Whenever we execute a shell command, it returns an integer value. When this value is 0, last command was a success. Any other number is the error code. We can see the return of last command by printing the value of ? variable.

To test this:

• Enter command ls
• Check it's return by command echo $?
• Enter some random letters and press Enter
• Check return by echo $?

You should get an error in the second time when you enter some random command

chinmay@CC-T480:~/Desktop$ ls
file
chinmay@CC-T480:~/Desktop$ echo $?
0    <-- Success
chinmay@CC-T480:~/Desktop$ a
a: command not found
chinmay@CC-T480:~/Desktop$ echo $?
127  <-- Error code

We will use this $? value in our prompt to see if our last command has passed or failed.

We also need to know about environment variable PROMPT_COMMAND. Bash provides an environment variable called PROMPT_COMMAND. The contents of this variable are executed as a regular Bash command just before Bash displays a prompt.

Since our bash prompt should contain status of last command, it should be set inside PROMPT_COMMAND.

First we will write a simple shell function to update the prompt with return code.

exit_code() {
    # Save the exit status of last command. This
    # needs to be done first otherwise exit status of
    # other commands will be stored in this variable.
    EXIT="$?"

    # PS1 needs to be reset or else it will
    # be appended every time to previous one.
    PS1=""

    # This will be final prompt, whatever set earlier will be
    # overwritten by this.
    export PS1="$EXIT \w $ "
}

Then we can add this function to our PROMPT_COMMAND

# exit_code Should be first command in `PROMPT_COMMAND' to be executed or # else return status will always be 0/true as it will hold return status of
# previous command.

# First we check if prompt command is empty or not.
# If empty, just add `exit_code' to it. If non empty,
# make it `exit_code:$PROMPT_COMMAND'
[ -n "$PROMPT_COMMAND" ] && PROMPT_COMMAND="exit_code;$PROMPT_COMMAND" ||
    PROMPT_COMMAND="exit_code"

After adding the above 2 snippets in your .bashrc, you get something like this:

chinmay@CC-T480:~$ exit_code() {
>     EXIT="$?"
>     PS1=""
>     export PS1="$EXIT \w $ "
> }
chinmay@CC-T480:~$ PROMPT_COMMAND="exit_code"
0 ~ $ ls Desktop/
file
0 ~ $ ^C
130 ~ $ a
a: command not found
127 ~ $ echo $?
127
0 ~ $ echo $?    # return of last echo command
0
0 ~ $ 

Note that the first number in our prompt is the return status of last command. For ls, we see a 0 as it ran as expected. When I ran command a, bash threw an error saying command not found and it's error code is 127 which we can see in next line. For pressing ctrl-c, we can see error code as 130.

Checkout designing a minimal bash prompt.