What is the dollar sign used for before a variable?

Serves to access values stored within variables. When you use the $ prefix you are wanting to access this value. Here's a simple example.

#!/bin/sh

NAME="Zara Ali"
echo $NAME

I declared a NAME variable and stored the value "Zara Ali". To ACCESS this value "Zara Ali", I need the prefix $

In Unix, you can choose the shell you want to use. Example: bash dash zsh sh ksh csh ... Each has its syntactic and semantic definition.

In various Linux and Mac distributions, the default shell is bash: let's assume we're talking Bash . Here are some conventions:

Identifiers

• By default the identifiers are commands, files, folders, arguments: there is a need to "tag" the variables in some way.

Definition of variables / assignment

• id=exp assigning a value to a variable
• id = exp executes the "id" command passing it as arguments "=" and "exp"
• id= date assigns empty value to id and executes date

About $ depending on the context, can mean a lot of different things:

• $id id value
• echo "o meu username é $USER" the value of the variable is expanded inside quoted strings (prints "my username is jj")
• $(comand) results in command stdout
• $((3 + 4 + $a )) results in the arithmetic calculation contained therein
• a=(v0 v1 v2 v3 v4 v5) ; echo ${a[2]} gives the second value of an array (v2)

A small example:

$ A=ano
$ echo "o $A passado foi $(( $(date +%Y) - 1))"
o ano passado foi 2016

Finally, remember that bash has:

• variables, arrays, dictionaries
• control structures (if, while, switch, for, ...)
• functions (including recursive)