00. Overview. Syscall → utility → shell → script → service

Reprise: operating system goals

• unification of resources interface

• separation of resources access

• logging (accounting) of resources utilization

(aka u/s/l)

Resources:

• CPU time
• RAM
• Devices

E. g.:

• Concurrent multitasking
  • Scheduling
  • Virtual memory
• Filesystems and file permissions
• Video rendering, sound, other input/output devices
• Networking
• …

Interfaces and tools

Supervisor mode (kernel space) and user mode (user space):

• hardware access restrictions
• virtual memory
• elimination of atomic resource concurrent usage

Interface levels:

1. Kernel: syscalls. Used primarily inside kernel code (∃ klibc, in fact). E. g. clone (in fact, huge amount of u/s/l) or in libc library.

2. Libc: library calls. Used when programming userspace utilities. E. g. fork or daemon, or exec.

3. Utilities: programs to perform syscalls, with goodies. E. g. env, xargs

4. Shell: scripting utilities usage. I/O redirection, process manupulation, programming language.
5. Services (daemons): task is too complex/too slow for shell scripting; back to programming

Also: daemons

• True Socrate's daemon

«Flower architecture»

• Thick circle is kernel space:
  • Kernel
  • Modules
• Outside is userspace
  • Utilities
  • Applications
  • Daemons

Shell (intro)

If there's time

Interface basecs:

• Control and data I/O flows
• Human readable

• Human writeable

• Data flow / control flow separation problem

So:

• Readable/writeable text is (almost) universal format
• Data /control join
• Interactive QA scenario

⇒ Command line interface (CLI)

Shell:

• CLI
• High-level domain-specific programming language
• Application integration shell (I/O streams and signals)

Shell commands:

• Internal (about some dozen)
• Utilities (thousands)

Command line conventions ...

Additional topics to discuss

• OS architecture: monolith/modular/mixed
• Packages et al
• ...