Bash – Execute ‘Clear’ Command Before Any Other Command

First preexec_invoke_exec has to be modified to prevent multiple executions of preexec. Also, modify preexec to take in account the actual number of lines in $PS1 :

preexec () { 
    # delete old prompt; one "\e[1A\e[K" per line of $PS1
    for (( i=0, l=$(echo -e $PS1 | wc -l) ; i < l ; i++ ))
    do             
        echo -ne "\e[1A\e[K"
    done
    # replacement for prompt
    echo -ne "\e[31;1m$ \e[0m"
    echo -n "$1"
    echo -ne "  \e[37;2m["
    echo -n "$2"
    echo -e "]\e[0m"
}

preexec_invoke_exec () {
    [ -n "$DONTCLEANPROMPT" ] && return
    DONTCLEANPROMPT=x
    [ -n "$COMP_LINE" ] && return  # do nothing if completing
    [ "$BASH_COMMAND" = "$PROMPT_COMMAND" ] && return # don't cause a preexec for $PROMPT_COMMAND
    local this_command=`history 1 | sed -e "s/^[ ]*[0-9]*[ ]*//g"`;
    local this_pwd=`pwd`
    preexec "$this_command" "$this_pwd"
}

trap 'preexec_invoke_exec' DEBUG

PROMPT_COMMAND='unset DONTCLEANPROMPT'

In order for preexec to be run again, DONTCLEANPROMPT has to be either unset or set to ''. This is done with PROMPT_COMMAND, which is run just before the primary prompt is issued. Therefore preexec will be run once and only once for every command line.

These strange escape sequences are color-changing commands.

The completion code runs the following command to list available commands:

git help -a|egrep '^  [a-zA-Z0-9]'

The output of git help -a looks like this:

add grep remote add--interactive hash-object remote-ext am help remote-fd … fsck-objects receive-pack write-tree gc reflog get-tar-commit-id relink

If grep is configured to print the matching part of the line in color, then command names that are in the first column will have their first letter highlighted:

$ git help -a|egrep --color=always '^  [a-zA-Z0-9]' | cat -v | head -n 1
^[[01;31m^[[K  a^[[m^[[Kdd                      grep                     remote

When bash sees this output, it thinks that ^[[01;31m^[[K, a^[[m^[[Kdd, grep and remote are possible commands. The first one won't turn up, the last two are correct, the second one is mangled.

You need to configure grep not to use colors when its output is not on a terminal. If you've aliased egrep to egrep --color=always (and ditto for grep and fgrep), change that to --color=auto. If you've set the GREP_OPTIONS variable somewhere, change --color=always to --color=auto there.