Shell脚本（BASH）中的多处理

Question

I've written a script and What I'm wanting to do is get it to run as fast as possible by using sub-processing or background-processing or what ever I need to use in order to speed up the computations.

So I was looking at using the & symbol in my script to simply speed up processing wherever I could, but when I put it in here, I don't get the results echo'ed to me.

This is the function that I have written and am trying to improve apon.

InTri(){
    while read line
    do
        V1X=$(echo "$line" | awk '{print $1}') &
        V1Z=$(echo "$line" | awk '{print $2}') &
        V2X=$(echo "$line" | awk '{print $3}') &
        V2Z=$(echo "$line" | awk '{print $4}') &
        V3X=$(echo "$line" | awk '{print $5}') &
        V3Z=$(echo "$line" | awk '{print $6}') &
        run=$(echo "$line" | awk '{print $7}') &
        wait
        echo "$V1X $V1Z $V2X $V2Z $V3X $V3Z $run"
    done < <(mysql -u root -ppassword LightCycle -N -e "SELECT V1X, V1Z, V2X, V2Z, V3X, V3Z, ID FROM Temp3 WHERE (V1X <= $x OR V2X <= $x OR V3X <= $x) AND (V1X >= $x OR V2X >= $x OR V3X >= $x) AND (V1Z <= $z OR V2Z <= $z OR V3Z <= $z) AND (V1Z >= $z OR V2Z >= $z OR V3Z >= $z);")
}

I've read the "man bash" (or what of it I could understand) but I don't understand why this is not working.

Help! :)

Thanks to anyone who can, your input is much appreciated.

Answer 1

Whatever happens in & line happens in a child process, that can't modify parent's environment. You'll probably need to open pipes and read from them in parent process. For tasks with comparable costs of execution with your example it isn't worth the effort.

In this example you can just do

cat <(echo "$line" | awk '{print $1}') <(echo "$line" | awk '{print $1}') ...

Instead of this whole loop body.

Answer 2

If you feel your program is getting slower due to those 7 awk commands, you can do something like this without any awk:

$ line="1 2 3 4 5 6 7"
$ a=($line)
$ echo ${a[0]}
1
$ echo ${a[1]}
2
$ echo ${a[@]}
1 2 3 4 5 6 7

where "a" is an array which contains all the elements of the line variable in it.

Answer 3

You might think that creating more processes will make it faster, but it might actually make it slower because of the overhead of creating new processes and all that piping.

Why not simply do the following?

mysql -u root -ppassword LightCycle -N -e "SELECT V1X, V1Z, V2X, V2Z, V3X, V3Z, ID FROM Temp3 WHERE (V1X <= $x OR V2X <= $x OR V3X <= $x) AND (V1X >= $x OR V2X >= $x OR V3X >= $x) AND (V1Z <= $z OR V2Z <= $z OR V3Z <= $z) AND (V1Z >= $z OR V2Z >= $z OR V3Z >= $z);" | awk '{print $1" "$2" "$3" "$4" "$5" "$6" "$7}'

Or if you want to read them into variables:

while read V1X V1Z V2X V2Z V3X V3Z run
do
  echo "$V1X $V1Z $V2X $V2Z $V3X $V3Z $run"
done < <(mysql -u root -ppassword LightCycle -N -e "SELECT V1X, V1Z, V2X, V2Z, V3X, V3Z, ID FROM Temp3 WHERE (V1X <= $x OR V2X <= $x OR V3X <= $x) AND (V1X >= $x OR V2X >= $x OR V3X >= $x) AND (V1Z <= $z OR V2Z <= $z OR V3Z <= $z) AND (V1Z >= $z OR V2Z >= $z OR V3Z >= $z);")

Answer 4

In order to get the script above working as it should, I just needed to remove the & before the "wait"

InTri(){
    while read line
    do
        V1X=$(echo "$line" | awk '{print $1}') &
        V1Z=$(echo "$line" | awk '{print $2}') &
        V2X=$(echo "$line" | awk '{print $3}') &
        V2Z=$(echo "$line" | awk '{print $4}') &
        V3X=$(echo "$line" | awk '{print $5}') &
        V3Z=$(echo "$line" | awk '{print $6}') &
        run=$(echo "$line" | awk '{print $7}') 
        wait
        echo "$V1X $V1Z $V2X $V2Z $V3X $V3Z $run"
    done < <(mysql -u root -ppassword LightCycle -N -e "SELECT V1X, V1Z, V2X, V2Z, V3X, V3Z, ID FROM Temp3 WHERE (V1X <= $x OR V2X <= $x OR V3X <= $x) AND (V1X >= $x OR V2X >= $x OR V3X >= $x) AND (V1Z <= $z OR V2Z <= $z OR V3Z <= $z) AND (V1Z >= $z OR V2Z >= $z OR V3Z >= $z);")
}