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Special Characters
There are several special characters in BASH that have a non-literal meaning. When we use these characters, BASH evaluates these characters and their meaning, but usually does not pass them on to the underlying commands. These are also called metacharacters.
Here are a few of those special characters, and what they do:
[whitespace]: Whitespace (spaces, tabs and newlines). BASH uses whitespace to determine where words begin and end. The first word of each command is used as the command name; any additional words become arguments to that command.
$: Expansion character. This character is used in most substitutions, including parameter expansion (variable substitution). More about this later.
'text': Single quotes protect the text inside from any kind of expansion by the shell and keep it from being split into multiple words or arguments. They also prevent the special meaning of all special characters inside.
"text": Double quotes protect the text inside from being split into multiple words or arguments, but they permit substitutions to occur. They prevent the special meaning of most special characters inside them -- basically, all except for the $.
#: Comment character. Any word beginning with # begins a comment that extends to the next newline. Comments are not processed by the shell.
;: Command separator. The semicolon is used to separate multiple commands from each other if the user chooses to keep them on the same line. It's basically the same thing as a newline.
\: Escape character. The backslash prevents the next character from being used in any special sort of way. This works inside double quotes, or outside of all quotes, but not inside single quotes.
~: The tilde is a shortcut for your home directory. By itself, or when followed by a /, it is the same as $HOME. When followed by a username, it means that user's home directory. Examples: cd ~john/bin; cp coolscript ~/bin
> or <: Redirection characters. These characters are used to modify (redirect) the input and/or output of a command. Redirections will be covered later.
|: Pipelines allow you to send the output of one command as the input to another command.
[[ expression ]]: Test expression. This evaluates the conditional expression as a logical statement, to determine whether it's "true" or "false".
{ commands; } : Command grouping. The commands inside the braces are treated as though they were only one command. It is convenient for places where BASH syntax requires only one command to be present, and you don't feel a function is warranted.
`command`, $(command): Command substitution (The latter form is highly preferred.) Command substitution executes the inner command first, and then replaces the whole `...` or $(...) with that command's standard output.
(command): Subshell Execution. This executes the command in a new bash shell, instead of in the current one. If the command causes side effects (like changing variables), those changes will have no effect on the current shell.
((expression)): Arithmetic Command. Inside the parentheses, operators such as +, -, * and / are seen as mathematical operators. This can be used for assignments like ((a=b+7)) as well as tests like if ((a < b)). More on this later.
$((expression)): Arithmetic Substitution. Comparable to the above, but this expression is replaced with the result of its arithmetic evaluation. Example: echo "The average is $(( (a+b)/2 ))".
Some examples:
$ echo "I am $LOGNAME"
I am lhunath
$ echo 'I am $LOGNAME'
I am $LOGNAME
$ # boo
$ echo An open\ \ \ space
An open space
$ echo "My computer is $(hostname)"
My computer is Lyndir
$ echo boo > file
$ echo $(( 5 + 5 ))
10
$ (( 5 > 0 )) && echo "Five is bigger than zero."
Five is bigger than zero.
In The Manual: Shell Syntax
Special Characters: Characters that have a special meaning to BASH. Usually their meaning is interpreted and then they are removed from the command before executing it.