Section 5.5. Metacharacters

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5.5. Metacharacters

Some characters are processed specially by the shell, and are known as metacharacters. All shells share a core set of common metacharacters, whose meanings are given in Figure 5-5.

Figure 5-5. Shell metacharacters.

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Symbol	Meaning
>	Output redirection; writes standard output to a file.
>>	Output redirection; appends standard output to a file.
<	Input redirection; reads standard input from a file.
*	File substitution wildcard; matches zero or more characters.
?	File substitution wildcard; matches any single character.
[...]	File substitution wildcard; matches any character between brackets.
`command`	Command substitution; replaced by the output from command.
|	Pipe symbol; sends the output of one process to the input of another.
;	Used to sequence commands.
| |	Conditional execution; executes a command if the previous one failed.
&&	Conditional execution; executes a command if the previous one succeeded.
(...)	Groups commands.
&	Runs a command in the background.
#	All characters that follow up to a newline are ignored by the shell and programs (i.e., a comment).
$	Expands the value of a variable.
\	Prevents special interpretation of the next character.
<<tok	Input redirection; reads standard input from script up to tok.

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When you enter a command, the shell scans it for metacharacters and processes them specially. When all metacharacters have been processed, the command is finally executed. To turn off the special meaning of a metacharacter, precede it by a \ character. Here's an example:

$ echo hi > file    ...store output of echo in "file". $ cat file          ...look at the contents of "file". hi $ echo hi \> file   ...inhibit > metacharacter. hi > file           ...> is treated like other characters. $ _                 ...and output comes to terminal instead 

This chapter describes the meaning of each metacharacter in the order listed in Figure 5-5.