Is there a good way to replace home directory with tilde in bash?

Question

Im trying to work with a path and replace the home directory with a tilde in bash, Im hoping to get it done with as little external programs as necessary. Is there a way to do it with just bash. I got

${PWD/#$HOME/\~} 

But thats not quite right. It needs to convert:

/home/alice to ~ /home/alice/ to ~/ /home/alice/herp to ~/herp /home/alicederp to /home/alicederp 

As a note of interest, heres how the bash source does it when converting the \w value in the prompt:

/* Return a pretty pathname.  If the first part of the pathname is    the same as $HOME, then replace that with `~'.  */ char * polite_directory_format (name)      char *name; {   char *home;   int l;    home = get_string_value ("HOME");   l = home ? strlen (home) : 0;   if (l > 1 && strncmp (home, name, l) == 0 && (!name[l] || name[l] == '/'))     {       strncpy (tdir + 1, name + l, sizeof(tdir) - 2);       tdir[0] = '~';       tdir[sizeof(tdir) - 1] = '\0';       return (tdir);     }   else     return (name); } 

Answer 1

I don't know of a way to do it directly as part of a variable substitution, but you can do it as a command:

[[ "$name" =~ ^"$HOME"(/|$) ]] && name="~${name#$HOME}" 

Note that this doesn't do exactly what you asked for: it replaces "/home/alice/" with "~/" rather than "~". This is intentional, since there are places where the trailing slash is significant (e.g. cp -R ~ /backups does something different from cp -R ~/ /backups).