Zsh script to prepend a file with string(s)

prepend

#!/bin/zsh
# ================================================
# PREPEND ========================================
# ================================================
# Prepends a string to a target file

# ------------------------------------------------
# TRAP->TERM -------------------------------------
# ------------------------------------------------
trap "exit 1" TERM

local TOP_PID=$$

# ------------------------------------------------
# UTILITY ----------------------------------------
# ------------------------------------------------
function _exit_fatal() {
  echo "fatal: $1"
  kill -s TERM $TOP_PID
}

# ------------------------------------------------
# MAIN -------------------------------------------
# ------------------------------------------------
#
# Validate target file
# ------------------------------------------------
local _target=$1

if [ $# -lt 1 ] ; then
  _exit_fatal "No target file given."
fi

test -e $_target || _exit_fatal "The target '$_target' does not exist."
test -f $_target || _exit_fatal "The target '$_target' is not a file."
test -w $_target || _exit_fatal "The target '$_target' is not writable."

# Validate string(s) to prepend
# ------------------------------------------------
if [ $# -lt 2 ] ; then
  _exit_fatal "No string(s) given to prepend to target file given."
fi

shift

local _to_prepend="$*"

# Create a temporary file for merger
# ------------------------------------------------
local _temporary_file_path=`mktemp "/tmp/\`basename $0\`.XXXX"`

if [[ $? != 0 ]] ; then
  _exit_fatal "Could not create a temporary file in '/tmp' with mktemp."
fi

# Prepend/merge the temporary file
# ------------------------------------------------
if ! ( echo $_to_prepend >> $_temporary_file_path && cat $_target >> $_temporary_file_path ) ; then
  _exit_fatal "Could not prepend the inputs to the temporary file at '$_temporary_file_path'."
fi

# Overwrite the original target
# ------------------------------------------------
if ! mv $_temporary_file_path $_target ; then
  _exit_fatal "Could not move/overwrite the original target file at '$_target' with the temporary file at '$_temporary_file_path'."
fi