Quoting directory names in bash prompt

Question

I want to quote the current directory in my prompt.

Eg, if I do:

mkdir $'new\nline'; cd $'new\nline'

I want my prompt to display $'new\nline', and NOT print a literal newline.

I'm seeing interesting behaviour trying to print backslashes (\) in bash 5.0.9:

p='\\n'       && echo -E "${p@P}"  # 2 slashes; output = '\n'
p='\\\\n'     && echo -E "${p@P}"  # 4 slashes; output = '\n'
p='\\\\\\n'   && echo -E "${p@P}"  # 6 slashes; output = '\\n'
p='\\\\\\\\n' && echo -E "${p@P}"  # 8 slashes; output = '\\n'

Note: ${parameter@P} is a string that is the result of expanding the value of parameter as if it were a prompt

Why is the output equivalent with (2 and 4), and (6 and 8) slashes?

Given this confusion, and:

mkdir '\\n' && cd '\\n'

I couldn't work out how to programatically transform \\n into a string such that it was displayed in a prompt either as: \\\\n or $'\\\\n', as well as handling the literal newline case.

---

How do I get directory names quoted in the prompt such that:

• ~ is displayed for $HOME and a leading ~/ for subdirectories
• Other paths are escaped only if required
• A copy-paste of the displayed string is a valid shell token referring to the current directory

Eg "$HOME/dir with spaces" should be displayed as either:

• ~/dir\ with\ spaces
• ~/$'dir with spaces'
• ~/'dir with spaces'

Answer 1

If I include \w in the prompt, and cd $'/tmp/new\nline', Bash shows just /tmp/newline in the prompt. It doesn't seem to print the literal newline, but that's also not an unambiguous output format.

${var@P} is meant to expand prompt-style escapes, like \u for username, \h for hostname and \w for the working directory, I doubt you want those here. Instead ${var@Q}, which quotes the output might be more useful?

Setting PS1='${PWD@Q}\$ ', I get the prompt: $'/tmp/new\nline'$, or '/tmp'$ if the path is nicer. An alternative might be PS1='$(printf "%q" "$PWD")\$ ', which gives a different quoting in some cases, and e.g. leaves the quotes out in the case of a "nice" path, so /tmp$.

---

To get the home directory shown as a tilde, one option would be to do that replacement manually:

set_ppath() {
    printf -v ppath "%q" "$PWD"
    ppath="${ppath/$HOME/"~"}";
}
PROMPT_COMMAND=set_ppath
PS1='$ppath\$ '

It still quotes the whole path if a part of it needs quoting, though. To work around that, I suppose you'd have to walk the path piece by piece.

---

I have no idea why ${var@P} collapses the backslashes like that, though.

Answer 2

Based on ilkkachu's answer, I came up with:

PROMPT_COMMAND=_prompt_bash_set

_prompt_bash_set() {
  # Working directory
  local prefix=''  # Used to contain '~' if $PWD directory is under $HOME
  local dir=$PWD

  # Replace leading $HOME with ~ if at beginning of string
  case "$PWD"/ in # Extra slash at end prevents substring match
    "$HOME"/)  # $PWD == $HOME itself
      prefix=\~  # no trailing /
      dir='' ;;
    "$HOME"/?*)  # $PWD is a subdir of $HOME
      prefix=\~/  # Add trailing / so that possibly quoted string goes after ~/ for path validity
      dir=${PWD#$HOME/} ;;   # Remove $HOME and / as this is now in $prefix
  esac

  if [[ $PWD != $HOME ]]; then  # $PWD != $HOME
    # Avoid ${PWD@Q} as it produced a literal \n inside $''
    printf -v dir '%q' "$dir"  # Quote directory minus $HOME

    # Replace a single slash with 4!?! slashes
    # https://unix.stackexchange.com/q/543473/143394
    dir=${dir//\\/\\\\\\\\}

    # Prevent command and variable substitution:
    dir=${dir//\`/\\\`}  # escape ` as \`
    dir=${dir//\$/\\\$}  # escape $ as \$
  fi

  PS1='\u@\h:'$dir'\$ '
}

This has the quoted directory as literal text in $PS1 rather than inclusion via a variable.

The advantage of doing it this way is that I can then go on to colour slashes in the directory which my eyes find easier to parse.