Wolfgang Resch - Notes

Use xargs to execute jobs in parallel

October 25, 2012

xargs can be used to construct and execute commands sequentially

seq 15 | xargs -n 1 echo

for example executes echo with arguments taken from stdin. In this case using 1 (-n) space separated argument per invocation.

xargs can execute these commands in parallel:

seq 15 | xargs -n 1 -P 2 echo

in this case running a maximum of 2 processes (-P or --max-procs) at any time. Of course, if executing in parallel, the results may be out of order.

Now, if a file with one command per line already exists, each line can be executed in parallel like so:

cat > test <<EOF
echo 1
echo 2
echo 3
echo 4

cat test | xargs -L 1 -I CMD -P 2 bash -c CMD

This works only if the commands don’t contain quotes. Files like the following example

cat > test <<EOF
mv "file 1" "file 2"
mv "file 2" "file 1"
echo -e "first\nsecond"
touch "file 1"
cat test | xargs -L 1 -I CMD bash -c CMD

result in errors or incorrect output.

In bash, this can be fixed by using printf’s "%q" format to escape quotes in each command

cat test | while read -r i; do printf "%q\n" "$i"; done | xargs -L 1 -I CMD bash -c CMD

This will even work for special characters (see \n above), but line continuations with backslash will fail.

More detail here .