#!/bin/bash -l
x=$(mktemp)
appendu $x<<'EOF'
aa
bb
EOF

y=$(mktemp)
tee -a $y<<'EOF'
aa
bb
EOF


output="$(
appendu $x<<'EOF'
cc
aa
caa
bb

dd
EOF
)"

good_output='cc
caa

dd'

[[ $output == "$good_output" ]] || { echo check1 failed; echo "$output != $good_output"; }

echo "$good_output" >> $y


diff $x $y || echo check2 failed diffing $x and $y

output=$(appendu $x "aa
ee" "bb")

good_output="aa
ee"

[[ $output == $good_output ]] || { echo check3 failed; echo "$output != $good_output"; }