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.
When used with the exec builtin,
redirections 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 allocates a file descriptor greater
than or equal to 10 and assigns it to {varname}.
If {varname} precedes
>&-
or
<&-,
the value of varname defines the file descriptor to close.
If {varname} is supplied, the redirection persists beyond
the scope of the command, which allows the shell programmer to
manage the file descriptor’s lifetime manually without using
the exec builtin.
The varredir_close shell option manages this behavior
(see The Shopt Builtin).
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, filename expansion, and word splitting. If it expands to more than one word, Bash reports an error.
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 made a copy of 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 uses them; otherwise it emulates them internally with the behavior described below.
/dev/fd/fdIf fd is a valid integer, duplicate file descriptor fd.
/dev/stdinFile descriptor 0 is duplicated.
/dev/stdoutFile descriptor 1 is duplicated.
/dev/stderrFile descriptor 2 is duplicated.
/dev/tcp/host/portIf 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/portIf 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 opens the file whose name results from
the expansion of word 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 opens the file whose name results from the expansion of word 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 command has been enabled, the redirection fails 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 is not enabled,
Bash attempts the redirection
even if the file named by word exists.
Redirecting output in this fashion opens the file whose name results from the expansion of word 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
This construct redirects both the standard output (file descriptor 1) and the standard error output (file descriptor 2) 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.
This construct appends both the standard output (file descriptor 1) and the standard error output (file descriptor 2) 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).
This type of redirection instructs the shell to read input from the current source until it reads a line containing only delimiter (with no trailing blanks). All of the lines read up to that point then become 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
The shell does not perform parameter and variable expansion, command substitution, arithmetic expansion, or filename expansion 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,
delimiter is word itself,
and the here-document text is treated similarly to a double-quoted string:
all lines of the here-document are subjected to
parameter expansion, command substitution, and arithmetic expansion,
the character sequence \newline is treated literally,
and ‘\’ must be used to quote the characters
‘\’, ‘$’, and ‘`’;
however, double quote characters have no special meaning.
If the redirection operator is ‘<<-’, the shell strips 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.
If the delimiter is not quoted, the
\<newline>
sequence is treated as a line continuation: the two lines are joined
and the backslash-newline is removed.
This happens while reading the here-document, before the check for
the ending delimiter, so joined lines can form the end delimiter.
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. Filename 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).
The redirection operator
[n]<&word
is used to duplicate input file descriptors. If word expands to one or more digits, file descriptor n is made to be a copy of that file descriptor. It is a redirection error if the digits in word do not specify a file descriptor open for input. If word evaluates to ‘-’, file descriptor n is closed. If n is not specified, this uses the standard input (file descriptor 0).
The operator
[n]>&word
is used similarly to duplicate output file descriptors. If n is not specified, this uses the standard output (file descriptor 1). It is a redirection error if the digits in word do not specify a file descriptor open for output. 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 ‘-’, this redirects the standard output and standard error as described previously.
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.
The redirection operator
[n]<>word
opens the file whose name is the expansion of word 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.