Howto do floating point compasiosn in an if-statement within a GNU parallel block? - bash

Question

I want to run a batch process in parallel. For this I pipe a list to parallel. When I've an if-statement, that compares two floating point numbers (taken form here), the code doesn't run anymore. How can this be solved.
LIMIT=25
ps | parallel -j2 '
echo "Do stuff for {} to determine NUM"
NUM=33.3333 # set to demonstrate
if (( $(echo "$NUM > $LIMIT" | bc -l) )); then
echo "react..."
fi
echo "Do stuff..."
'
Prints:
Do stuff for \ \ PID\ TTY\ \ \ \ \ \ \ \ \ \ TIME\ CMD to determine NUM
Do stuff...
(standard_in) 2: syntax error
#... snipp

Answer 1

LIMIT is not set inside parallel shell. Running echo "$NUM > $LIMIT" | bc -l exapands to echo "123 > " | bc -l which results in syntax error reported by bc. You need to export/pass/put it's value to the shell run from inside parallel. Try this:
LIMIT=25
ps | parallel -j2 '
LIMIT="'"$LIMIT"'"
echo "Do stuff for {} to determine NUM"
NUM=33.3333 # set to demonstrate
if (( $(echo "$NUM > $LIMIT" | bc -l) )); then
echo "react..."
fi
echo "Do stuff..."
'
Or better use env_parallel, designed for such problems.
Side note: GNU parallel was designed for executing jobs in parallel using one or more computers. For scripts running on one computer it is better to stick with the xargs command, which is more commonly available (so you don't need to install some package each time you move your script to another machine).

Answer 2

While GNU Parallel is designed to deal correctly with commands spanning multiple lines, I personally find that hard to read. I prefer using a function:
doit() {
arg="$1"
echo "Do stuff for $a to determine NUM"
NUM=33.3333 # set to demonstrate
if (( $(echo "$NUM > $LIMIT" | bc -l) )); then
echo "react..."
fi
echo "Do stuff..."
}
export -f doit
LIMIT=25
export LIMIT
ps | parallel -j2 doit
Instead of the exports you can use env_parallel:
ps | env_parallel -j2 doit
If your environment is too big, use env_parallel --session before starting:
#!/bin/bash
env_parallel --session
# Define functions and variables _after_ running --session
doit() {
[...]
}
LIMIT=25
ps | env_parallel -j2 doit