不能在bash中使用while和pipe中的变量

while read line; do
  echo "$line" | awk '{print $1}'
done< file.txt

export my_var="hello"
cat file.txt | while read line; do
  echo "$line $my_var"
done

my_var="hello"
env my_var="$my_var" cat file.txt | while read line; do
  echo "$line $my_var"
done

my_var="hello"
while read line; do
  echo "$line $my_var"
done < <(cat file.txt)