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(bashref.info)Redirections

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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.

Redirecting Input
-----------------

   Redirection of input causes the file whose name results from the
expansion of WORD to be opened for reading on file descriptor `n', or
the standard input (file descriptor 0) if `n' is not specified.

   The general format for redirecting input is:
     [n]<WORD

Redirecting Output
------------------

   Redirection of output causes the file whose name results from the
expansion of WORD to be opened for writing on file descriptor `n', or
the standard output (file descriptor 1) if `n' is not specified.  If
the file does not exist it is created; if it does exist it is truncated
to zero size.

   The general format for redirecting output is:
     [n]>[|]WORD

   If the redirection operator is `>', and the `noclobber' option to
the `set' builtin has been enabled, the redirection will fail if the
file whose name results from the expansion of WORD exists and is a
regular file.  If the redirection operator is `>|', or the redirection
operator is `>' and the `noclobber' option is not enabled, the
redirection is attempted even if the file named by WORD exists.

Appending Redirected Output
---------------------------

   Redirection of output in this fashion causes the file whose name
results from the expansion of WORD to be opened for appending on file
descriptor `n', or the standard output (file descriptor 1) if `n' is
not specified.  If the file does not exist it is created.

   The general format for appending output is:
     [n]>>WORD

Redirecting Standard Output and Standard Error
----------------------------------------------

   Bash allows both the standard output (file descriptor 1) and the
standard error output (file descriptor 2) to be redirected to the file
whose name is the expansion of WORD with this construct.

   There are two formats for redirecting standard output and standard
error:
     &>WORD

and
     >&WORD

Of the two forms, the first is preferred.  This is semantically
equivalent to
     >WORD 2>&1

Here Documents
--------------

   This type of redirection instructs the shell to read input from the
current source until a line containing only WORD (with no trailing
blanks) is seen.  All of the lines read up to that point are then used
as the standard input for a command.

   The format of here-documents is as follows:
     <<[-]WORD
             HERE-DOCUMENT
     DELIMITER

   No parameter expansion, command substitution, arithmetic expansion,
or filename expansion is performed on WORD.  If any characters in WORD
are quoted, the DELIMITER is the result of quote removal on WORD, and
the lines in the here-document are not expanded.  If WORD is unquoted,
all lines of the here-document are subjected to parameter expansion,
command substitution, and arithmetic expansion.  In the latter case,
the character sequence `\newline' is ignored, and `\' must be used to
quote the characters `\', `$', and ``'.

   If the redirection operator is `<<-', then all leading tab
characters are stripped from input lines and the line containing
DELIMITER.  This allows here-documents within shell scripts to be
indented in a natural fashion.

Duplicating File Descriptors
----------------------------

   The redirection operator
     [n]<&WORD

is used to duplicate input file descriptors.  If WORD expands to one or
more digits, the file descriptor denoted by `n' is made to be a copy of
that file descriptor.  If the digits in WORD do not specify a file
descriptor open for input, a redirection error occurs.  If WORD
evaluates to `-', file descriptor `n' is closed.  If `n' is not
specified, the standard input (file descriptor 0) is used.

   The operator
     [n]>&WORD

is used similarly to duplicate output file descriptors.  If `n' is not
specified, the standard output (file descriptor 1) is used.  If the
digits in WORD do not specify a file descriptor open for output, a
redirection error occurs.  As a special case, if `n' is omitted, and
WORD does not expand to one or more digits, the standard output and
standard error are redirected as described previously.

Opening File Descriptors for Reading and Writing
------------------------------------------------

   The redirection operator
     [n]<>WORD

causes the file whose name is the expansion of WORD to be opened for
both reading and writing on file descriptor `n', or on file descriptor
0 if `n' is not specified.  If the file does not exist, it is created.