2. Variable expansions

1. We have already seen that sometimes we need to surround variable names in braces, to avoid any confusion:

   a="foo"

   echo "$a_file"

   echo "${a}_file"

2. Get a default value if the variable is unset (or empty):
   ${parameter:-word}

   foo=

   echo ${foo:-"substitute value if unset"}

   echo $foo

   foo=bar

   echo ${foo:-"substitute value if unset"}

   echo $foo

3. Assign a default value if the variable is unset (or empty):
   ${parameter:=word}

   foo=

   echo ${foo:="default value if unset"}

   echo $foo

   foo=bar

   echo ${foo:="default value if unset"}

   echo $foo

   Note: Positional and other special parameters cannot be assigned this way.

4. Exit with an error message if the parameter is unset or empty:
   ${parameter:?word}

   foo=

   echo ${foo:?"parameter is empty"}

   echo $?

   foo=bar

   echo ${foo:?"parameter is empty"}

   echo $?

5. Return the given value only if the parameter is not empty:
   ${parameter:+word}

   foo=

   echo ${foo:+"substitute value if set"}

   foo=bar

   echo ${foo:+"substitute value if set"}

6. Return the names of variables that begin with a prefix:
   ${!prefix*} or ${!prefix@}

   echo ${!BASH*}

   echo ${!BASH@}

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