假如有一个file文件
Hello, World!
然后再bash下面执行
cat <> file
输出的结果是：
Hello, World!
请问这中间到底发生了什么？
Update
想了一下 cat <> file相当于cat 0<> file。之后在网上找到了如下信息：
Use program <> file to open file for both reading and writing.
I/O Redirection
[j]<>filename
# Open file "filename" for reading and writing,
#+ and assign file descriptor "j" to it.
# If "filename" does not exist, create it.
# If file descriptor "j" is not specified, default to fd 0, stdin.
#
# An application of this is writing at a specified place in a file.
echo 1234567890 > File # Write string to "File".
exec 3<> File # Open "File" and assign fd 3 to it.
read -n 4 <&3 # Read only 4 characters.
echo -n . >&3 # Write a decimal point there.
exec 3>&- # Close fd 3.
cat File # ==> 1234.67890
3.6 Redirections
The redirection operator
[n]<>word
causes the file whose name is the expansion of word to be opened for both reading and writing on file descriptor n, or on file descriptor 0 if n is not specified. If the file does not exist, it is created.
正如题主自己所说，cat <> file 相当于 cat 0<> file，即以读写方式打开 file，并且把 fd 设为 0。
因为 0 是 stdin，所以只是写 cat <> file 并没有什么实际意义，和 cat < file 在表现上没什么区别，只是多了些副作用，包括需要拥有 file 的写权限，如果不存在 file 会自动创建一个新的空文件等。