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--- jaysnote:redirection [2018/12/11 16:00]
jaylee
+++ jaysnote:redirection [2021/06/22 23:14]
@@ 行 1: / 行 1: @@
-<markdown>
-# man bash redirection
-## REDIRECTION
-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.
-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
-```bash
-ls > dirlist 2>&1
-```
-directs both standard output and standard error to the file dirlist, while the command
-```bash
-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:
-* /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.
-Note that the exec builtin command can make redirections take effect in the current shell.
-### 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:
-```bash
-[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
-) 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:
-```bash
-[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:
-```bash
-[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:
-```bash
-&>word
-```
-and
-```bash
->&word
-```
-Of the two forms, the first is preferred.  This is semantically equivalent to
-```bash
->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](Duplicating File Descriptors )) 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:
-```bash
-&>>word
-```
-This is semantically equivalent to
-```bash
->>word 2>&1
-```
-(see [Duplicating File Descriptors below](#Duplicating File Descriptors)).
-### 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 for a command.
-The format of here-documents is:
-```bash
-<<[-]word
-here-document
-delimiter
-```
-No parameter and variable expansion, command substitution, arithmetic expansion, or pathname 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 \<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:
-```bash
- <<<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.
-### Duplicating File Descriptors
-The redirection operator
-```bash
-[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
-```bash
-[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
-```bash
-[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
-```bash
-[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
-```bash
-[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.
-</markdown>

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