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 $*
