Bash Shell 重定向(一):原文
2015-09-17 11:03
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From: http://www.gnu.org/software/bash/manual/bashref.html#Redirections
3.6 Redirections
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 changethe 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 10
and assign it to {varname}. If >&- or <&- is preceded by {varname}, the value of
varname defines the file descriptor to close.
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 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:
/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.
3.6.1 Redirecting Input
Redirection of input causes the file whose name results from the expansion ofword to be opened for reading on file descriptor
n, or the standard input (file descriptor 0) if
nis not specified.
The general format for redirecting input is:
[n]<word
3.6.2 Redirecting Output
Redirection of output causes the file whose name results from the expansion ofword 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
noclobberoption to the
setbuiltin 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
noclobberoption is not enabled, the redirection is attempted even if the file named by
word exists.
3.6.3 Appending Redirected Output
Redirection of output in this fashion causes the file whose name results from the expansion ofword 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
3.6.4 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 ofword.
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.
3.6.5 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 ofword.
The format for appending standard output and standard error is:
&>>word
This is semantically equivalent to
>>word 2>&1
(see Duplicating File Descriptors below).
3.6.6 Here Documents
This type of redirection instructs the shell to read input from the current source until a line containing onlyword (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:
<<[-]word here-document delimiter
No parameter and variable 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, the character sequence
\newlineis 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.
3.6.7 Here Strings
A variant of here documents, the format is:<<< word
The word undergoes brace expansion, 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
to the command on its standard input.
3.6.8 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.
3.6.9 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.
3.6.10 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.
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