metacharacters.md

Metacharacters

characters with "special meaning" The Bash manual defines metacharacters as: a character that, when unquoted, separates words. A metacharacter is a blank (space, tab) or one of the following characters: | & ; ( ) < >

Metacharacters per the Bash Manual

Metacharacter	Description
[whitespace]	Whitespace (spaces, tabs and newlines). Bash uses whitespace to determine where words begin and end.
|	Pipe the output of one command to the input of another command.
&	1. Run command in the background, and continue to the next. 2. used in redirection.
;	Command separator. Used for multiple commands on one line.
( command )	Subshell Execution.
> or <	Redirection characters. redirect the input and/or output of a command.

Other characters with special meaning

Character	Description
=	1. Variable Assignment. 2. "is equal to"
$	Substitution (variables, command substitution).
$variable	Parameter expansion, short form.
${variable}	Parameter expansion, long form. No spaces around braces, else Bash thinks you want command grouping.
'text'	Single quotes protect the text inside from any kind of expansion, or word-splitting.
"text"	Double quotes protect the text inside from word-splitting, but they permit substitutions to occur.
\	Escape character. prevents the next character from having special meaning. Does not work within single quotes.
\	Line continuation, if last character on line, escapes newline.
#	Comment character. Any word beginning with # begins a comment that extends to the next newline.
&&	AND. Run the command to the right ONLY IF the command on the left succeeded.
||	OR. Run the command to the right ONLY IF the command on the left failed.
;; ;& ;;&	terminates a list of commands in a case statement.
~	A shortcut for home directory.
>>	Redirection: Append output to a file. If the file does not exist, it will be created.
<<	Redirection. "Here Doc"
<<<	Redirection. "Here String"
[abc]	Match nne of the characters within the square brackets.
[ expr ]	Same as the "test" command. Brackets must be surrounded by whitespace, operators such as < and > escaped.
[[ expression ]]	Test keyword. This evaluates the conditional expression as a logical statement, to determine whether it's "true" or "false".
{ commands; }	Command grouping.
command, $(command)	Command substitution (The latter form is highly preferred.)
((expression))	Arithmetic Command. Equivalent to the "let" builtin command.
$((expression))	Arithmetic Substitution.