The return status is zero. The commands in arg are to be read and executed when the shell receives signal sigspec . If arg is the null string, then the signal specified by each sigspec is ignored by the shell and commands it invokes. If a sigspec is 0 or EXIT , arg is executed when the shell exits. Signals ignored upon entry to the shell cannot be trapped or reset.
Trapped signals are reset to their original values in a child process when it is created.
The return status is zero unless a sigspec does not specify a valid signal. If mode is omitted, the current value of the mask is printed. The return status is zero if the mode is successfully changed or if no mode argument is supplied, and non-zero otherwise. Each variable or function name is removed. Readonly variables and functions may not be unset.