Launch new terminal instance with zsh using custom prompt string

Question

I want to be able to launch a terminal emulator (preferably gnome-terminal) with interactive zsh running inside using a custom prompt string. I don't want to modify my default prompt string in .zshrc because I use that most often and it is just occasionally I want to launch a terminal with a custom zsh prompt string (more specifically, I want the time to show up in the right hand side prompt).

If I already have a terminal window open, I can solve this by running:

export RPS1=%T zsh

But I can't figure out the syntax for making this work together with launching a new terminal instance. This is what I have tried with the error message below each command:

gnome-terminal -- export RPS1=%T zsh
# Error: Failed to execute child process “export” (No such file or directory)

gnome-terminal -- sh -c export RPS1=%T zsh
# Prints all the options, like with `set`
# and then exits with "The child process
# exited normally with status 0".

gnome-terminal -- sh -c "export RPS1=%T zsh"
# The child process exited normally with status 0.

gnome-terminal -- sh -c "export RPS1=%T; zsh"
# This start the terminal with zsh, but the RSP1 is not changed

I considered reading a custom rc file, like in this answer, but there does not seem to be a way of doing this in zsh without using source, which was giving me similar problems as above.

Answer 1

In zshrc, allow for overriding RPS1 by setting it as:

# If rps1 is not set, use a default value
RPS1=${rps1-"Your usual RPROMPT"}

• Now you can launch a command with a different RPS1 by setting rps1 in its environment, for example:

rps1=%T gnome-terminal

• The parameter expansion form ${param-word} allows RPS1 to be set to null:

rps1= gnome-terminal

Parameters used by the shell (such as RPS1) needn't be exported. They are used to set up the shell, so should be set once in the rc file.

If a program (e.g. gnome-terminal) is launched with rps1 in its environment, any subsequent programs launched from it will see rps1 in their environment. To avoid this, either of the following could be added to zshrc: unset rps1 (after using it to set RPS1) or typeset +x rps1.

---

BTW, the following will export two shell variables:

export RPS1=%T zsh

• The above is export with two arguments, resulting in RPS1=%T & zsh='' being created in the environment
• To set RPS1 in the current shell, simply do:

RPS1=%T

Answer 2

When you run the shell, it's processing your /~.zshrc, which is overwriting the prompt.

One option would be to do something like this in your ~/.zshrc:

MY_PROMPT="${MY_PROMPT:-"> "}" # Set the value to "> " if it isn't currently set
RPS1="${MY_PROMPT}"

Then, if you start a shell, you'll get:

> 
> MY_PROMPT="my new prompt > " zsh
my new prompt >

Note here that it's important that I chose a non-standard variable (here MY_PROMPT) since standard variables might already contain default values before the shell processes your ~/.zshrc.