"Shebang" function for Zsh

shebang.zsh

# shebang [-iv] [-t interpreter] [-I extension] [-J [extension]] [filename]
#
# If no filename is given, print a shebang. If a filename is given, append a shebang to the file and print its contents. If a filename is given and the -I or -J options are specified, append a shebang to the file in place.
#
# OPTIONS
#   -i
#     Interactive mode; ask for confirmation first.
#   -I
#     Modify "filename" in place (as in `sed -i`) using specified extension. Extension is mandatory.
#   -J
#     Same as -I, but extension is mandatory only if the string "gsed" cannot be found in the output of `which sed`. Overridden by -I.
#   -t
#     Specify an interpreter, e.g. `shebang -t zsh` produces "#!/usr/bin/env zsh"
#   -v 
#     Print (to stderr) the interpreter being used.

shebang () {
  local interpreter
  local inplace
  local inplace2
  local verbose
  local interactive
  local input_shebang
  local shebang
  local continue
  local sed_command
  local gsed_avail
  zparseopts -D t:=interpreter I:=inplace J::=inplace2 v=verbose -i=interactive
  (( $? )) && return 1
  
  if [[ -n $1 ]]; then
    input_shebang=$(sed -n '1 { /^#!/ p; }' "$1")
    if (( $? )); then
      echo "Unable to read $1." >&2
      return 1
    fi
    if [[ -n "$input_shebang" ]]; then
      echo "$1 already has a shebang." >&2
      return 1
    fi
  fi

  if [[ (-z "$interpreter") && (-z "$1") ]]; then
    echo "The -t option is mandatory if no argument is supplied." >&2
    return 1
  fi

  if [[ (-z "$interpreter") && (-n "$1") ]]; then
    interpreter=$(filename=$(basename "$1"); [[ "$filename" = *.* ]] && echo "${filename##*.}" || echo '')  # grab extension
    interpreter=${interpreter:#(* *|* | *)}  # "extensions" with whitespace probably aren't legit
    interpreter=${interpreter:-sh}  # assume sh if no extension and no -t option
  else
    interpreter="$interpreter[2]"
  fi

  shebang="#!/usr/bin/env $interpreter"

  if [[ -n $verbose ]]; then
    echo "Using interpreter '$interpreter'" >&2
  fi

  if [[ -n $interactive ]]; then
    read -q "continue?Shebang will be '$shebang'. Ok? y/n: "
    [[ $continue == n ]] && return 1
  fi

  if [[ -z "$1" ]]; then
    echo $shebang
  else
    gsed_avail=$(command which -s gsed && echo 1)
    echo $gsed_avail

    if [[ -n $gsed_avail ]]; then
      sed_command="1 i\\$shebang\n"
      if [[ -n $inplace ]]; then
        gsed "${inplace/-I/-i}" "$sed_command" "$1"
      elif [[ -n $inplace2 ]]; then
        gsed "${inplace2/-J/-i}" "$sed_command" "$1"
      else
        gsed "$sed_command" "$1"
      fi
    else
      sed_command='1 i\
REPLACEME\n'  # need to use single quotes to preserve the line break
      sed_command=${sed_command/REPLACEME/$shebang}  # work around the single quotes
      if [[ -n $inplace ]]; then
        sed "${inplace/-I/-i}" "$sed_command" "$1"
      elif [[ (-n $inplace2) && (-z ${inplace2#-J}) ]]; then
        echo "'-J' was given without an argument, but 'gsed' is unavailable. Specify an explicit extension or use -I."
      elif [[ -n ${inplace2#-J} ]]; then
        sed "${inplace2/-J/-i}" "$sed_command" "$1"
      else
        sed "$sed_command" "$1"
      fi
    fi
  fi
}