wait command in shell script

Question

fork(){
    i=0
    count=0
    while (($count<5))
    do
        fpfunction &
        pid=$!

        #Ensure the pid is available 
        if [  -z "$pid" ]; then
        echo "IS NULL"
        else
        Array[$i]=$pid
        echo "PID is $pid"
        fi

        wait [$pid]
        i=$((i+1))
        count=$(( count+1 ))
    done

    echo "PID: ${Array[*]}"

}

The fpfunction() only echo a string, so I dont write it here.

My understanding to wait() is it will wait a child process to complete and then return a value.

I put wait() in my script because I want to ensure all child processes are finished.

However, the error occured

not a pid or valid job spec

Is my understanding wrong? please let me know how to fix this issue.

Thanks a lot.

I think the asynchronous command execution is pointless here! Here a sub-process started in the background and then immediately waited to finish, so instead of the lot of code use simply fpfunction to start a sub-process and wait for it. If You want to limit the number of process started I suggest to use make. — TrueY
– TrueY, Commented Jul 17, 2013 at 10:29

paxdiablo · Accepted Answer · 2013-07-17 02:49:53Z

5

If the process ID is, for example, 42 then your wait instruction becomes:

wait [42]

which is not what you want - the square brackets are a problem. It should be:

wait $pid

as per the following transcript:

pax> date ; sleep 60 &
Wednesday 17 July  10:43:51 WST 2013
[1] 5200

pax> pid=$! ; echo $pid
5200

pax> wait [$pid]
bash: wait: '[5200]': not a pid or valid job spec

pax> wait $pid ; date
[1]+ Done sleep 60
Wednesday 17 July  10:44:51 WST 2013

And, just as an aside (irrelevant to the answer), wait() is better suited to the C wait() function. If you're using wait from the shell, you would normally leave off the parentheses.

edited Jul 17, 2013 at 2:49

answered Jul 17, 2013 at 2:43

paxdiablo

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