Bash prompt execute command every time a new prompt is displayed

Make sure that the prompt_subst option is turned on. If necessary, add the following line to your ~/.zshrc:

setopt prompt_subst

This tells zsh to reevaluate the prompt string each time it's displaying a prompt. Then, make sure you are assigning PS1 (or some other variable that is used by the prompt theme) as desired:

PS1='${PWD/#$HOME/~}'

The single quotes protect the special characters such as $ from being evaluated when you set the variable.

export PS1="…`__git_ps1`…"

With `__git_ps1` inside double quotes, this command runs the command __git_ps1 and assigns its output (and other surrounding text) to the variable PS1. Thus your prompt is showing the branch that was determined when your .bash_profile was executed.

You need to run __git_ps1 each time bash displays a prompt. (Actually you don't need to run it again until the git information has changed, but that's difficult to detect.) There are two ways to do that.

• Include the literal text `__git_ps1` in the PS1 variable. Make sure that bash is configured to perform shell expansions on the prompt string, with the promptvars option turned on; that's the case by default but it can be turned off with shopt -u promptvars.

  PS1='\n\[…\]$(__git_ps1)\[…\]\$ '
  

• Update the prompt content by a command run from the PROMPT_COMMAND variable.

  update_PS1 () {
    PS1="\\n\\[…\\]$(__git_ps1)\[…\]\\$ "
  }
  shopt -u promptvars
  PROMPT_COMMAND=update_PS1
  

By the way, the prompt is a shell configuration, not a global setting, so you should set it in ~/.bashrc, not in ~/.bash_profile.