exec dup2

Not all properties of the process would change across an exec call. In other words, the new process inherits a number of properties from the calling process:

• process ID and parent process ID
• real user ID and reall group ID
• supplementary group IDs
• process group ID
• session ID
• controlling terminal
• time left on alarm clock
• current working directory
• root directory
• file mode creation mask
• file locks
• process signal mask
• pending signals
• resource limits
• file descriptors without close-on-exec flag set

Obviously, here we care the most about open file descriptors. While we say that exec replaces the old process with a new one, most open file descriptors would remain open.