Characters used with commands

Character: Usage
|: Pipes the output from one command to a second command; separates commands in a pipeline.
||: Separates two commands. If the command preceding || fails, it runs the following command (Boolean OR operator).
>: Redirects stdout.
<: Redirects stdin.
&: Runs a command in the background, if placed at the end of a command line.
>&: Used for redirecting stdout and stderr.
&&: Separates two commands. If the command preceding && succeeds, it runs the following command (Boolean AND operator).
;: Separates sequential commands; allows you to enter more than one command on the same line.
( ): Around a sequence of commands, groups those commands that are to run as a separate process in a subshell environment. The commands run in a separate execution environment: changes to variables, the working directory, open files, and so on, will not remain in effect after the last command finishes.
(␠) is also used to group mathematical operations.
{ }: Around a sequence of commands, groups those commands that are run in the current shell environment. Changes to variables will affect the current shell.
Both { and } are reserved words to the shell. To make it possible for the shell to recognize these symbols, you must enter a blank or <newline> after the {, and a semicolon or <newline> before the }.
#: Following a command in a shell script, indicates the beginning of a comment.
$: At the beginning of a string, indicates that it is a variable name.
\: In general, the backslash character turns off the special meaning of the character that follows it. For more information, see Using a special character without its special meaning.
' ': A pair of single quotation marks turns off the special meaning of all characters within the quotation marks. For more information, see Using a special character without its special meaning.
" ": A pair of double quotation marks turns off the special meaning of the characters within the quotation marks, except that !event, $var, and `cmd` will show history, variable, and command substitution. See Using a special character without its special meaning for more information.