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While aliases let you create convenient names for commands, they don't really let you change the shell's behavior. Options are one way of doing this. A shell option is a setting that is either "on" or "off." While several options relate to arcane shell features that are of interest only to programmers, those that we will cover here are of interest to all users.
The basic commands that relate to options are set -o optionname and set +o optionname. You can change more than one option with the one set command by preceding each optionname with a -o or +o. The use of plus (+) and minus (-) signs is counterintuitive: the - turns the named option on, while the + turns it off. The reason for this incongruity is that the dash (-) is the conventional UNIX way of specifying options to a command, while the use of + is an afterthought.
Most options also have one-letter abbreviations that can be used in lieu of the set -o command; for example, set -o noglob can be abbreviated set -f. These abbreviations are carryovers from the Bourne shell. Like several other "extra" bash features, they exist to ensure upward compatibility; otherwise, their use is not encouraged.
Table 3-1 lists the options that are useful to general UNIX users. All of them are off by default except as noted.
There are several other options (21 in all; Appendix B lists them). To check the status of an option, just type set -o. bash will print a list of all options along with their settings.
bash 2.0 introduced a new built-in for configuring shell behaviour, shopt. This built-in is meant as a replacement for option configuration originally done through environment variables and the set command. 
The shopt -o functionality is a duplication of parts of the set command and is provided for completeness on the part of shopt, while retaining backward compatibility by its continued inclusion in set.
The format for this command is shopt options option-names. Table 3-2 lists shopt's options.
The default action is to unset (turn off) the named options. If no options and arguments are given, or the -p option is used, shopt displays a list of the settable options and the values that they currently have. If -s or -u is also given, the list is confined to only those options that are set or unset, respectively.
A list of the most useful option names is given in Table 3-3. A complete list is given in Appendix B.
We'll look at the use of the various options later in this chapter.
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