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$ set -o feature $ set +o feature turn the feature on turn the feature off
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Three of the most common features are described here: ignoreeof, noclobber, and noglob. Table 13-2 lists these different features, as well as the set command. Setting ignoreeof enables a feature that prevents you from logging out of the user shell with a CTRL-D. CTRLD is used not only to log out of the user shell, but also to end user input entered directly into the standard input. CTRL-D is used often for the Mail program or for utilities such as cat. You could easily enter an extra CTRL-D in such circumstances and accidentally log yourself out.
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The ignoreeof feature prevents such accidental logouts. In the next example, the ignoreeof feature is turned on using the set command with the -o option. The user can now only log out by entering the logout command.
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$ set -o ignoreeof $ ctrl-d Use exit to logout $
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Setting noclobber enables a feature that safeguards existing files from redirected output. With the noclobber feature, if you redirect output to a file that already exists, the file will not be overwritten with the standard output. The original file is preserved. Situations may occur in which you use, as the name for a file to hold the redirected output, a name you have already given to an existing file. The noclobber feature prevents you from accidentally overwriting your original file. In the next example, the user sets the noclobber feature on and then tries to overwrite an existing file, myfile, using redirection. The system returns an error message.
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$ set -o noclobber $ cat preface > myfile myfile: file exists $
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At times, you may want to overwrite a file with redirected output. In this case, you can place an exclamation point after the redirection operator. This will override the noclobber feature, replacing the contents of the file with the standard output.
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$ cat preface >! myfile
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Setting noglob enables a feature that disables special characters in the user shell. The characters *, , [], and ~ will no longer expand to matched filenames. This feature is helpful if you have special characters as part of the name of a file. In the next example, the user needs to reference a file that ends with the character, answers . First, the user turns off special characters using the noglob feature. Now the question mark on the command line is taken as part of the filename, not as a special character, and the user can reference the answers file.
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Environment Variables and Subshells: export
When you log into your account, Linux generates your user shell. Within this shell, you can issue commands and declare variables. You can also create and execute shell scripts. When you execute a shell script, however, the system generates a subshell. You then have two shells, the one you logged into and the one generated for the script. Within the script shell, you could execute another shell script, which would have its own shell. When a script has finished execution, its shell terminates and you return to the shell from which it was executed. In this sense, you can have many shells, each nested within the other. Variables you define within a shell are local to it. If you define a variable in a shell script, then, when the script is run, the variable is defined with that script's shell and is local to it. No other shell can reference that variable. In a sense, the variable is hidden within its shell.
You can define environment variables in all types of shells including the BASH, the Z shell, and the TCSH shell. The strategy used to implement environment variables in the BASH shell, however, is different from that of the TCSH shell. In the BASH shell, environment variables are exported. That is to say, a copy of an environment variable is made in each subshell. For example, if the EDITOR variable is exported, a copy is automatically defined in each subshell for you. In the TCSH shell, on the other hand, an environment variable is defined only once and can be directly referenced by any subshell. In the BASH shell, an environment variable can be thought of as a regular variable with added capabilities. To make an environment variable, you apply the export command to a variable you have already defined. The export command instructs the system to define a copy of that variable for each new shell generated. Each new shell will have its own copy of the environment variable. This process is called exporting variables. Thinking of exported environment variables as global variables is a mistake. A new shell can never reference a variable outside of itself. Instead, a copy of the variable with its value is generated for the new shell. Note You can think of exported variables as exporting their values to a shell, not to themselves. For those familiar with programming structures, exported variables can be thought of as a form of "call by value."
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