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$ set -o noclobber $ cat myletter >! oldletter
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Although the redirection operator and the filename are placed after the command, the redirection operation is not executed after the command. In fact, it is executed before the command. The redirection operation creates the file and sets up the redirection before it receives any data from the standard output. If the file already exists, it will be destroyed and replaced by a file of the same name. In effect, the command generating the output is executed only after the redirected file has been created. In the next example, the output of the ls command is redirected from the screen device to a file. First the ls command lists files and, in the next command, ls redirects its file list to the listf file. Then the cat command displays the list of files saved in listf. Notice the list of files in listf includes the listf filename. The list of filenames generated by the ls command includes the name of the file created by the redirection operation- in this case, listf. The listf file is first created by the redirection operation, and then the ls command lists it along with other files. This file list output by ls is then redirected to the listf file, instead of being printed on the screen.
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$ ls mydata intro preface $ ls > listf $ cat listf mydata intro listf preface
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Errors occur when you try to use the same filename for both an input file for the command and the redirected destination file. In this case, because the redirection operation is executed first, the input file, because it exists, is destroyed and replaced by a file of the same name. When the command is executed, it finds an input file that is empty. In the cat command shown next, the file myletter is the name for both the destination file for redirected output and the input file for the cat operation. As shown in the next example, the redirection operation is executed first, destroying the myletter file and replacing it with a new and empty myletter file. Then the cat operation is executed and attempts to read all the data in the myletter file. However, nothing new is now in the myletter file.
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$ cat myletter > myletter
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You can also append the standard output to an existing file using the >> redirection operator. Instead of overwriting the file, the data in the standard output is added at the end of the file. In the next example, the myletter and oldletter files are appended to the alletters file. The alletters file will then contain the contents of both myletter and oldletter.
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$ cat myletter >> alletters $ cat oldletter >> alletters
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The Standard Input
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Many Linux commands can receive data from the standard input. The standard input itself receives data from a device or a file. The default device for the standard input is the keyboard. Characters typed into the keyboard are placed in the standard input, which is then directed to the Linux command. The cat command without a filename argument reads data from standard input. When you type in data on the keyboard, each character will be placed in the standard input and directed to the cat command. The cat command then sends the character to the standard output-the screen device-which displays the character on the screen. If you combine the cat command with redirection, you have an easy way of saving what you have typed to a file. As shown in the next example, the output of the cat operation is redirected to the mydat file. The mydat file will now contain all the data typed in at the keyboard. The cat command, in this case, still has no file arguments. It will receive its data from the standard input, the keyboard device. The redirection operator redirects the output of the cat command to the file mydat. The cat command has no direct contact with any files; it is simply receiving input from the standard input and sending output to the standard output.
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