7

I have a function prereq() that may be invocated several times but should not actually be executed more than once on the same running thread of the script by selecting other options from the menu (each option on the menu will have prereq() as part of the code):

# Pre-requirements
prereq ()
{
        echo
        echo "########################## CHECKING PRE-REQUIREMENTS ##########################"
        echo "#                                                                             #"
        echo "Required packages: hdparm, fio, sysbench, iperf3 and sshpass"
        sleep 2
        echo
        for pack in hdparm fio sysbench iperf3 sshpass; do
                echo "Checking and if needed install '$pack'..."
                if ! rpm -qa | grep -qw "$pack"; then
                        yum install -y $pack > /dev/null
                else
                        echo "$pack is already installed, skipping..."
                        echo
                fi
        done
        echo "###############################################################################"
echo
}

The function is executed like below:

select opt in "${options[@]}"
do
        case $opt in
        "CPU Stress Test (local)")
                sleep 2
                prereq                ===>> HERE IS!
                cpu
                cleanup
                echo
                break
        ;;
        "Memory Stress Test (local)")
                sleep 2
                prereq                ===>> HERE IS!
                memory
                cleanup
                echo
                break
                .
                .
                .

I need to execute prereq() just once even if selecting other options from the menu that's invocate prereq() because each function can be executed once and the purpose of the script is done, and it can be exited.

I am planning to have a variable on prereq() as a flag and if it's executed, the flag is checked every time that prereq() is invocated on any option from the menu.

Furthermore, I appreciate any help! Thanks!

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4
  • 2
    then again, why not run prereq once before the whole select command? On the other hand, it looks like it checks things that not all the functions need, like hddparm which probably isn't involved in the CPU and memory tests shows. Commented Feb 25, 2021 at 13:41
  • You are right! I just needed to put 'prereq' before select commmand. And you are right about checking/installing packages not needed on functions... do you have any suggestion on how to split prereq depending on function selected? Thank you!! Commented Feb 25, 2021 at 15:30
  • This looks like it's complicated enough that you'd be happier writing it in Perl or Python, even though you need to run a bunch of shell commands. Commented Feb 26, 2021 at 21:18
  • What is the reason for the sleep 2 commands? Commented Mar 4, 2021 at 22:24

2 Answers 2

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18

Since shell variables are global unless you declare them as local inside a function (a mean trap by the way), you can simply define a variable

prereq_done=0

on top of the script and then modify the prereq() function to check for it at the beginning (exit if it is already set), and set it to 1 at the end:

prereq()
{
  if (( prereq_done == 1 )); then return; fi

  < your code here >

  prereq_done=1
}
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3
  • @rafaelvileladias You're welcome! As for your question stated in a comment, you could call the function as prereq "$opt" and then make an if or case check inside prereq() (where you would refer to it as $1). Alternatively (remember, all variables are global), just check "$opt" also inside prereq() to make decisions based on the value ... Commented Feb 25, 2021 at 15:37
  • ...should not actually be executed more than once on the same running thread of the script... If this is actually a multi-threaded script, then you will need to much more ingenious! Commented Mar 4, 2021 at 0:48
  • 1
    @JeremyBoden I did read that part of the text too, but I do not see any actual indication of parallelism in the code shown by the OP. You may also want to define what you would consider a multi-threaded script in this context - one that launches tasks as background processes? Commented Mar 4, 2021 at 8:33
13

Let the function redefine itself. Put this somewhere* in the function body:

prereq () { :; }

The first execution of prereq will turn the function into a no-op.

There's a downside. People usually don't expect a shell function to be redefined; let alone to be redefined by itself. You should at least place a comment stating that # this function turns itself into no-op after first use. Without such comment an approach with a flag (e.g. prereq_done variable) is more readable thanks to explicit code needed to handle the flag.

*Most likely at the beginning; but if the original function ever exits early in a way that shouldn't count as "the function has been executed" then you may want to place the redefinition after this point. If at the end then remember it will affect the exit status of the function.

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