How to iterate a for loop and create a customized string

Question

I have a bash script where I'm using a for loop to generate a list of numbers (increasing by one).

Within my loop I use echo in combination with tee to build out my string.

I then use sed to take my "list" (input as file) of numbers and then format it.

Since I use a for loop to generate this list (text file), is there a better and more efficient way for me to create my string (it's being appended as it's being looped) to include values double quoted without having to write to a file first?

Here's my code:

#!/usr/local/bin/bash

if [[ $# != 3 ]]; then
   echo "Usage:   ./crIPRange.sh <octet> <start#> <ending#>" 2>&1
   echo "Example: ./crIPRange.sh 10.1.2 100 150" 2>&1
   exit 1
fi

_octet="$1"
_startIP="$2"
_IPList="List1.out"
_IPListFinal="List2.out"

for (( c=$2; c<=$3; c++ ))
do
  echo "${_octet}.$c" | tee >> ${_IPList}
  sed -E 's/^(.*)$/"\1"/' ${_IPList} | sed -e ':a' -e 'N' -e '$!ba' -e 's/\n/,/g' |  tee > ${_IPListFinal}
done

In other words, I'm thinking I don't necessarily need that step of writing it to a file as a list?

So in the end, I have (2) output files and I really just need my final output. Would I use something like printf?

Benjamin W. · Accepted Answer · 2017-05-05 14:20:52Z

1

This looks like a brutally complicated way to make a comma separated list. For every single entry, your sed command goes through the complete file, just to replace a single newline by a comma. Moving sed to outside the loop and executing it just once would already improve efficiency a bit.

But you don't need sed at all (and no intermediate files, at that). This solution populates an array with the elements we want, then prints it as a single string with IFS set to a comma:

#!/bin/bash

(( $# != 3 )) && echo "Wrong number of arguments" >&2

subnet=$1
from=$2
to=$3

for (( i = from; i <= to; ++i )); do
    arr+=("\"$subnet.$i\"")
done

IFS=,

echo "${arr[*]}"

Notice that your comparison if [[ $# != 3 ]] uses the != meant for strings, but should use -ne for numbers. Or even easier, (( )) made for exactly that.

Usage and output:

$ ./octet 1.2.3 10 15
"1.2.3.10","1.2.3.11","1.2.3.12","1.2.3.13","1.2.3.14","1.2.3.15"

Or, if you don't want a function, you can just use brace expansion and tr:

$ echo \"1.2.3.{10..15}\" | tr ' ' ,
"1.2.3.10","1.2.3.11","1.2.3.12","1.2.3.13","1.2.3.14","1.2.3.15"

This cannot be parameterized, though.

edited May 5, 2017 at 14:20

answered May 5, 2017 at 13:52

Benjamin W.

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4 Comments

noober Over a year ago

It was a hack as it always is as i'm learning to script. Appreciate the input. Thanks for pointing out the bad practice as well as providing an alternative solution.

noober Over a year ago

How do I double quote the output?: "1.2.3.10","1.2.3.11","1.2.3.12"

Benjamin W. Over a year ago

@noober Updated. Notice (for future reference) that it's good practice to show example input and expected output in your questions – then people don't have to guess the intention of the provided code.

noober Over a year ago

will do for future reference. thanks for your input!

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