Unix sees :
files -  object
processes - actions

When a command is executed, it becomes a process 
Files - file-system 
types - regular, directory, symbolic, pipe, etc.
ownership - user, group, other.
permissions - read, write, execute, SUID, SGID, sticky bit. 

Processes - process control mechanism
access based on ownership and type.

real - person who 'created' the command being run.
effective - person executing the command being run.

execution control

foreground/background
priority
suspension and termination


Process interaction.
redirection (batch) - takes input from file or saves to file.
piping - allows output of one process to connect to input of another.


Command format : command [options] [arguments]

Command - shell build-in, binary program, shell-script

options - usually preceded by -, modify command behavior. 

arguments - provide data to program. May be filename or actual data.

Depending on command, options and arguments may be optional.


File-system view from the login shell (bash).


The home directory is the active or current working directory you are "in" 
on login.

Home is specified in system's password 'file'.
Doesn't change unless modified in the password system.

echo $HOME

Ways to return home from another directory.

cd $HOME
   - uses the environmental variable set on login.
cd
   - by itself should return you to home in the bash shell.
cd ~
   - tilde refers to your absolute home path in bash and can be used 
     with commands.


Current working directory

Current working directory is the directory list that a command will use 
when accessing a file if no additional path information is provided.

To change current working directory, use cd and a valid directory 
path and name as an argument :

cd bin

To identify current working directory, use one of the following :

echo $PWD
  - variable set by the bash shell each time you change the working directory.
  - useful in scripts.

pwd
  - command that prints current working dircctory.

bash shell built-in
/bin/pwd is a separate binary executable (original).