What is this script doing?

Question

#!/bin/bash

echo 123456789 > out.txt
exec 3<> out.txt
read -n 4 <&3
echo -n 5 >&3
exec 3>&-

Was asked the content of out.txt at the end of script, in an interview written exam. I did run the script afterwords and it gave me 123456789. Yet I have no idea what is going on in the script, especially the parts with the exec statements. I looked up the manpage and google search results for exec and could not find anything on the 3<> bit. Could somebody, well versed in shell scripting, explain what is going on here?

Answer 1

echo 123456789 > out.txt writes the string 123456789 in out.txt file.

The exec 3<>out.txt construct opens the file out.txt for reading < and writing > and attaches it to file descriptor #3.

read -n 4 <&3 reads 4 characters.

echo -n 5 >&3 writes 5 (replacing 5 by 5).

exec 3>&- closes file descriptor #3.

Resulting in

cat out.txt
123456789

Section about execint bash(1) states that:

exec [-cl] [-a name] [command [arguments]] If command is specified, it replaces the shell. [...] If command is not specified, any redirections take effect in the current shell [...].