What Exec Does in Shell Scripts

If you think the answer is:

The exec() family of functions replaces the current process image with a new process image.

you are only partially right… That’s only true if you don’t redirect:

Redirections are a special case, and exec does not destroy the current shell process, but bash will no longer print output to the screen, writing it to the file instead.

or from man bash itself (scroll down):

If command is not specified, any redirections take effect in the current shell […]

Seeing is believing

Put the following in a file called test.bash

echo "before: $$"
exec > stdout.log
echo "after: $$"

and run it:

> bash test.bash
before: 78511
> ls
stdout.log  test.bash
> cat stdout.log
after: 78511

Comments:

• the “after” echo ran (after the exec!)
• echo was redirected to stdout.log
• it shows the same process ID (the $$ variable)

There are more examples of what’s possible:

# Open myinfile.txt for reading ("<") on file descriptor 3.
exec 3< myinfile.txt
# "-u 3" tells read to get its data from file descriptor 3
read -u 3 mydata
# Close ("&-") the open read descriptor ("<") number 3
exec 3<&-

What about the usual exec behavior?

That’s still in effect:

> ls
child.bash  parent.bash
> cat parent.bash
echo "from parent, before: $$"
exec bash child.bash
echo "from parent, after: $$"
> cat child.bash
echo "from child: $$"
> bash parent.bash
from parent, before: 78601
from child: 78601

Comments:

• parent.bash ran until the exec, then it was replaced by child.bash
• the process ID is the same – the content of the parent process was replaced by its child process