The general format for the exec command:

Code:

    exec command [redirections]

overlays the current shell script with the command named by command and performs the input and/or output redirections specified by redirections for that command. When no command is specified, the exec command performs the specified redirections for the current script. So, in this case:

Code:

exec > log.txt 2>&1

no command is specified, so the standard output of the remaining commands in this script that do not otherwise redirect their standard output will be written to the file named log.txt and any data written to standard error by the remaining commands in this script that do not otherwise redirect their standard error output will be sent to the same file. Since the redirection to log.txt uses ">" (rather than ">>"), data in log.txt (if there is any) will be discarded before continuing with the following commands.
That is what the exec command above did.
The shell variable $? is the exit status of the previous command. It is being saved in the variable ec because I need to use the value in the next two commands.
This increments the value of the shell variable i as the last of the commands in the while loop:

Code:

while condition
do      commands
done

After incrementing i, [ -f cmd$i ] (the condition in this while loop) tests whether or not there is a regular file named cmdx where x is the current value of the shell variable i? If cmdx does exist, the commands in the loop are executed again with the updated value of i.