Unix

Unix is user-friendly — it's just choosy about who its friends are.

The Art of UNIX Programming (2003) by Eric S. Raymond

Created in 1969 at AT&T Bell Labs to run on departmental computers for the PDP-11. It used 16-bit words and had 64KiB/128KiB RAM.

It was initially written in ASM, but sparked the creation of C which was designed to make Unix development easier.

Unix gives you just enough rope to hang yourself -- and then a couple of more feet, just to be sure.

Eric Allman

Philosophy

• smaller kernel, a master process which provides services to system and user processes
• a simple datatype (text) and simple interprocess communication
• allows chaining of processes
• seperation between ordinary users and superusers using privileges
• files
• almost everything is a file
  • processes are files
  • directories are files that point to other files
• files are just sequences of bytes

Basics

• filenames
• extensions are conventional, not required
• filenames can be any non-whitespace character (sus)
• current dir is ., parent is ..
• dotfiles are hidden
• commands
• <commandname> [<options>] [<objects>]
• shell
• the program that handles command interpretation
• shell locates commands and objects
• you can chain commands into shell scripts
  • first was Bourne Shell sh in 1976
  • followed by Bourne Again Shell bash in 1989
  • Korn Shell ksh in 1983

Environment Variables

Environment variables are defined by the system and will be will be passed to the shell.

• system path is stored in env var PATH, list of dirs seperated by :, usually contains the following paths
• /bin
  • top level system commands
  • cat, echo
• /usr/bin
  • system level commands that generally come with the system but may need to be installed
  • python, gcc
  • usr stands for Unix System Resources (though it originally meant user, since it used to be the user's directory, see The bin Split)
• /usr/local/bin
  • programs that you have installed for all users of your system locally
  • pip3, ps2pdf
• ~/bin
  • a users programs
• .
  • current directory
• CDPATH
• all locations that cd can direct you to
• similar to PATH
• PS1
• shell prompt text
• umask 077
• permissions applied to files on creation

More environment variables can be setup in the ~/.profile executable.