Shebang in Bash Scripts – Why Use /bin/sh Instead of /bin/bash?

Your question is a bit rambling. I'll answer what seems to be the central part, on the difference between ksh and bash that you observe.

You have encountered what is probably the #1 incompatibility between ksh and bash when it comes to scripts. ATT ksh (both ksh88 and ksh93) and zsh execute the last (rightmost) command in a pipeline in the parent shell, whereas other shells (Bourne, ash, bash, pdksh, mksh) execute all the commands including the last one in a subshell.

Here is a simple test program:

msg="a subshell"
true | msg="the parent shell"
echo "This shell runs the last command of a pipeline in $msg"

In ATT ksh and zsh, the second assignment to msg is executed in the parent shell so the effect is visible after the pipeline. In other shells, this assignment is executed in a subshell so the first assignment remains in place in the parent.

A workaround is to execute the rest of the script in the pipeline. This is a common idiom for reading data and doing some processing afterward:

output_some_stuff | {
  var=
  while IFS= read -r line; do
    var=$(process "$line")
  done
  use "$var"
}

You appear to have run into a ksh bug. I recommend upgrading to a non-buggy version. If that isn't possible, try Stephane Chazelas's workaround. While you can try running your scripts in bash, it is not (and does not pretend to be) a drop-in replacement for ksh; there are plenty of ksh features that bash doesn't have (and vice versa). Bash and ksh are only compatible in their POSIX core and some other central features (in particular arrays, [[ … ]], and local variables in functions declared by typeset).

You could also try zsh, which when invoked as ksh behaves in a way that's a bit closer to ksh than bash is. You may nonetheless run into incompatibilities.

Things that may help you:

The preexec hook function is called before every command line is executed (not before every command).

preexec() echo will execute: $1

precmd before each prompt.

precmd() echo executed: $history[$[HISTCMD-1]]

You can extend a function like this:

functions[git]='
  (){ '$functions[git]'; } "$@"; local myret=$?
  do-extra-stuff
  return $myret'

Or:

functions[git-old]=$functions[git]
git() {
  git-old "$@"
  local myret=$?
  do-extra-stuff || return $myret
}

The DEBUG trap is executed after each command:

trap 'echo $ZSH_DEBUG_CMD returned with $?' DEBUG

You can redefine the accept-line widget (mapped to Enter) to do stuff in there:

accept-line() {
  zle -R "going to run: $BUFFER"
  sleep 1
  zle .$WIDGET
}

zle -N accept-line