The below as either .zshrc or .bashrc inserts a header at the top of the history of every shell session.

in bash it's silent on standard out as desired and the header inserts before the first command of each session.

in zsh the header appears both on standard out and in the history *after* the first command of the session.

Changing "preexec" to "precmd" seems like it ought to put the header before the first command of the session.

But instead it breaks history, nothing appears in history.  Looks like a bug..

HISTTIMEFORMAT="%F%a%T  "
if [ $ZSH_NAME ]
then emulate -R sh              #whether or not history breaks is consistent whether both,
     setopt extendedhistory     #either, or neither of the emulate&setopt commands are included
else shopt -s histappend
fi
HIShead(){ echo ===header $WINDOW $SUDO_USER;}
HISTwin(){ [[ $WINDo = $WINDOW ]]||{ WINDo=$WINDOW;HISneww;};}
if [ $ZSH_NAME ]
then #precmd(){ HISTwin;}       #breaks history
     preexec(){ HISTwin;}       #inserts header after not before first command
     HISneww(){ fc -eHISplic;}
     HISplic(){ echo \#$(HIShead)>|$1;}         #zsh, insert a header whenever WINDOW is new
else HISneww(){ history -s $(HIShead);}         #bash, same
     PROMPT_COMMAND=HISTwin
fi