========================= misread question ===========================

To edit a file, you can use ed, the standard editor:

line=' bcm2708.w1_gpio_pin=20'
file=/boot/cmdline.txt
if ! grep -q -x -F -e "$line" <"$file"; then
    ed -s "$file" < <(printf '%s\n' 1 a "$line" . 1,2j w q)
fi

ed's commands:

• 1: go to line 1
• a: append (this will insert after the current line)
• We're in insert mode and we're inserting the expansion of $line
• .: stop insert mode
• 1,2j join lines 1 and 2
• w: write
• q: quit