BASH supports a $PROMPT_COMMAND environment variable that defines a command to be executed before any first-level interactive prompt. I'm looking for a ZSH equilvalent of that.
The documentation says that there's a function precmd I can define to achive that; however, I have no idea how to define it from an environment variable.
I've considered passing an environment variable that would make ZSH read a file containing the definition of that function, but ZSH doesn't seem to support such things: it only reads global files and then per-user files. I can replace them but I cannot add to them without modifying the files, which I cannot do.
So how do I define a pre-prompt hook in ZSH via an environment variable, like I'd do using $PROMPT_COMMAND in BASH?
2 Answers
The simplest approach to emulate bash's $PROMPT_COMMAND which comes to my mind is to use the precmd hook, as you already figured out. Define it as
precmd() { eval "$PROMPT_COMMAND" } and you can do something like that:
$ PROMPT_COMMAND='echo Hello, it is now $(date)' Hello, it is now Mon, Mar 31, 2014 7:08:00 PM $ whoami user Hello, it is now Mon, Mar 31, 2014 7:08:21 PM $ Please note the single quotes in that example, otherwise $(date) will get expanded too early, i.e. already when defining $PROMPT_COMMAND and not when called before the prompt.
If you want to preserve (and don't want to alter) the existing definition, you can use that approach:
$ prmptcmd() { eval "$PROMPT_COMMAND" } $ precmd_functions=(prmptcmd) With that the prmptcmd functions is executed after the existing precmd() function.
Finally, here is a way which is suitable for use in a program package, which neither should modify user or system files nor can enter the commands interactive.
An example to spawn a bash session could be
PROMPT_COMMAND="echo foo" bash To spawn zsh you can use
ZDOTDIR=/program/dir zsh which causes /program/dir/.zshrc to be sourced. In this file the precmd() hook can be defined as explained above. If you want the user's settings in addition include source $HOME/.zshrc etc. in the program's .zshrc, too. This setup is maintainable, as no files outside the program directory are modified.
As a last addition, here is a proof of concept how to keep the newuser welcome, too. Use the following code in your /program/dir/.zshenv rc config file:
echo define precmd, traps, etc. autoload -Uz zsh-newuser-install if [[ ! -e "$HOME/.zshrc" ]]; then zsh-newuser-install -f mv $ZDOTDIR/.zshrc $HOME/.zshrc else builtin source $HOME/.zshrc fi As @mypy states, Zsh's precmd works similarly to Bash's PROMPT_COMMAND.
Here's an example that works for Bash or Zsh and doesn't use eval:
## ~/myprompt.sh # 'ZSH_VERSION' only defined in Zsh # 'precmd' is a special function name known to Zsh [ ${ZSH_VERSION} ] && precmd() { myprompt; } # 'BASH_VERSION' only defined in Bash # 'PROMPT_COMMAND' is a special environment variable name known to Bash [ ${BASH_VERSION} ] && PROMPT_COMMAND=myprompt # function called every time shell is about to draw prompt myprompt() { if [ ${ZSH_VERSION} ]; then # Zsh prompt expansion syntax PS1='%{%F{red}%}%n%{%f%}@%{%F{red}%}%m %{%F{cyan}%}%~ %{%F{white}%}%# %{%f%}' elif [ ${BASH_VERSION} ]; then # Bash prompt expansion syntax PS1='\[\e[31m\]\u\[\e[0m\]@\[\e[31m\]\h \[\e[36m\]\w \[\e[37m\]\$ \[\e[0m\]' fi } Run from shell init scripts:
## ~/.bashrc . ~/myprompt.sh and:
## ~/.zshrc . ~/myprompt.sh The prompts here are just examples. One can definitely do much trickier stuff.
