A script which defines actions before and after each command execution

.bash_hooks

#!/bin/bash

# NOTES
# To activate hooks, run:
# . ./.bash_hooks

# To clear hooks, run:
# trap '' DEBUG
# PROMPT_COMMAND=

# To add a new action to the preceding hook
# define a function
# add it into __hooks__before_each_command

# To add a new action to the subsequent hook
# define a function
# add it into __hooks__after_each_command

filename="$(basename "$BASH_SOURCE")"

# deactivate hooks to initialize (activate at the end)
unset __HOOKS__ACTIVATED

########## START OF PRECEDING HOOK ##########
# define functions and add them into __hooks__before_each_command
function __hooks__set_git_clone_flag() {
    if [ 'git' = "$1" ] && [ 'clone' = "$2" ]; then
        echo "Setting __HOOKS__ON_GIT_CLONE=true in $filename"
        export __HOOKS__ON_GIT_CLONE=true
    fi
}

function __hooks__before_each_command() {
    local last_command=( $BASH_COMMAND )
    
    if [ -z "$__HOOKS__ACTIVATED" ]; then
        return
    fi

    # before each command
    __hooks__set_git_clone_flag "${last_command[@]}";

    if [ -z "$__HOOKS__EXECUTING_LINE" ]; then
        return
    fi

    unset __HOOKS__EXECUTING_LINE
    # before each first command of a comamnd line
}
########## END OF PRECEDING HOOK ##########



########## START OF SUBSEQUENT HOOK ##########
# define functions and add them into __hooks__after_each_command
function __hooks__prevent_duplicate_actions() {
    export __HOOKS__EXECUTING_LINE=true
}

function __hooks__unset_git_clone_flag() {
    unset __HOOKS__ON_GIT_CLONE
}

function __hooks__after_each_command() {
    if [ -z "$__HOOKS__ACTIVATED" ]; then
        return
    fi
    # after each command
    __hooks__prevent_duplicate_actions
    __hooks__unset_git_clone_flag
    
    if [ "$PREVPWD" != "$PWD" ]; then
        # after changing directories
        # do something here
        export PREVPWD="$PWD"
    fi
}
########## END OF SUBSEQUENT HOOK ##########



########## START REGISTERING HOOKS ##
function __hooks__register_function() {
    local f="$1"; shift
    local regex="^ *$f *$"
    # filter the given function name out of PROMPT_COMMAND to avoid invalid format and duplicate execution
    # awk splits PROMPT_COMMAND by ";", extract elements that UNMATCH to the given regex and join with ";"
    # avoided using ORS to skip appending ";" to the last record
    PROMPT_COMMAND="$(
        echo "$PROMPT_COMMAND" |
        awk -v regex="$regex" 'BEGIN { RS=";" } length($1) > 0 && $1 !~ regex { printf "%s%s", delim, $1; delim=";" }'
    )"
    # append the function name
    export PROMPT_COMMAND="${PROMPT_COMMAND:+$PROMPT_COMMAND;}$f"
}

# register the preceding hook
echo 'registering the preceding hook..'
trap '__hooks__before_each_command' DEBUG
# register the subsequent hook
echo 'registering the subsequent hook..'
__hooks__register_function '__hooks__after_each_command'
unset __hooks__register_function
# initialize PREVPWD
echo 'initializing $PREVPWD..'
export PREVPWD="$PWD"
unset filename
# activate hooks
echo 'activating hooks..'
export __HOOKS__ACTIVATED=true
########## FINISH REGISTERING HOOKS ##########