Configure shell to always print prompt on new line, like zsh

Question

If a command's output does not end with a \n, the next prompt appears, awkwardly, immediately afterwards:

$ echo -n hai
hai$

I just noticed a colleague whose shell (zsh, for what it's worth) is configured to print a % (with background and foreground colours inverted for emphasis) followed by a \n in such cases:

$ echo -n hai
hai%
$

I'd like to do the same. I use Bash. Is this possible? If so, what would I add to my ~/.bashrc?

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UPDATE

I've spent several hours gaining an understanding of how gniourf_gniourf's solution works. I'll share my findings here, in case they are of use to others.

• ESC[6n is the control sequence introducer for accessing the cursor position (http://en.wikipedia.org/wiki/ANSI_escape_code).

• \e is not a valid representation of ESC when using echo on OS X (https://superuser.com/q/33914/176942). \033 can be used instead.

• IFS is Bash's internal field separator (http://tldp.org/LDP/abs/html/internalvariables.html#IFSREF).

• read -sdR looks like shorthand for read -s -d -R, but in fact the "R" is not a flag, it's the value of the -d (delimiter) option. I decided to write read -s -d R instead to avoid confusion.

• The double-parentheses construct, (( ... )), permits arithmetic expansion and evaluation (http://tldp.org/LDP/abs/html/dblparens.html).

Here's the relevant snippet from my .bashrc:

set_prompt() {
  # CSI 6n reports the cursor position as ESC[n;mR, where n is the row
  # and m is the column. Issue this control sequence and silently read
  # the resulting report until reaching the "R". By setting IFS to ";"
  # in conjunction with read's -a flag, fields are placed in an array.
  local curpos
  echo -en '\033[6n'
  IFS=';' read -s -d R -a curpos
  curpos[0]="${curpos[0]:2}"  # strip leading ESC[
  (( curpos[1] > 1 )) && echo -e '\033[7m%\033[0m'

  # set PS1...
}

export PROMPT_COMMAND=set_prompt

Note: The curpos[0]="${curpos[0]:2}" line is unnecessary. I included it so this code could be used in a context where the row is also relevant.

Answer 1

A little trick using PROMPT_COMMAND:

The value of the variable PROMPT_COMMAND is examined just before Bash prints each primary prompt. If PROMPT_COMMAND is set and has a non-null value, then the value is executed just as if it had been typed on the command line.

Hence, if you put this in your .bashrc:

_my_prompt_command() {
    local curpos
    echo -en "\E[6n"
    IFS=";" read -sdR -a curpos
    ((curpos[1]!=1)) && echo -e '\E[1m\E[41m\E[33m%\E[0m'
}
PROMPT_COMMAND=_my_prompt_command

you'll be quite good. Feel free to use other fancy colors in the echo "%" part. You can even put the content of that in a variable so that you can modify it on the fly.

The trick: obtain the column of the cursor (with echo -en "\E[6n" followed by the read command) before printing the prompt and if it's not 1, print a % and a newline.

Pros:

• pure bash (no external commands),
• no subshells,
• leaves your PS1 all nice and clean: if you want to change your PS1 sometimes (I do this when I work in deeply nested directory — I don't like having prompts that run on several miles), this will still work.

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As tripleee comments, you could use stty instead of echoing a hard-coded control sequence. But that uses an external command and is not pure bash anymore. Adapt to your needs.

---

Regarding your problem with the ugly character codes that get randomly printed: this might be because there's still some stuff in the tty buffer. There might be several fixes:

1. Turn off and then on the echo of the terminal, using stty.

   set_prompt() {
       local curpos
       stty -echo
       echo -en '\033[6n'
       IFS=';' read -d R -a curpos
       stty echo
       (( curpos[1] > 1 )) && echo -e '\033[7m%\033[0m'
   }
   PROMPT_COMMAND=set_prompt
   

   the main difference is that the echo/read combo has been wrapped with stty -echo/stty echo that respectively disables and enables echoing on terminal (that's why the -s option to read is now useless). In this case you won't get the cursor position correctly and this might lead to strange error messages, or the % not being output at all.

2. Explicitly clear the tty buffer:

   set_prompt() {
       local curpos
       while read -t 0; do :; done
       echo -en '\033[6n'
       IFS=';' read -s -d R -a curpos
       (( curpos[1] > 1 )) && echo -e '\033[7m%\033[0m'
   }
   PROMPT_COMMAND=set_prompt
   

3. Just give up if the tty buffer can't be cleaned:

   set_prompt() {
       local curpos
       if ! read -t 0; then
           echo -en '\033[6n'
           IFS=';' read -s -d R -a curpos
           (( curpos[1] > 1 )) && echo -e '\033[7m%\033[0m'
       # else
       #     here there was still stuff in the tty buffer, so I couldn't query the cursor position
       fi
   }
   PROMPT_COMMAND=set_prompt
   

As a side note: instead of reading in an array curpos, you can directly obtain the position of the cursor in variables, say, curx and cury as so:

IFS='[;' read -d R _ curx cury

If you only need the y-position cury:

IFS='[;' read -d R _ _ cury