Variable expansion Modifiers (parameter expansion)

Modifier	Value
${variable:–word}	If variable is set and is non-null, substitute its value; otherwise, substitute word.
${variable:=word}	If variable is set or is non-null, substitute its value; otherwise, set it to word. The value of variable is substituted permanently. Positional parameters may not be assigned in this way.
${variable:+word}	If variable is set and is non-null, substitute word; otherwise, substitute nothing.
${variable:?word}	If variable is set and is non-null, substitute its value; otherwise, print word and exit from the shell. If word is omitted, the message parameter null or not set is printed.
${variable:offset}	Gets the substring of the value in variable starting at offset, where offset starts at 0 to the end of the string.
${variable:offset:length}	Gets the substring of the value in variable starting at offset, length characters over.

Using the colon with any of the modifiers (–, =, +, ?) checks whether the variable is not set or is null; without the colon, a variable set to null is considered to be set.