preexec.bash -- Bash support for ZSH-like 'preexec' and 'precmd' functions. The 'preexec' function is executed before each interactive command is executed, with the interactive command as its argument. The 'precmd' function is executed before each prompt is displayed.

preexec.bash

#!/bin/bash

# preexec.bash -- Bash support for ZSH-like 'preexec' and 'precmd' functions.

# The 'preexec' function is executed before each interactive command is
# executed, with the interactive command as its argument.  The 'precmd'
# function is executed before each prompt is displayed.

# To use, in order:

#  1. source this file
#  2. define 'preexec' and/or 'precmd' functions (AFTER sourcing this file),
#  3. as near as possible to the end of your shell setup, run 'preexec_install'
#     to kick everything off.

# Note: this module requires 2 bash features which you must not otherwise be
# using: the "DEBUG" trap, and the "PROMPT_COMMAND" variable.  preexec_install
# will override these and if you override one or the other this _will_ break.

# This is known to support bash3, as well as *mostly* support bash2.05b.  It
# has been tested with the default shells on MacOS X 10.4 "Tiger", Ubuntu 5.10
# "Breezy Badger", Ubuntu 6.06 "Dapper Drake", and Ubuntu 6.10 "Edgy Eft".


# Copy screen-run variables from the remote host, if they're available.

if [[ "$SCREEN_RUN_HOST" == "" ]]
then
    SCREEN_RUN_HOST="$LC_SCREEN_RUN_HOST"
    SCREEN_RUN_USER="$LC_SCREEN_RUN_USER"
fi

# This variable describes whether we are currently in "interactive mode";
# i.e. whether this shell has just executed a prompt and is waiting for user
# input.  It documents whether the current command invoked by the trace hook is
# run interactively by the user; it's set immediately after the prompt hook,
# and unset as soon as the trace hook is run.
preexec_interactive_mode=""

# Default do-nothing implementation of preexec.
function preexec () {
    true
}

# Default do-nothing implementation of precmd.
function precmd () {
    true
}

# This function is installed as the PROMPT_COMMAND; it is invoked before each
# interactive prompt display.  It sets a variable to indicate that the prompt
# was just displayed, to allow the DEBUG trap, below, to know that the next
# command is likely interactive.
function preexec_invoke_cmd () {
    precmd
    preexec_interactive_mode="yes"
}

# This function is installed as the DEBUG trap.  It is invoked before each
# interactive prompt display.  Its purpose is to inspect the current
# environment to attempt to detect if the current command is being invoked
# interactively, and invoke 'preexec' if so.
function preexec_invoke_exec () {
    if [[ -n "$COMP_LINE" ]]
    then
        # We're in the middle of a completer.  This obviously can't be
        # an interactively issued command.
        return
    fi
    if [[ -z "$preexec_interactive_mode" ]]
    then
        # We're doing something related to displaying the prompt.  Let the
        # prompt set the title instead of me.
        return
    else
        # If we're in a subshell, then the prompt won't be re-displayed to put
        # us back into interactive mode, so let's not set the variable back.
        # In other words, if you have a subshell like
        #   (sleep 1; sleep 2)
        # You want to see the 'sleep 2' as a set_command_title as well.
        if [[ 0 -eq "$BASH_SUBSHELL" ]]
        then
            preexec_interactive_mode=""
        fi
    fi
    if [[ "preexec_invoke_cmd" == "$BASH_COMMAND" ]]
    then
        # Sadly, there's no cleaner way to detect two prompts being displayed
        # one after another.  This makes it important that PROMPT_COMMAND
        # remain set _exactly_ as below in preexec_install.  Let's switch back
        # out of interactive mode and not trace any of the commands run in
        # precmd.

        # Given their buggy interaction between BASH_COMMAND and debug traps,
        # versions of bash prior to 3.1 can't detect this at all.
        preexec_interactive_mode=""
        return
    fi

    # In more recent versions of bash, this could be set via the "BASH_COMMAND"
    # variable, but using history here is better in some ways: for example, "ps
    # auxf | less" will show up with both sides of the pipe if we use history,
    # but only as "ps auxf" if not.
    local this_command=`history 1 | sed -e "s/^[ ]*[0-9]*[ ]*//g"`;

    # If none of the previous checks have earlied out of this function, then
    # the command is in fact interactive and we should invoke the user's
    # preexec hook with the running command as an argument.
    preexec "$this_command"
}

# Execute this to set up preexec and precmd execution.
function preexec_install () {

    # *BOTH* of these options need to be set for the DEBUG trap to be invoked
    # in ( ) subshells.  This smells like a bug in bash to me.  The null stderr
    # redirections are to quiet errors on bash2.05 (i.e. OSX's default shell)
    # where the options can't be set, and it's impossible to inherit the trap
    # into subshells.

    set -o functrace > /dev/null 2>&1
    shopt -s extdebug > /dev/null 2>&1

    # Finally, install the actual traps.
    PROMPT_COMMAND=preexec_invoke_cmd
    trap 'preexec_invoke_exec' DEBUG
}

# Since this is the reason that 99% of everybody is going to bother with a
# pre-exec hook anyway, we'll include it in this module.

# Change the title of the xterm.
function preexec_xterm_title () {
    local title="$1"
    echo -ne "\e]0;$title\007"
}

function preexec_screen_title () {
    local title="$1"
    echo -ne "\ek$1\e\\"
}

# Abbreviate the "user@host" string as much as possible to preserve space in
# screen titles.  Elide the host if the host is the same, elide the user if the
# user is the same.
function preexec_screen_user_at_host () {
    local RESULT=""
    if [[ "$SCREEN_RUN_HOST" == "$SCREEN_HOST" ]]
    then
        return
    else
        if [[ "$SCREEN_RUN_USER" == "$USER" ]]
        then
            echo -n "@${SCREEN_HOST}"
        else
            echo -n "${USER}@${SCREEN_HOST}"
        fi
    fi
}

function preexec_xterm_title_install () {
    # These functions are defined here because they only make sense with the
    # preexec_install below.
    function precmd () {
        preexec_xterm_title "${TERM} - ${USER}@${SCREEN_HOST} `dirs -0` $PROMPTCHAR"
        if [[ "$TERM" == screen ]]
        then
            preexec_screen_title "`preexec_screen_user_at_host`${PROMPTCHAR}"
        fi
    }

    function preexec () {
        preexec_xterm_title "${TERM} - $1 {`dirs -0`} (${USER}@${SCREEN_HOST})"
        if [[ $TERM == screen ]]
        then
            local cutit="$1"
            local cmdtitle=`echo "$cutit" | cut -d " " -f 1`
            if [[ "$cmdtitle" == "exec" ]]
            then
                local cmdtitle=`echo "$cutit" | cut -d " " -f 2`
            fi
            if [[ "$cmdtitle" == "screen" ]]
            then
                # Since stacked screens are quite common, it would be nice to
                # just display them as '$$'.
                local cmdtitle="${PROMPTCHAR}"
            else
                local cmdtitle=":$cmdtitle"
            fi
            preexec_screen_title "`preexec_screen_user_at_host`${PROMPTCHAR}$cmdtitle"
        fi
    }

    preexec_install
}

This code allows doing things such as setting term window titles to the current running process (well, technically, to what you just typed on the cmdline). I didn't write this, but finding the original source to link to is difficult, so hopefully this will make it more likely for others to find this lovely work around to one of BASH'es failings.