POSIX (IEEE Std 1003.1 and Open Group Base Specifications)

1. General concepts

   Filenames, pathname resolution, file access permissions and thread-safety.

2. Environment variables

   PATH, HOME, LANG, TZ and other standard variables.

3. Regular expressions

   Basic and extended regular expression grammar.

4. Utility conventions

   Argument syntax guidelines for getopt-style utilities.

5. General terminal interface

   Canonical and non-canonical modes and termios.

6. Shell command language

   Quoting, expansions, redirection and shell grammar.

7. Shell word expansions

   Parameter expansion, command substitution, field splitting and pathname expansion.

8. Signals concepts

   Signal delivery, masks and async-signal-safe functions.

9. Threads

   pthread semantics, mutexes and cancellation.

10. Sockets

    Socket address families and connection semantics.

11. Error numbers

    errno values and their meaning.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.