Split number string arbitrarily using bash into fixed number of variables

Question

I have a string with 3000 elements (NOT in series) in bash,

sections='1 2 4 ... 3000'

I am trying to split this string into x chunks of length n. I want x to be typically between 3-10. Each chunk may not be of the same length.

Each chunk is the input to a job.

Looking at https://unix.stackexchange.com/questions/122499/bash-split-a-list-of-files and using bash arrays, my first attempt looks like this:

#! /bin/bash

nArgs=10
nChunkSize=10

z="0 1 2 .. 1--"

zs=(${z// / })
echo ${zs[@]}

for i in $nArgs; do
  echo "Creating argument: "$i
  startItem=$i*$nChunkSize
  zArg[$i] = ${zs[@]:($startItem:$chunkSize}
done

echo "Resulting args"

for i in $nArgs; do
 echo "Argument"${zArgs[$1]}
done

The above is far from working I'm afraid. Any pointers on the ${zs[@]:($startItem:$chunkSize} syntax?

For an input of 13 elements:

z='0 1 2 3 4 5 6 7 8 10 11 12 15'

nChunks=3 and nArgs=4 I would like to obtain an array with 3 elements, zs with content

zs[0] = '0 1 2 3'
zs[1] = '4 5 6 7'
zs[2] = '8 10 11 12 15'

Each zs will be used as arguments to subsequent jobs.

Please add sample input and your desired output for that sample input to your question. — Cyrus
– Cyrus, Commented Jan 27, 2017 at 20:27
Use shellcheck.net to identify and correct the more glaring errors. — chepner
– chepner, Commented Jan 27, 2017 at 20:36

Charles Duffy · Accepted Answer · 2017-01-27 20:50:06Z

2

First note: This is a bad idea. It won't work reliably with arbitrary (non-numeric) contents, as bash doesn't have support for nested arrays.

output=( )

sections_str='1 2 4 5 6 7 8 9 10 11 12 13 14 15 16 3000'
batch_size=4
read -r -a sections <<<"$sections_str"
for ((i=0; i<${#sections[@]}; i+=batch_size)); do
  current_pieces=( "${sections[@]:i:batch_size}" )
  output+=( "${current_pieces[*]}" )
done

declare -p output # to view your output

Notes:

zs=( $z ) is buggy. For example, any * inside your list will be replaced with a list of filenames in the current directory. Use read -a to read into an array in a reliable way that doesn't depend on shell configuration other than IFS (which can be controlled scoped to just that one line with IFS=' ' read -r -a).
${array[@]:start:count} expands to up to count items from your array, starting at position start.

edited Jan 27, 2017 at 20:50

answered Jan 27, 2017 at 20:43

Charles Duffy

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1 Comment

Totte Karlsson Over a year ago

This seem to do it for me! I will not have any non-numeric content. Thanks!!

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