Eval error Argument list too long when expandind a command - bash

Question

I got some code from here that works pretty well until I get "Argument list too long"
I am NOT a developer and pretty old too :) so if it is not much to ask please explain.
Is there a way the expand DIRCMD like eval does and pass each of the commands one at the time so eval does not break?
for (( ifl=0;ifl<$((NUMFIRSTLEVELDIRS));ifl++ )) { FLDIR="$(get_rand_dirname)"
FLCHILDREN="";
for (( ird=0;ird<$((DIRDEPTH-1));ird++ )) {
DIRCHILDREN=""; MOREDC=0;
for ((idc=0; idc<$((MINDIRCHILDREN+RANDOM%MAXDIRCHILDREN)); idc++)) {
CDIR="$(get_rand_dirname)" ;
# make sure comma is last, so brace expansion works even for 1 element? that can mess with expansion math, though
if [ "$DIRCHILDREN" == "" ]; then
DIRCHILDREN="\"$CDIR\"" ;
else
DIRCHILDREN="$DIRCHILDREN,\"$CDIR\"" ;
MOREDC=1 ;
fi
}
if [ "$MOREDC" == "1" ] ; then
if [ "$FLCHILDREN" == "" ]; then
FLCHILDREN="{$DIRCHILDREN}" ;
else
FLCHILDREN="$FLCHILDREN/{$DIRCHILDREN}" ;
fi
else
if [ "$FLCHILDREN" == "" ]; then
FLCHILDREN="$DIRCHILDREN" ;
else
FLCHILDREN="$FLCHILDREN/$DIRCHILDREN" ;
fi
fi
}
cd $OUTDIR
DIRCMD="mkdir -p $OUTDIR/\"$FLDIR\"/$FLCHILDREN"
eval "$DIRCMD"
echo "$DIRCMD"
}
I tried echo $DIRCMD but do not get the expanded list of commands
'echo mkdir -p /mnt/nvme-test/rndpath/"r8oF"/{"rc","XXKR","p0H"}/{"5Dw0K","oT","rV","coU","uo"}/{"3m5m","uEdA","w4SJp","49"}'

Answer 1

I had trouble following the code, but if I understood it correctly, you dynamically generate a mkdir -p command with a brace expansion:
'mkdir -p /mnt/nvme-test/rndpath/"r8oF"/{"rc","XXKR","p0H"}/{"5Dw0K","oT","rV","coU","uo"}/{"3m5m","uEdA","w4SJp","49"}'
Which then fails when you eval it due to your OS' maximum argument limit.
To get around that, you can instead generate a printf .. command since this is a Bash builtin and not subject to the argument limit, and feed its output to xargs:
dircmd='printf "%s\0" /mnt/nvme-test/rndpath/"r8oF"/{"rc","XXKR","p0H"}/{"5Dw0K","oT","rV","coU","uo"}/{"3m5m","uEdA","w4SJp","49"}'
eval "$dircmd" | xargs -0 mkdir -p
If your xargs doesn't support -0, you can instead use printf "%s\n" and xargs mkdir -p, though it won't behave as well if your generated names contain spaces and such.
If this is for benchmarking, you may additionally be interested to know that you can now use xargs -0 -n 1000 -P 8 mkdir -p to run 8 mkdirs in parallel, each creating 1000 dirs at a time.