http://www.manabii.info/2012/05/bash-wn.html のスクリプトを PROMPT_COMMAND を使うように変更

woo_nyaa.bash

#!/bin/bash
# usage: . ./woo_nyaa.bash

LANG="ja_JP.UTF-8"
WOO_NYAA_STRINT=(
    "(」・ω・)」うー！"
    "(／・ω・)／にゃー！"
    "(」・ω・)」うー！"
    "(／・ω・)／にゃー！"
    "(」・ω・)」うー！"
    "(／・ω・)／にゃー！"
    "・・・・・・"
    "Let's＼(・ω・)／にゃー！"
)

woo_nyaa_index_succ () {
    WOO_NYAA_INDEX=$(( ($WOO_NYAA_INDEX + 1) % ${#WOO_NYAA_STRINT[@]} ))
}

PROMPT_COMMAND=woo_nyaa_index_succ
PS1='${WOO_NYAA_STRINT[$WOO_NYAA_INDEX]} \$ '