There's no excuse for using #!/usr/bin/env ...

foo

test

install_script.bash

#!/usr/bin/env bash
# WARNING: if this file starts with #!/usr/bin/env, please use it to install itself,
# and then use that to install other files.

#
# 1. No write permission on runnable programs
# 2. No partially-written files during installation; pivot into place once complete
# 3. No .sh etc suffices on runnable programs
# 4. Set the #! line at installation, to use a set path, not /usr/bin/env
#

verbose=0

sources=()
bindir=

declare -A interp=(
                [.pl]=perl
                [.py]=python
                [.pyc]=python
                [bash]=$BASH
                [sh]=/bin/sh
            )
default_lang=.bash
owner=
group=
mybin=$HOME/bin
sysbin=/usr/local/bin

match_prefix() {
    local i= j= k= n=0
    k=$1 ; shift
    for i do [[ "$i" = "$k"* ]] && { j=$i ; ((++n)) ; } ; done
    if ((n==1)) ; then printf '%s' "$j" ; else printf '%s' "$k" ; false ; fi
}

while (($#))
do  case $( match_prefix "$1" --help --admin --dir --group --interpreter --owner --quiet --user --verbose --bash --perl --php --python ) in
    (-h|--help) cat <<EndOfHelp ; exit 0 ;;
$0 [options | files] ...
 --admin --me
 --bindir PATH
 --group GID
 --owner UID
 --verbose --quiet
EndOfHelp
    (-B|--bash)         interp[bash]=$2 ; shift ;;
    (-H|--php)          interp[php]=$2 ; shift ;;
    (-P|--perl)         interp[perl]=$2 ; shift ;;
    (-S|--sh)           interp[sh]=$2 ; shift ;;
    (-Y|--python)       interp[python]=$2 ; shift ;;
    (-a|--admin)        bindir=$sysbin ; : ${owner:=root} ${group:=root} ;;
    (-b|--bindir)       bindir=$2 ; shift ;;
    (-g|--group)        group=$2 ; shift ;;
    (-i|--interpreter)  interp[${2##*/}]=$2 ; shift ;;
    (-m|--me)           bindir=$mybin owner= group= ;;
    (-o|--owner)        owner=$2 ; shift ;;
    (-q|--quiet)        verbose=0 _v= _vv= ;;
    (-v|--verbose)      ((++verbose)) ; _vv=$_v _v=${_v:--}v ;;

    # Don't change bundled-options boiler-plate, except for ──╮
    #   ╭──── the list of option letters which take args ←────╯
    #   ↓
    (-[dgio]?*)         set x "${1:0:2}"  "${1:2}" "${@:2}" ;;
    (-[^-]?*)           set x "${1:0:2}" "-${1:2}" "${@:2}" ;;
    (--*=*)             set x "${1%%=*}" "${1#*=}" "${@:2}" ;;

    # boiler-plate error handling
    (-*)                echo >&2 "Invalid option '$1'" ; exit 64 ;;

    # permutable non-option args...
    (*)                 sources+=("$1") ;;
    esac
    shift
done

: ${bindir:=$mybin}

for p in "${sources[@]}"
do
    n=${p##*/}
    r=${n%%.*}
    s=${n#"$r"}
    t=$bindir/$r
    tt=$bindir/.$n.$$

    read -r l < "$p" || break

    if [[ $l = '#!'* ]]
    then
        # use basename of existing #! line
        i=${l#'#!'}
        i=${i#"${i%%[^ ]*}"}
        i=${i#/usr/bin/env}
        i=${i#"${i%%[^ ]*}"}
        i=${i%%' '*}
        i=${i##*/}
        ((verbose>1)) && echo "hashbang filetype $i"
    else
        # look up suffix
        i=${interp[${s:-$default_lang}]:=${s#.}}
        ((verbose>1)) && echo "suffix filetype $i"
    fi
    # if not an absolute path, look up in $PATH (and cache result)
    [[ $i = /* ]] ||
        i=${interp[$i]:=$(type -p "$i")} || break

    ((verbose>1)) && echo "filetype path $i"

    [[ $p -ef $t ]] && {
        echo "Source '$p' and target '$t' are same file; won't overwrite"
        continue
    }
    sed -e "
            1{
                s@^#! */[^ ]*/${i##*/} *\( [^ ].*|\)@#!$i\1@;tz;
                s@^#! */usr/bin/env *${i##*/}@#!$i@;tz;
                s@^@#!$i\n@;tz;
                :z
            }
        " < "$p" > "$tt" || {
        echo "Cannot create '$tt'"
        continue
    }
    if
        chmod 555 "$tt" &&
        { [[ -z $owner$group ]] ||
        chown $owner:$group "$tt" ; } &&
        mv -f "$tt" "$t"
    then
        ((verbose)) && echo "Installed '$p' as '$t' with interpreter '$i'"
    else
        rm -f "$tt"
        echo "Could not install '$p' as '$t'"
    fi
done