How to get result from background process linux shell script? - linux

Question

For example let's say I want to count the number of lines of 10 BIG files and print a total.
for f in files
do
#this creates a background process for each file
wc -l $f | awk '{print $1}' &
done
I was trying something like:
for f in files
do
#this does not work :/
n=$( expr $(wc -l $f | awk '{print $1}') + $n ) &
done
echo $n

Answer 1

I finally found a working solution using anonymous pipes and bash:
#!/bin/bash
# this executes a separate shell and opens a new pipe, where the
# reading endpoint is fd 3 in our shell and the writing endpoint
# stdout of the other process. Note that you don't need the
# background operator (&) as exec starts a completely independent process.
exec 3< <(./a.sh 2&1)
# ... do other stuff
# write the contents of the pipe to a variable. If the other process
# hasn't already terminated, cat will block.
output=$(cat <&3)

Answer 2

You should probably use gnu parallel:
find . -maxdepth 1 -type f | parallel --gnu 'wc -l' | awk 'BEGIN {n=0} {n += $1} END {print n}'
or else xargs in parallel mode:
find . -maxdepth 1 -type f | xargs -n1 -P4 wc -l | awk 'BEGIN {n=0} {n += $1} END {print n}'
Another option, if this doesn't fit your needs, is to write to temp files. If you don't want to write to disk, just write to /dev/shm. This is a ramdisk on most Linux systems.
#!/bin/bash
declare -a temp_files
count=0
for f in *
do
if [[ -f "$f" ]]; then
temp_files[$count]="$(mktemp /dev/shm/${f}-XXXXXX)"
((count++))
fi
done
count=0
for f in *
do
if [[ -f "$f" ]]; then
cat "$f" | wc -l > "${temp_files[$count]}" &
((count++))
fi
done
wait
cat "${temp_files[#]}" | awk 'BEGIN {n=0} {n += $1} END {print n}'
for tf in "${temp_files[#]}"
do
rm "$tf"
done
By the way, this can be though of as a map-reduce with wc doing the mapping and awk doing the reduction.

Answer 3

You could write that to a file or better, listen to a fifo as soon as data arrives.
Here is a small example on how they work:
# create the fifo
mkfifo test
# listen to it
while true; do if read line <test; then echo $line; fi done
# in another shell
echo 'hi there'
# notice 'hi there' being printed in the first shell
So you could
for f in files
do
#this creates a background process for each file
wc -l $f | awk '{print $1}' > fifo &
done
and listen on the fifo for sizes.