Variable Substitution
Variable values can be accessed and manipulated using variable expansion. Basic expansion is done by preceding the variable name with the $ character. Other types of expansion can be used to return portions or the length of variables, use default or alternate values, assign default or alternate values, and more.
Variable Expansion Format
| ${variable} | value of variable |
| ${#variable} | length of variable |
| ${variable:–word} | value of variable if set and not null, else print word. If: is omitted, variable is only checked if it is set. |
| ${variable:=word} | value of variable if set and not null, else variable is set to word, then expanded. If: is omitted, variable is only checked if it is set. |
| ${variable:?} | value of variable if set and not null, ... |
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