The DEBUG variable enables three types of debugging output dependant on the settings of various bits.
bit value purpose
0 1 Display all commands executed
1 2 Display all alias substitutions made
2 4 Display all function calls and return values
DEBUG 3 will show all command executed and all alias expansions.
DEBUG 7 will show EVERYTHING. It is pretty messy so you might
want to set HOLD_MODE on when you use this.
Keep in mind that flush will flush pending client output as
well, in case you get more than you bargained for.