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CHAPTER 3 ES SEN TIAL UN IX (AN D LINUX) FOR THE ORA CLE DBA
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Redirecting Input and Output
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When using a UNIX window on your PC or a UNIX workstation, the keyboard is the standard way to input a command to the shell, and the terminal is the standard location for the output of the commands. Any resulting errors are called standard errors and are usually displayed on the screen.
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It s common to use the terms standard input, standard output, and standard error to refer to the standard input and output locations in the UNIX shell.
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However, you can also use a previously written file as input, or you can have UNIX send output to a file instead of the screen. This process of routing your input and output through files is called input and output redirection. You can redirect output to a special location called /dev/null when you want to get rid of the output. When you use /dev/null as the output location, all messages issued during the execution of a program are simply discarded and not recorded anywhere on the file system. The following example shows how redirecting a file s output to /dev/null make its contents disappear. $ $ $ $ cat testfile1 This is the first line of testfile1 cat testfile1 > /dev/null cat /dev/null
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In this example, the first cat command shows you the output of testfile1. However, after redirecting the cat command s output to /dev/null, the output of the cat command disappears.
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Redirecting the output of the cat command tends to defeat the purpose of running the command in the first place, but there will be other situations, such as when running a script, when you don t want to see the output of all the commands.
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Table 3-2 summarizes the key redirection operators in most versions of UNIX.
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Table 3-2. Input/Output Redirection in UNIX
Redirection Operator
< > >> << 2 >
Description
Redirects standard input to a command Redirects standard output to a file Appends standard output to a file Appends standard input to a file Redirects standard error
In the following example, the date command s output is stored in file1, and file2 in turn gets the output of file1: $ date > file1 $ file1 < file2
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You can achieve the same result with the use of the UNIX pipe (|): $ date | file2 The pipe command, which uses the pipe symbol (|), indicates that the shell takes the output of the command before the | symbol and makes it the input for the command after the | symbol.
Protecting Files from Being Overwritten
You can use the noclobber shell variable to avoid accidentally overwriting an existing file. It s a good idea to include this variable in your shell startup file, such as the .cshrc file, as shown here: set noclobber The noclobber command is very handy when you re redirecting output to a file.
Navigating Files and Directories in UNIX
As you might have inferred, files and directories in UNIX are pretty much the same as in the Windows system. In this section, you ll learn all about the UNIX file system and directory structure, and you ll learn about the important UNIX directories. You ll also learn some important file-handling commands.
Files in the UNIX System
Files are the basic data storage unit on most computer systems, used to store user lists, shell scripts, and so on. Everything in UNIX/Linux, including hardware devices, is treated as a file. The UNIX file system is hierarchical, with the root directory, denoted by a forward slash (/), as the starting point at the top.
Tip
In Oracle, everything is in a table somewhere; in UNIX, everything is in a file somewhere.
Files in a typical UNIX system can be one of the following three types: Ordinary files: These files can contain text, data, or programs. A file cannot contain another file. Directories: Directories contain files. Directories can also contain other directories because of the UNIX tree directory structure. Special files: These files are not used by ordinary users to input their data or text; rather, they are for the use of input/output devices, such as printers and terminals. The special files are called character special files if they contain streams of characters, and they are called block special files if they work with large blocks of data.
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