$ ls * doc1 doc2 document docs mydoc myletter yourletter $ rm * $ ls $ in Software

Drawing QR Code JIS X 0510 in Software $ ls * doc1 doc2 document docs mydoc myletter yourletter $ rm * $ ls $

$ ls * doc1 doc2 document docs mydoc myletter yourletter $ rm * $ ls $
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Use the * wildcard character carefully and sparingly with the rm command. The combination can be dangerous. A misplaced * in an rm command without the -i option could easily erase all your files. The first command in the next example erases only those files with a .c extension. The second command, however, erases all files. Notice the space between the asterisk and the period in the second command. A space in a command line functions as a delimiter, separating arguments. The asterisk is considered one argument, and the .c another argument. The asterisk by itself matches all files and, when used as an argument with the rm command, instructs rm to erase all your files.
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$ rm *.c $ rm * .c
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The question mark, , matches only a single incomplete character in filenames. Suppose you want to match the files doc1 and docA, but not document. Whereas the asterisk will match filenames of any length, the question mark limits the match to just one extra character. The next example matches files that begin with the word "doc" followed by a single differing letter:
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$ ls doc1 docA document $ ls doc doc1 docA
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Whereas the * and wildcard characters specify incomplete portions of a filename, the brackets, [], enable you to specify a set of valid characters to search for. Any character placed within the brackets will be matched in the filename. Suppose you want to list files beginning with "doc", but only ending in 1 or A. You are not interested in filenames ending in 2 or B, or any other character. Here is how it's done:
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$ ls doc1 doc2 doc3 docA docB docD document $ ls doc[1A] doc1 docA
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You can also specify a set of characters as a range, rather than listing them one by one. A dash placed between the upper and lower bounds of a set of characters selects all characters within that range. The range is usually determined by the character set in use. In an ASCII character set, the range "a-g" will select all lowercase alphabetic characters from a through g, inclusive. In the next example, files beginning with the pattern "doc" and ending in characters 1 through 3 are selected. Then, those ending in characters B through E are matched.
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$ ls doc1 $ ls docB doc[1-3] doc2 doc3 doc[B-E] docD
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You can combine the brackets with other wildcard characters to form flexible matching operators. Suppose you only want to list filenames ending in either a .c or .o extension, but no other extension. You can use a combination of the asterisk and brackets: * [co]. The asterisk matches all filenames, and the brackets match only filenames with extension .c or .o.
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$ ls *.[co] main.c main.o calc.c
At times, a wildcard character is actually part of a filename. In these cases, you need to quote the character by preceding it with a backslash to reference the file. In the next example, the user needs to reference a file that ends with the character, answers . The is, however, a wildcard character and would match any filename beginning with "answers" that has one or more characters. In this case, the user quotes the with a preceding backslash to reference the filename.
$ ls answers\ answers
Standard Input/Output and Redirection
When UNIX was designed, a decision was made to distinguish between the physical implementation and the logical organization of a file. Physically, UNIX files are accessed in randomly arranged blocks. Logically, all files are organized as a continuous stream of bytes. Linux, as a version of UNIX, has this same organization. Aside from special system calls, the user never references the physical structure of a file. To the user, all files have the same organization-a byte stream. Any file can be easily copied or appended to another because all files are organized in the same way. In this sense, only one standard type of file exists in Linux, the byte-stream file. Linux makes no implementational distinction between a character file and a record file, or a text file and a binary file. This logical file organization extends to input and output operations. The data in input and output operations is organized like a file. Data input at the keyboard is placed in a data stream arranged as a continuous set of bytes. Data output from a command or program is also placed in a data stream and arranged as a continuous set of bytes. This input data stream is referred to in Linux as the standard input, while the output data stream is called the standard output. There is also a separate output data stream reserved solely for error messages, called the standard error (see the section on the standard error later in this chapter). Because the standard input and standard output have the same organization as that of a file, they can easily interact with files. Linux has a redirection capability that lets you easily move data in and out of files. You can redirect the standard output so that, instead of displaying the output on a screen, you can save it in a file. You can also redirect the standard input away from the keyboard to a file, so that input is read from a file instead of from your keyboard. When a Linux command is executed that produces output, this output is placed in the standard output data stream. The default destination for the standard output data stream is a device-in this case, the screen. Devices, such as the keyboard and screen, are treated as files. They receive and send out streams of bytes with the same organization as that of a byte-stream file. The screen is a device that displays a continuous stream of bytes. By default, the standard output will send its data to the screen device, which will then display the data. For example, the ls command generates a list of all filenames and outputs this list to the standard output. Next, this stream of bytes in the standard output is directed to the screen device. The list of filenames is then printed on the screen. The cat command also sends output to the standard output. The contents of a file are copied to the standard output whose default destination is the screen. The contents of the file are then displayed on the screen.
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