Conditional emoji on terminal success and failure

Bash has some variables that let you control the prompt:

• PROMPT_COMMAND
• PS1
• PS2
• PS3
• PS4

In this specific scenario, only PROMPT_COMMAND (code executed before printing the primary prompt) and PS1 (the primary prompt) are helpful.

And the variable ? let you know the exit status of the last command executed. For example:

command

if [[ "${?}" == '0' ]]; then
  echo 'OK'
else
  echo 'ERROR'
fi

So you just need to take advantage of these handy features:

# Using PROMPT_COMMAND
PROMPT_COMMAND='if [[ "${?}" == "0" ]]; then printf "[OK]"; else printf "[ERROR]"; fi'

# Using PS1
PS1='$(if [[ "${?}" == "0" ]]; then printf "[OK]"; else printf "[ERROR]"; fi)\$ '

Both ways would print something like this (assuming your initial prompt is $):

[OK]$ false
[ERROR]$ true
[OK]$ 

Just replace [OK] and [ERROR] with your desired emojis.

You can read the Controlling the Prompt section of Bash manual to learn more about this topic.