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As mentioned earlier, if you want the value of a variable, you precede it with a $ (for example, $CITY). This is really just shorthand for the notation ${CITY}; curly braces are used when the value of the parameter needs to be placed next to other text without a space. Bash has special rules that allow you to expand the value of a variable in different ways. Going into all of the rules is probably overkill for a quick introduction to shell scripts, but the following list presents some common constructs you're likely to see in bash scripts that you find on your Linux system.

 ${var:-value}: If variable is unset or empty, expand this to value.

 ${var#pattern}: Chop the shortest match for pattern from the front of var's value.

 ${var##pattern}: Chop the longest match for pattern from the front of var's value.

 ${var%pattern}: Chop the shortest match for pattern from the end of var's value.

 ${var%%pattern}: Chop the longest match for pattern from the end of var's value.

Try typing the following commands from a shell to test how parameter expansion works:

 $ THIS="Example" $ THIS=${THIS:-"Not Set"} $ THAT=${THAT:-"Not Set"} $ echo $THIS Example $ echo $THAT Not Set

In the examples here, the THIS variable is initially set to the word Example. In the next two lines, the THIS and THAT variables are set to their current values or to Not Set, if they are not currently set. Notice that because I just set THIS to the string Example, when I echo the value of THIS it appears as Example. However, because THAT was not set, it appears as Not Set.