21sh/redirections

• 3.4.1 Redirecting Input
• 3.4.2 Redirecting Output
• 3.4.3 Here-Document
• 3.4.4 Duplicating an Input File Descriptor
• 3.4.5 Duplicating an Output File Descriptor

3.4.1 Redirecting Input

Input redirection shall cause the file whose name results from the expansion of word to be opened for reading on the designated file descriptor, or standard input if the file descriptor is not specified.

The format for redirecting input is:

[n]<word

Where the optional n represents the file descriptor number. If the number is omitted, the redirection shall refer to standard input (file descriptor 0).

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3.4.2 Redirecting Output

The two general formats for redirecting output are:

[n]>word

Where the optional n represents the file descriptor number. If the number is omitted, the redirection shall refer to standard output (file descriptor 1).

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3.4.3 Here-Document

The here-document shall be treated as a single word that begins after the next and continues until there is a line containing only the delimiter and a , with no characters in between. Then the next here-document starts, if there is one. The format is as follows:

[n]<<word
    here-document
delimiter

Where the optional n represents the file descriptor number. If the number is omitted, the here-document refers to standard input (file descriptor 0). It is unspecified whether the file descriptor is opened as a regular file, a special file, or a pipe. Portable applications cannot rely on the file descriptor being seekable.

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3.4.4 Duplicating an Input File Descriptor

The redirection operator:

[n]<&word

shall duplicate one input file descriptor from another, or shall close one. If word evaluates to one or more digits, the file descriptor denoted by n, or standard input if n is not specified, shall be made to be a copy of the file descriptor denoted by word; if the digits in word do not represent a file descriptor already open for input, a redirection error shall result.

If word evaluates to -, file descriptor n, or standard input if n is not specified, shall be closed.

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3.4.5 Duplicating an Output File Descriptor

The redirection operator:

[n]>&word

shall duplicate one output file descriptor from another, or shall close one. If word evaluates to one or more digits, the file descriptor denoted by n, or standard output if n is not specified, shall be made to be a copy of the file descriptor denoted by word; if the digits in word do not represent a file descriptor already open for output, a redirection error shall result.

If 'word' evaluates to -, file descriptor n, or standard output if n is not specified, is closed.

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