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Bash Reference Manual

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3.6 Redirections

Before a command is executed, its input and output may be redirected using a special notation interpreted by the shell. Redirection may also be used to open and close files for the current shell execution environment. The following redirection operators may precede or appear anywhere within a simple command or may follow a command. Redirections are processed in the order they appear, from left to right.

In the following descriptions, if the file descriptor number is omitted, and the first character of the redirection operator is `<', the redirection refers to the standard input (file descriptor 0). If the first character of the redirection operator is `>', the redirection refers to the standard output (file descriptor 1).

The word following the redirection operator in the following descriptions, unless otherwise noted, is subjected to brace expansion, tilde expansion, parameter expansion, command substitution, arithmetic expansion, quote removal, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.

Note that the order of redirections is significant. For example, the command
 
ls > dirlist 2>&1
directs both standard output (file descriptor 1) and standard error (file descriptor 2) to the file dirlist, while the command
 
ls 2>&1 > dirlist
directs only the standard output to file dirlist, because the standard error was duplicated as standard output before the standard output was redirected to dirlist.

Bash handles several filenames specially when they are used in redirections, as described in the following table:

/dev/fd/fd
If fd is a valid integer, file descriptor fd is duplicated.

/dev/stdin
File descriptor 0 is duplicated.

/dev/stdout
File descriptor 1 is duplicated.

/dev/stderr
File descriptor 2 is duplicated.

/dev/tcp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a TCP connection to the corresponding socket.

/dev/udp/host/port
If host is a valid hostname or Internet address, and port is an integer port number or service name, Bash attempts to open a UDP connection to the corresponding socket.

A failure to open or create a file causes the redirection to fail.


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