Shell and variables

Use variables

The shell suports 2 types of variables : environment variables and shell variables.
Environment variables :
- Are available system-wide and are inherited by all spawned child processes and shells.
- Their values differ from user to user (an obvious example is $USER).
- Environment variables for the current shell can be listed using env.
- Environment variables can be modified and new variables added using env VARIABLE=value.
Shell variables :
- Shell variables are variables that apply only to the current shell instance.
- Each shell (zsh, bash, etc ...) has its own set of predefined internal shell variables.
- Shell variables for the current shell can be listed using set.
- Shell variables can be modified and new variables added using set VARIABLE=value.

Note : similar to shell commands, environment variables are accessibles from within shell scripts.

environment	contains
`$0`	The name of the running executable
`$1 - $9`	The first 9 arguments that were passed to it
`$#`	Total number of arguments that were passed
`$@`	All the arguments that were passed
`$?`	The exit status of the most recently run process
`$$`	The PID (process id) of the current executable instance
`$USER`	The username of the user running the process
`$HOSTNAME`	The hostname of the machine the process is running on
`$SECONDS`	The number of seconds since the process has started
`$RANDOM`	Returns a different random number each time is it referred to
`$LINENO`	Returns the current line number in the context of the shell or script

User defined variables are most often used in shell scripts.
A variable is set using VARIABLE=value (no space on either side of the equals sign).
A variable is referred to using $VARIABLE.
Any variable included in a command will have the shell expansion replace it with its value before running the command.
The scope of a variable is the process it is created in.
As a result, referencing the same variable in different scripts requires the variable to be exportd as an environment variable.
Setting an environment variable in a script won't change its value outside of the script unless it is exported again.

Lists are actually lists of words that can either be :
1. A series of strings, separated by spaces, ex : Stan Kyle Cartman.
2. A range of numbers, separated by 2 dots, ex : {1..5} or {10..0} :
  - It evaluates as an increasing series of numbers if the right value is greater than the left value.
  - It evaluates as a decreasing series otherwise ...
  - It is possible to specify the increment/decrement value by adding two more dots (..) and the value to use.
  - For instance, {10..0..2} will evaluate to 10 8 6 4 2 0.
lists are not arrays : for instance range numbers can't be assigned to variables without being expanded into an array first.