Parameter expansion for defaults and errors

The $ character introduces parameter expansion, and braces around the parameter name are optional but protect it from characters immediately following it; braces are required for positional parameters with more than one digit, so ${11} is the eleventh positional parameter while $11 is the first parameter followed by a literal 1. A leading ! introduces indirect expansion, using the value of the named parameter as the name of the parameter to expand. ${parameter:-word} substitutes word when parameter is unset or null, and omitting the colon tests only for unset. ${parameter:=word} assigns word to an unset or null parameter; positional and special parameters may not be assigned this way. ${parameter:?word} writes word to standard error and a non-interactive shell exits with a non-zero status when parameter is null or unset. ${parameter:+word} substitutes word only when parameter is set and not null. In each of these forms word undergoes tilde expansion, parameter expansion, command substitution, and arithmetic expansion. Scope: Bash. Reviewed October 4, 2026.