Changing Your PS1's Prompt Based on The Previous Command's Return Value

You may find it useful for your PS1 to signal if a previous command returned 0 or not. One neat way to do this that doesn’t add clutter is to have your PS1 change colour based on the previous command’s return value.

I find that it also works well for scrollback, allowing me to at a quick glance where things went wrong. It can also draw attention to nonzero return values for programs that don’t properly signal that they didn’t properly exit to a user.

Using the man page for the console escape codes as reference, I wanted the PS1 to be red or code 31 when there is a nonzero return value, and green or 32 for a zero return value.

I also keep my PS1 prompt minimal being just a $ character, but it should be trivial to adapt your own prompt.

Setting the Colours #

The green and red prompts for the $ character can be set like so

# green prompt
PS1="\[\e[32m\]$ \[\e[0m\]"
# red prompt
PS1="\[\e[31m\]$ \[\e[0m\]"

where

• \e is the escape sequence
• \e[0m resets the attributes to the default at the end of the prompt.
• The \[ and \] are used to wrap non-printing control sequences (in our case the colour escape sequences), so word wrapping doesn’t break.

Colour Switching on Return Value #

We can use special shell variable $? that gives us the return value of the last executed command. What we want is to return the string 32 (green) if it’s zero, and 31 (red) otherwise. Using bash conditionals, we can test this

$ true
$ [[ $? = 0 ]] && printf 32 || printf 31
32
$ false
$ [[ $? = 0 ]] && printf 32 || printf 31
31

Putting it together #

Inserting the conditional into the PS1 where the colour sequence number was as a subshell we have

PS1='\[\e[$([[ $? = 0 ]] && printf 32 || printf 31)m\]$ \[\e[0m\]'

(Note that we use single quotes so that the subshell isn’t evaluated when setting the PS1 variable)

Now whenever you run a command and it returns 0 the prompt should be green, and if it’s nonzero the prompt will turn red.