The use of
exec exemplifies the spirit of UNIX in that it provides a very simple way to start new processes.
fork call basically makes a duplicate of the current process, identical in almost every way (not everything is copied over, for example, resource limits in some implementations but the idea is to create as close a copy as possible).
The new process (child) gets a different process ID (PID) and has the the PID of the old process (parent) as its parent PID (PPID). Because the two processes are now running exactly the same code, they can tell which is which by the return code of
fork - the child gets 0, the parent gets the PID of the child. This is all, of course, assuming the
fork call works - if not, no child is created and the parent gets an error code.
exec call is a way to basically replace the entire current process with a new program. It loads the program into the current process space and runs it from the entry point.
exec are often used in sequence to get a new program running as a child of a current process. Shells typically do this whenever you try to run a program like
find - the shell forks, then the child loads the
find program into memory, setting up all command line arguments, standard I/O and so forth.
But they're not required to be used together. It's perfectly acceptable for a program to
fork itself without
execing if, for example, the program contains both parent and child code (you need to be careful what you do, each implementation may have restrictions). This was used quite a lot (and still is) for daemons which simply listen on a TCP port and fork a copy of themselves to process a specific request while the parent goes back to listening.
Similarly, programs that know they're finished and just want to run another program don't need to
exec and then
wait for the child. They can just load the child directly into their process space.
Some UNIX implementations have an optimized
fork which uses what they call copy-on-write. This is a trick to delay the copying of the process space in
fork until the program attempts to change something in that space. This is useful for those programs using only
fork and not
exec in that they don't have to copy an entire process space.
exec is called following
fork (and this is what happens mostly), that causes a write to the process space and it is then copied for the child process.
Note that there is a whole family of
exec calls (
execve and so on) but
exec in context here means any of them.
The following diagram illustrates the typical
fork/exec operation where the
bash shell is used to list a directory with the
| pid=7 |
| ppid=4 |
| bash |
| calls fork
| pid=7 | forks | pid=22 |
| ppid=4 | ----------> | ppid=7 |
| bash | | bash |
| waits for pid 22 | calls exec to run ls
| | pid=22 |
| | ppid=7 |
| | ls |
| pid=7 | | exits
| ppid=4 | <---------------+
| bash |