How to expand PS1?

I have a shell script that runs the same command in several directories (fgit). For each directory, I would like it to show the current prompt + the command which will be run there. How do I get the string that corresponds to the decoded (expanded)PS1? For example, my default PS1 is

${debian_chroot:+($debian_chroot)}\[\e[1;32m\]\u\[\e[0m\]@\[\e[1;32m\]\h\[\e[0m\]:\[\e[1;34m\]\w\[\e[0m\]$(__git_ps1 ' (%s)')$

and I'd like to echo the resulting prompt username@hostname:/path$, preferably (but not necessarily) with the nice colors. A cursory look at the Bash manual didn't reveal any definite answer, and echo -e $PS1 only evaluates the colors.

Since Bash 4.4 you can use the @P expansion:

First I put your prompt string in a variable myprompt using read -r and a quoted here-doc:

read -r myprompt <<'EOF'
${debian_chroot:+($debian_chroot)}\[\e[1;32m\]\u\[\e[0m\]@\[\e[1;32m\]\h\[\e[0m\]:\[\e[1;34m\]\w\[\e[0m\]$(__git_ps1 ' (%s)')$ 
EOF

To print the prompt (as it would be interpreted if it were PS1), use the expansion ${myprompt@P}:

$ printf '%s\n' "${myprompt@P}"
gniourf@rainbow:~$
$

(In fact there are some \001 and \002 characters, coming from \[ and \] that you can't see in here, but you can see them if you try to edit this post; you'll also see them in your terminal if you type the commands).

To get rid of these, the trick sent by Dennis Williamson on the bash mailing list is to use read -e -p so that these characters get interpreted by the readline library:

read -e -p "${myprompt@P}"

This will prompt the user, with the myprompt correctly interpreted.

To this post, Greg Wooledge answered that you might as well just strip the \001 and \002 from the string. This can be achieved like so:

myprompt=${myprompt@P}
printf '%s\n' "${myprompt//[$'\001'$'\002']}"

To this post, Chet Ramey answered that you could also turn off line editing altogether with set +o emacs +o vi. So this will do too:

( set +o emacs +o vi; printf '%s\n' "${myprompt@P}" )

One great advantage of open source software is that the source is, well, open :-)

Bash itself does not provide this functionality but there are various tricks you can use to provide a subset (such as substituting \u with $USER and so on). However, this requires a lot of duplication of functionality and ensuring that the code is kept in sync with whatever bash does in future.

If you want to get all the power of prompt variables (and you don't mind getting your hands dirty with a bit of coding (and, if you do mind, why are you here?)), it's easy enough to add to the shell itself.

If you download the code for bash (I'm looking at version 4.2), there's a y.tab.c file which contains the decode_prompt_string() function:

char *decode_prompt_string (string) char *string; { ... }

This is the function that evaluates the PSx variables for prompting. In order to allow this functionality to be provided to users of the shell itself (rather than just used by the shell), you can follow these steps to add an internal command evalps1.

First, change support/mkversion.sh so that you won't confuse it with a "real" bash, and so that the FSF can deny all knowledge for warranty purposes :-) Simply change one line (I added the -pax bit):

echo "#define DISTVERSION \"${float_dist}-pax\""

Second, change builtins/Makefile.in to add a new source file. This entails a number of steps.

(a) Add $(srcdir)/evalps1.def to the end of DEFSRC.

(b) Add evalps1.o to the end of OFILES.

evalps1.o: evalps1.def $(topdir)/bashtypes.h $(topdir)/config.h \
           $(topdir)/bashintl.h $(topdir)/shell.h common.h

Third, add the builtins/evalps1.def file itself, this is the code that gets executed when you run the evalps1 command:

This file is evalps1.def, from which is created evalps1.c.
It implements the builtin "evalps1" in Bash.

Copyright (C) 1987-2009 Free Software Foundation, Inc.

This file is part of GNU Bash, the Bourne Again SHell.

Bash is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation, either version 3 of the License, or
(at your option) any later version.

Bash is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with Bash.  If not, see <http://www.gnu.org/licenses/>.

$PRODUCES evalps1.c

$BUILTIN evalps1
$FUNCTION evalps1_builtin
$SHORT_DOC evalps1
Outputs the fully interpreted PS1 prompt.

Outputs the PS1 prompt, fully evaluated, for whatever nefarious purposes
you require.
$END

#include <config.h>
#include "../bashtypes.h"
#include <stdio.h>
#include "../bashintl.h"
#include "../shell.h"
#include "common.h"

int
evalps1_builtin (list)
     WORD_LIST *list;
{
  char *ps1 = get_string_value ("PS1");
  if (ps1 != 0)
  {
    ps1 = decode_prompt_string (ps1);
    if (ps1 != 0)
    {
      printf ("%s", ps1);
    }
  }
  return 0;
}

The bulk of that is the GPL licence (since I modified it from exit.def) with a very simple function at the end to get and decode PS1.

Lastly, just build the thing in the top level directory:

./configure
make

The bash executable that appears can be renamed to paxsh, though I doubt it will ever become as prevalent as its ancestor :-)

And running it, you can see it in action:

pax> mv bash paxsh

pax> ./paxsh --version
GNU bash, version 4.2-pax.0(1)-release (i686-pc-linux-gnu)
Copyright (C) 2011 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

pax> ./paxsh

pax> echo $BASH_VERSION
4.2-pax.0(1)-release

pax> echo "[$PS1]"
[pax> ]

pax> echo "[$(evalps1)]"
[pax> ]

pax> PS1="\h: "

paxbox01: echo "[$PS1]"
[\h: ]

paxbox01: echo "[$(evalps1)]"
[paxbox01: ]

When you put one of the PSx variables into the prompt, echoing $PS1 simply gives you the variable, while the evalps1 command evaluates it and outputs the result.

Now, granted, making code changes to bash to add an internal command may be considered by some to be overkill but, if you want an perfect evaluation of PS1, it's certainly an option.

Why don't you just process the $PS1 escape substitutions yourself? A series of substitutions such as these:

p="${PS1//\\u/$USER}"; p="${p//\\h/$HOSTNAME}"

By the way, zsh has the ability to interpret prompt escapes.

print -P '%n@%m %d'

p=${(%%)PS1}

I like the idea of fixing Bash to make it better, and I appreciate paxdiablo's verbose answer on how to patch Bash. I'll have a go sometime.

However, without patching Bash source-code, I have a one-liner hack that is both portable and doesn't duplicate functionality, because the workaround uses only Bash and its builtins.

x="$(PS1=\"$PS1\" echo -n | bash --norc -i 2>&1)"; echo "'${x%exit}'"

Note that there's something strange going on with tty's and stdio seeing as this also works:

x="$(PS1=\"$PS1\" echo -n | bash --norc -i 2>&1 > /dev/null)"; echo "'${x%exit}'"

So although I don't understand what's going on with the stdio here, my hack is working for me on Bash 4.2, NixOS GNU/Linux. Patching the Bash source-code is definitely a more elegant solution, and it should be pretty easy and safe to do now that I'm using Nix.

How to expand PS1?

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