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

*!@#$^&*))0_)()?><M;sdf234

EOF

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

*!@#$^&*))0_)()?><M;sdf234

EOF


output=$(appendu $x<<'EOF'
foo
abc
*(@#$%)

*!@#$^&*))0_)()?><M;sdf234
EOF
)

good_output='foo
*(@#$%)'

[[ $output == "$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 "abc
bd" "abc")

good_output="abc
bd"

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