope, including base definitions, system interfaces, shell command language, and utilities
- POSIX as the portable baseline rather than a statement that all Unix-like implementations behave identically
- https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap04.html
Supports
- Pathname resolution, including absolute pathnames beginning at root and relative pathnames resolved from the current working directory
- Directory entries, file hierarchy concepts, process identifiers, and process group identifiers
- https://pubs.opengroup.org/onlinepubs/9799919799/basedefs/V1_chap03.html
Supports
- Definitions of shell, shell script, signal, file descriptor, standard input, standard output, standard error, process, and symbolic link
- Definitions of core Unix terms used throughout the course
- https://pubs.opengroup.org/onlinepubs/9799919799/functions/V2_chap02.html
Supports
- Predefined standard input, standard output, and standard error streams at program startup
- POSIX signal actions and the fact that SIGKILL cannot be caught or ignored
- https://pubs.opengroup.org/onlinepubs/9799919799/utilities/V3_chap02.html
Supports
- Shell processing sequence: token recognition, parsing, word expansion, redirection, execution, and exit status
- Quoting, word expansion, simple commands, pipelines, redirections, exit status, and job control
- Pipeline connection from one command standard output to the next command standard input
- https://pubs.opengroup.org/onlinepubs/9799919799/utilities/contents.html
Supports
- Official index for POSIX shell utilities and their normative utility pages
- Portability guidance that starts from the standardized utility contract
- https://www.gnu.org/software/bash/manual/bash.html
Supports
- Bash as a command language interpreter and programming language
- Interactive and non-interactive shell behavior, builtins, command execution, quoting, redirection, exit status, and job control
- Bash relationship to the POSIX Shell and Utilities specification
- https://www.gnu.org/software/coreutils/manual/html_node/index.html
Supports
- GNU implementation documentation for common file and text utilities
- Distinction between a GNU utility implementation manual and the POSIX utility baseline
- https://www.kernel.org/doc/html/latest/filesystems/proc.html
Supports
- Linux proc filesystem as an interface for process and kernel runtime information
- https://man.openbsd.org/intro
Supports
- OpenBSD manual organization as a second Unix-family implementation reference