Why didn't the shell command execute in order as I expect?

Question

I have written 3 shell scripts named s1.sh s2.sh s3.sh. They have the same content:

#!/bin/ksh
echo $0 $$

and s.sh invoke them in order:

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Answer 1

They have been executing in order (at least starting in order - Notice the ids are incrementing). You open 3 separate threads for 3 separate programs. One (for some reason) is faster than the other. If you want them in sequence, take the execs and &s out of exec ./s1.sh &.