Before a command is executed, its input and output may be
redirected using a special notation interpreted by the shell.
Redirection allows commands' file handles to be duplicated,
opened, closed, made to refer to different files, and can change
the files the command reads from and writes to. Redirection may
also be used to modify file handles in 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.
Each redirection that may be preceded by a file descriptor number
may instead be preceded by a word of the form {varname}. In this
case, for each redirection operator except >&- and <&-, the shell
will allocate a file descriptor greater than or equal to 10 and
assign it to varname. If >&- or <&- is preceded by {varname},
the value of varname defines the file descriptor to close. If
{varname} is supplied, the redirection persists beyond the scope
of the command, allowing the shell programmer to manage the file
descriptor himself.
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 and variable expansion,
command substitution, arithmetic expansion, quote removal,
pathname 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 and standard error 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 from the 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. If the
operating system on which bash is running provides these special
files, bash will use them; otherwise it will emulate them
internally with the behavior described below.
/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 the corresponding TCP 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 the corresponding UDP socket.
A failure to open or create a file causes the redirection to
fail.
Redirections using file descriptors greater than 9 should be used
with care, as they may conflict with file descriptors the shell
uses internally.
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 to the set
builtin command 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
This construct 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.
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
When using the second form, word may not expand to a number or -.
If it does, other redirection operators apply (see Duplicating
File Descriptors below) for compatibility reasons.
Appending Standard Output and Standard Error
This construct allows both the standard output (file descriptor
1) and the standard error output (file descriptor 2) to be
appended to the file whose name is the expansion of word.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
(see Duplicating File Descriptors below).
Here Documents
This type of redirection instructs the shell to read input from
the current source until a line containing only delimiter (with
no trailing blanks) is seen. All of the lines read up to that
point are then used as the standard input (or file descriptor n
if n is specified) for a command.
The format of here-documents is:
[n]<<[-]word
here-document
delimiter
No parameter and variable expansion, command substitution,
arithmetic expansion, or pathname expansion is performed on word.
If any part of word is 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, 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.
Here Strings
A variant of here documents, the format is:
[n]<<<word
The word undergoes tilde expansion, parameter and variable
expansion, command substitution, arithmetic expansion, and quote
removal. Pathname expansion and word splitting are not
performed. The result is supplied as a single string, with a
newline appended, to the command on its standard input (or file
descriptor n if n is specified).
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. If word evaluates to -, file
descriptor n is closed. As a special case, if n is omitted, and
word does not expand to one or more digits or -, the standard
output and standard error are redirected as described previously.
Moving File Descriptors
The redirection operator
[n]<&digit-
moves the file descriptor digit to file descriptor n, or the
standard input (file descriptor 0) if n is not specified. digit
is closed after being duplicated to n.
Similarly, the redirection operator
[n]>&digit-
moves the file descriptor digit to file descriptor n, or the
standard output (file descriptor 1) if n is not specified.
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.
