Customize zsh's prompt when displaying previous command exit code

Question

Zsh includes the ability to display the return code/exit code of the previous command in the prompt by using the %? escape sequence.

However I would like to have the following prompt:

user@host ~ [%?] % 

when the exit code is different from 0 and:

user@host ~ % 

when exit code is 0.

If I use %? alone it is always displayed, even if %? is 0. In addition I want the square brackets but only when the exit code not 0.

What is the simplest way to do this?

Answer 1

Add this in the position in PS1 where you want the exit code to appear:

%(?..[%?] ) 

It's a conditional expression. The part between the two dots (nothing in this case) is output if the expression before the first dot is true. The part after the second dot is output if it's false.

For example:

PS1='%n%m %~ %(?..[%?] )%# ' 

Answer 2

Alternatively, you can:

setopt PRINT_EXIT_VALUE 

to always print a newline showing previous return value.

I don't prefer this for ordinary use, but it is often good for debugging shell scripts.