exec is a built-in shell command that forces a binary to be executed by the currently running shell process instead of forking the process and running the binary on that child process.

When you run a command on a shell-script, it forks a child process and runs the command there. On a syscall level this is the classic:

if( (pid=fork()) == 0) {
    exec(command);
    exit();
}
wait();

And this is usually what we want, because we will keep running commands after that one. Nevertheless, sometimes this is a problem, like when:

• we have a program that’s going to monitor a given process, and it doesn’t run properly if there’s an intermediate shell process but we need to run this second process via a shel-script for whatever reasons (to initialize some variables, run the program with nice, whatever)
• on MacOS we’re running a program via a shell-script and get two icons on the dock, one for the shell and another one for the program

Running a command with exec forces the shell not to fork, but to run the command directly over the shell process. An important thing to note here is that the shell-script will end there, no further commands of the shell script will be executed as the shell process will be substituted by the command process, so to speak.

#!/bin/sh

# initialize variables, parse command-line parameters, etc.
export IP=$1

exec nice command $*