Log and debug -- without XDEBUGFD so it works with bash version <4.1

logging and tracing.sh

#!/bin/bash -eu

tag=$(dirname $(readlink -f $0))/$(basename $0)
# redirects output to logger, -s print it to stderr as well, -t add script name as a tag
# http://stackoverflow.com/questions/3173131/redirect-copy-of-stdout-to-log-file-from-within-bash-script-itself
exec > >( logger -s -t "${tag} [INFO ]" )

# trap handler: print last error - filename, line, return code to stderr
function error-trap-handler()
{
  FILENAME=$(dirname $(readlink -f $0))/$(basename $0)
  # filanem handled by output redirections as well but it does not work for included files
  LASTLINE="$1" # argument 1: last line of error occurence
  LASTERR="$2" # argument 2: error code of last command
  echo "${FILENAME} at line ${LASTLINE}: exit status of the last command: ${LASTERR}" >&2
}

# to print all commands with timestamps: export DEBUG=1
if [ "$DEBUG" == "1" ] ; then
        echo "DEBUG MODE is ON!"
        set -x
        echo "Traps: "
        trap -p
        echo "Flags: $-"
        export PS4='+ $(date "+%F %T") ${BASH_SOURCE}:${LINENO} ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
        exec 2> >( logger -s -p user.debug -t "${tag} [DEBUG]" )
else
        trap 'error-trap-handler ${LINENO} $?' ERR
        # redirect std err
        exec 2> >( logger -s -p user.err -t "${tag} [ERR  ]" )
fi

# If set, any trap on ERR is inherited by shell functions, command substitutions, 
# and commands executed in a subshell environment.
set -E