Shell command language

This section introduces commands and, more importantly, special characters that let you

• find and manipulate a group of files by using pattern matching

• run a command in the background or at a specified time

• run a group of commands sequentially

• redirect standard input and output (of files and other commands)

• terminate running programs

Characters with special meanings in the shell language

Character	Function
? [ ]	The asterisk, question mark, and brackets allow you to specify filenames by pattern matching.
&	The ampersand places commands in background mode, leaving your terminal free for other tasks.
;	The semicolon separates multiple commands on one command line.
\	The backslash turns off the meaning of special characters such as ? [ ] & ; > < and |.
' . . . '	Single quotes turn off the delimiting meaning of a space and the special meaning of all special characters.
" . . . "	Double quotes turn off the delimiting meaning of a space and the special meaning of all special characters except $ and `.
>	The greater than sign redirects the output of a command into a file (replacing the existing contents).
<	The less than sign redirects the input for a command to come from a file.
>>	Two greater than signs redirect the output of a command to be added to the end of an existing file.
|	The vertical bar, or pipe, makes the output of one command the input of another command.
` . . . `	A pair of grave accents around a command embedded on a command line makes the output of the embedded command an argument on the larger command line.
$	The dollar sign retrieves the value of positional parameters and user-defined variables. It is also the default shell prompt.