zsh: update prompt with current time when a command is started

Question

I have a zsh prompt I rather like: it evaluates the current time in precmd and displays that on the right side of the prompt:

[Floatie:~] ^_^ 
cbowns%                      [9:28:31 on 2012-10-29]

However, this isn't exactly what I want: as you can see below, this time is actually the time the previous command exited, not the time the command was started:

[Floatie:~] ^_^ 
cbowns% date                           [9:28:26 on 2012-10-29]
Mon Oct 29 09:28:31 PDT 2012
[Floatie:~] ^_^ 
cbowns% date                           [9:28:31 on 2012-10-29]
Mon Oct 29 09:28:37 PDT 2012
[Floatie:~] ^_^ 
cbowns%                                [9:28:37 on 2012-10-29]

Is there a hook in zsh to run a command just before the shell starts a new command so I can update the prompt timestamp then? (I saw Constantly updated clock in zsh prompt?, but I don't need it constantly updated, just updated when I hit enter.)

(The ^_^ is based on the previous command's return code. It shows ;_; in red when there's a nonzero exit status.)

Answer 1

I had a struggle to make this:

It displays the date on the right side when the command has been executed. It does not overwrite the command shown. Warning: it may overwrite the current RPROMPT.

strlen () {
    FOO=$1
    local zero='%([BSUbfksu]|([FB]|){*})'
    LEN=${#${(S%%)FOO//$~zero/}}
    echo $LEN
}

# show right prompt with date ONLY when command is executed
preexec () {
    DATE=$( date +"[%H:%M:%S]" )
    local len_right=$( strlen "$DATE" )
    len_right=$(( $len_right+1 ))
    local right_start=$(($COLUMNS - $len_right))

    local len_cmd=$( strlen "$@" )
    local len_prompt=$(strlen "$PROMPT" )
    local len_left=$(($len_cmd+$len_prompt))

    RDATE="\033[${right_start}C ${DATE}"

    if [ $len_left -lt $right_start ]; then
        # command does not overwrite right prompt
        # ok to move up one line
        echo -e "\033[1A${RDATE}"
    else
        echo -e "${RDATE}"
    fi

}

Sources:

• http://www.tldp.org/HOWTO/Bash-Prompt-HOWTO/x361.html
• https://stackoverflow.com/a/10564427/238913

Answer 2

This is in fact possible without resorting to strange hacks. I've got this in my .zshrc

RPROMPT='[%D{%L:%M:%S %p}]'

TMOUT=1

TRAPALRM() {
    zle reset-prompt
}

The TRAPALRM function gets called every TMOUT seconds (in this case 1), and here it performs a prompt refresh, and does so until a command starts execution (and it doesn't interfere with anything you type on the prompt before hitting enter). I know you don't need it constantly refreshed but it still gets the job done without needing a line for itself!

Source: http://www.zsh.org/mla/users/2007/msg00944.html (It's from 2007!)

Answer 3

zsh will run the preexec function just before executing a line. It would be simple to have that output the current time, a simple version would be just:

preexec() { date }

Modifying an existing prompt would be much more challenging.

Answer 4

You can remap the Return key to reset the prompt before accepting the line:

reset-prompt-and-accept-line() {
    zle reset-prompt
    zle accept-line
}

zle -N reset-prompt-and-accept-line

bindkey '^m' reset-prompt-and-accept-line

Answer 5

Building off @vitaŭt-bajaryn's cool ZSH style answer:

I think overriding the accept-line function is probably the most idiomatic zsh solution:

function _reset-prompt-and-accept-line {
  zle reset-prompt
  zle .accept-line     # Note the . meaning the built-in accept-line.
}
zle -N accept-line _reset-prompt-and-accept-line

Answer 6

You can use ANSI escape sequences to write over the previous line, like this:

preexec () {
  DATE=`date +"%H:%M:%S on %Y-%m-%d"`
  C=$(($COLUMNS-24))
  echo -e "\033[1A\033[${C}C ${DATE} "
}