How to run iterations asynchronously in shell script - linux

Question

I have a few .csv files like below.
xyz0900#1#-1637746436.csv
xxx0900#1#-1637746436.csv
zzz0900#2#-1637746439.csv
yyy0900#1#-1637746436.csv
sss0900#2#-1637746439.csv
I have written a script to perform below tasks:
Get the large file based on the pattern which we have passed as a argument to the script.
Merge all other files which are having same pattern and create a new file
Remove duplicate header from new file.
Move new file to the destination based on the parameter passed as a argument.
Example: I am passing "1637746436#home/dest1,1637746436#home/dest2" as
a second argument to the script. Below script will fetch the
pattern(1637746436). Get the bigger file and merge all other
files(having same pattern) with it. New file will be get created and same will be moved to the destination(home/dest1).
The below script will perform the pattern matching and execution sequentially.
How to make 'for loop iteration' should be executed parallelly? I mean pattern matching of "1637746436#home/dest1,1637746436#home/dest2" should be performed simultaneously(not one after another).
Please help on this.
$merge.sh /home/dummy/17 "1637746436#home/dest1,1637746439#home/dest2"
#!/bin/bash
current=`pwd`
source=$1
destination=$2
echo "$destination" | tr "," "\n" > $current/out.txt
cat out.txt | cut -d "#" -f1 > $current/pattern.txt
for var in `cat pattern.txt`
do
getBiggerfile=$(ls -Sl $source/*$var.csv | head -1)
cd $source
getFileName=$(echo $getBiggerfile | cut -d " " -f9-)
newFileName=$(echo $getFileName | cut -d "#" -f1)
cat *$var.csv >> $getFileName
header=$(head -n 1 $getFileName)
(printf "%s\n" "$header";
grep -vFxe "$header" $getFileName
) > $newFileName.csv
rm -rf *$var.csv
cd $current
for var1 in `cat out.txt`
do
target=`echo $var1 | cut -d "#" -f2`
id=$(echo $var1 | cut -c-10)
if [ $id = $var ]
then
mv $newFileName.csv $target
fi
done
done

Answer 1

The cleanest would be to make the internals of the loop a function, and call the function inside the loop, putting it in the background (child processes), then wait for the background (child) processes to finish:
function do_the_thing(){
source="$1"
current="$2"
var="$3"
getBiggerfile=$(ls -Sl $source/*$var.csv | head -1)
cd $source
getFileName=$(echo $getBiggerfile | cut -d " " -f9-)
newFileName=$(echo $getFileName | cut -d "#" -f1)
cat *$var.csv >> $getFileName
header=$(head -n 1 $getFileName)
(printf "%s\n" "$header";
grep -vFxe "$header" $getFileName
) > $newFileName.csv
rm -rf *$var.csv
cd $current
for var1 in `cat out.txt`
do
target=`echo $var1 | cut -d "#" -f2`
id=$(echo $var1 | cut -c-10)
if [ $id = $var ]
then
mv $newFileName.csv $target
fi
done
}
for var in `cat pattern.txt`
do
do_the_thing "$source" "$current" "$var" &
done
wait