重定向

重定向

重定向指的是将命令行输出写入指定位置。

• cmd1 | cmd2：Pipe; take standard output of cmd1 as standard input to cmd2.
• > file：Direct standard output to file.
• < file：Take standard input from file.
• >> file：Direct standard output to file; append to file if it already exists.
• >| file：Force standard output to file even if noclobber is set.
• n>| file：Force output to file from file descriptor n even if noclobber is set.
• <> file：Use file as both standard input and standard output.
• n<> file：Use file as both input and output for file descriptor n.
• << label：Here-document; see text.
• n > file：Direct file descriptor n to file.
• n < file：Take file descriptor n from file.
• n >> file：Direct file descriptor n to file; append to file if it already exists.
• n>&：Duplicate standard output to file descriptor n.
• n<&：Duplicate standard input from file descriptor n.
• n>&m：File descriptor  n is made to be a copy of the output file descriptor.
• n<&m：File descriptor  n is made to be a copy of the input file descriptor.
• &>file：Directs standard output and standard error to file.
• <&-：Close the standard input.
• >&-：Close the standard output.
• n>&-：Close the output from file descriptor  n.
• n<&-：Close the input from file descriptor  n.
• n>&word：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.
• n<&word：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.
• n>&digit-：Moves the file descriptor digit to file descriptor  n, or the standard output (file descriptor 1) if n is not specified.
• 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.

>用来将标准输出重定向到指定文件。

$ ls -l /usr/bin > ls-output.txt

如果重定向后的指定文件已经存在，就会被覆盖，不会有任何提示。

如果命令没有任何输出，那么重定向之后，得到的是一个长度为0的文件。因此，>具有创建新文件或改写现存文件、将其改为长度0的作用。