How can I find the program I'm hiding in bash

You can always get the path to the second foo with:

foo=$(type -Pa foo | tail -n+2 | head -n1)

(provided file paths don't contain newline characters).

Beware that may be a relative path which would stop to be valid after you run cd.

You could then do:

hash -p "$foo" foo

So that foo be invoked when you run foo.

I don't think there's an easier way to do this robustly than enumerating the directories in PATH. It isn't hard.

#!/bin/bash
set -f; IFS=:
for d in $PATH; do
  if [[ -f $d/foo && -x $d/foo && ! $d/foo -ef /home/bob/bin/foo ]]; then
    exec "$d/foo" "$@"
    exit 126
  fi
done
echo "$0: foo (real) not found in PATH"
exit 127

I assume there are no empty entries in PATH. Empty PATH entries are evil, write . explicitly (or better don't include it at all).

If you'll only ever run foo from the command line and not from other programs, make it a function instead of a script. From the function, run command foo to hide the function.

If foo is important, then it should be configured in the script. This I think is good practice, because you then EXPLICITLY peg it to the script. I do not like indirection, implicit or hidden stuff ... scripts should be very simple if they are to be deployed.

Otherwise if you insist on finding it, then it can be found by doing

whereisfoo="$(which foo)"

If this is not enough, then I fear what you may be doing is too complex.

Here’s an approach that I believe is slightly less messy than Gilles’ answer (although I concede that to be a subjective judgment).

#!/bin/sh
this_dir=$(dirname "$0")        # Alternatively, hard-code this as this_dir=$HOME/bin
redacted_PATH=$(echo ":${PATH}:" | sed -e "s:\:$this_dir\::\::" -e "s/^://" -e 's/:$//')
if obscured_prog=$(PATH=$redacted_PATH which foo)
then
            ⋮           # Munging to do before running /usr/bin/foo
        "$(obscured_prog)" argument(s)          # may be "$@", but might not be.
            ⋮           # Munging to do after running /usr/bin/foo
else
        echo "$0: foo (real) not found in PATH."
            ⋮           # Any code that you want to do anyway.
fi

This constructs redacted_PATH to be $PATH minus the $HOME/bin directory where this private copy of foo lives.  echo ":${PATH}:" adds colons to the beginning and end of $PATH, so every component of it will be preceded and followed by a colon — even the first and last one.  The sed searches for

: $this_dir :

(with spaces added for “clarity”) and replaces it with

:

i.e., it excises $this_dir from ":${PATH}:".  It then removes the colons from the beginning and end.

Then we temporarily set PATH to $redacted_PATH and search for foo using which.  If successful, we get a full path to it (e.g., /bin/foo or /usr/bin/foo), which we use to run the real (public/shared/system) copy of foo.  Since we changed PATH only temporarily, /bin/foo has access to the user’s ambient $PATH, and so, if /bin/foo runs brillig, it can find $HOME/bin/brillig (if it exists).

This will have a problem if $HOME/bin appears in $PATH multiple times, but that’s not too hard to remedy.