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Korn Shell: Unix and Linux Programming Manual, Third Edition, The by Anatole Olczak

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Substitution

The first step the shell takes in executing a simple-command is to perform substitutions on the words of the command. There are three kinds of substitution: parameter, command and arithmetic. Parameter substitutions, which are described in detail in the next section, take the form $name or ${...}; command substitutions take the form $(command) or ‘command‘; and arithmetic substitutions take the form $((expression)). If a substitution appears outside of double quotes, the results of the substitution are generally subject to word or field splitting according to the current value of the IFS parameter. The IFS parameter specifies a list of characters which are used to break a string up into several words; any characters from the set ...

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