用文件描述符覆盖文件的内容

Question

如何通过文件描述符完全覆盖文件内容？

已经有一个问题问同样的问题，但没有明确答案：Bash, overwrite using file descriptor?。

实施例

script.sh：

#!/bin/sh -e -x

echo "Hello world, this is a test." >&3

sleep 1

cat ./state

sleep 1

echo "Hello." >&3

sleep 1

cat ./state

parent.sh：

#!/bin/sh -e -x

echo "" > ./state
exec 3> ./state

./script.sh

运行parent.sh，预期输出：

+ echo ''
+ exec
+ ./script.sh
+ echo 'Hello world, this is a test.'
+ sleep 1
+ cat ./state
Hello world, this is a test.
+ sleep 1
+ echo Hello.
+ sleep 1
+ cat ./state
Hello.

实际输出：

+ echo ''
+ exec
+ ./script.sh
+ echo 'Hello world, this is a test.'
+ sleep 1
+ cat ./state
Hello world, this is a test.
+ sleep 1
+ echo Hello.
+ sleep 1
+ cat ./state
Hello world, this is a test.
Hello.