Input/output

The standard input and standard output of a command may be redirected with the following syntax:

< name: Open file name (which is first variable, command and filename expanded) as the standard input.
<< word: Read the shell input up to a line which is identical to word. word is not subjected to variable, filename or command substitution, and each input line is compared to word before any substitutions are done on this input line. Unless a quoting `\', `"', `' or ``' appears in word variable and command substitution is performed on the intervening lines, allowing `\' to quote `$', `\' and ``'. Commands which are substituted have all blanks, tabs, and newlines preserved, except for the final newline which is dropped. The resultant text is placed in an anonymous temporary file which is given to the command as standard input.

> name
>! name
>& name

>&! name: The file name is used as standard output. If the file does not exist then it is created; if the file exists, its is truncated, its previous contents being lost.

If the shell variable noclobber is set, then the file must not exist or be a character special file (e.g. a terminal or `/dev/null') or an error results. This helps prevent accidental destruction of files. In this case the `!' forms can be used to suppress this check.

The forms involving `&' route the diagnostic output into the specified file as well as the standard output. name is expanded in the same way as `<' input filenames are.

>> name
>>& name
>>! name

>>&! name: Like `>', but appends output to the end of name. If the shell variable noclobber is set, then it is an error for the file not to exist, unless one of the `!' forms is given.

A command receives the environment in which the shell was invoked as modified by the input-output parameters and the presence of the command in a pipeline. Thus, unlike some previous shells, commands run from a file of shell commands have no access to the text of the commands by default; rather they receive the original standard input of the shell. The `<<' mechanism should be used to present inline data. This permits shell command scripts to function as components of pipelines and allows the shell to block read its input. Note that the default standard input for a command run detached is not the empty file /dev/null, but the original standard input of the shell. If this is a terminal and if the process attempts to read from the terminal, then the process will block and the user will be notified (see Jobs).

Diagnostic output may be directed through a pipe with the standard output. Simply use the form `|&' rather than just `|'.

The shell cannot presently redirect diagnostic output without also redirecting standard output, but `(command > output-file) >& error-file' is often an acceptable workaround. Either output-file or error-file may be `/dev/tty' to send output to the terminal.

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