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Variable substitution

Variable substitution happens when a $ prefixed word is found in a command. There are three types of variable substitution:

• $name
  • where name is a scalar variable. name is simply substituted in the word for its value. name can contain only letters, digits, or underscores.
• $name(index)
  • where name is the name of an array variable and index is the index into it. This is substituted by the value of the array element. name must contain only letters, digits, or underscores. index has variable, command, and backslash substitution performed on it too.
• ${name}
  • where name can have any characters in it except closing curly bracket. This is more or less the same as $name substitution except it is used to get around the restrictions in the characters that can form name.

An example of variable substitution is:

set a 1
set b $a

which sets the value of variable a to 1, and then sets the value of variable b to the value of variable a.