How to nest loops correctly - bash

Question

I have 2 scripts, #1 and #2. Each work OK by themselves. I want to read a 15 row file, row by row, and process it. Script #2 selects rows. Row 0 is is indicated as firstline=0, lastline=1. Row 14 would be firstline=14, lastline=15. I see good results from echo. I want to do the same with script #1. Can't get my head around nesting correctly. Code below.
#!/bin/bash
# script 1
filename=slash
firstline=0
lastline=1
i=0
exec <${filename}
while read ; do
i=$(( $i + 1 ))
if [ "$i" -ge "${firstline}" ] ; then
if [ "$i" -gt "${lastline}" ] ; then
break
else
echo "${REPLY}" > slash1
fold -w 21 -s slash1 > news1
sleep 5
fi
fi
done
# script2
firstline=(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14)
lastline=(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
for ((i=0;i<${#firstline[#]};i++))
do
echo ${firstline[$i]} ${lastline[$i]};
done

Answer 1

Your question is very unclear, but perhaps you are simply looking for some simple function calls:
#!/bin/bash
script_1() {
filename=slash
firstline=$1
lastline=$2
i=0
exec <${filename}
while read ; do
i=$(( $i + 1 ))
if [ "$i" -ge "${firstline}" ] ; then
if [ "$i" -gt "${lastline}" ] ; then
break
else
echo "${REPLY}" > slash1
fold -w 21 -s slash1 > news1
sleep 5
fi
fi
done
}
# script2
firstline=(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14)
lastline=(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
for ((i=0;i<${#firstline[#]};i++))
do
script_1 ${firstline[$i]} ${lastline[$i]};
done
Note that reading the file this way is extremely inefficient, and there are undoubtedly better ways to handle this, but I am trying to minimize the changes from your code.

Answer 2

Update: Based on your later comments, the following idiomatic Bash code that uses sed to extract the line of interest in each iteration solves your problem much more simply:
Note:
- If the input file does not change between loop iterations, and the input file is small enough (as it is in the case at hand), it's more efficient to buffer the file contents in a variable up front, as is demonstrated in the original answer below.
- As tripleee points out in a comment: If simply reading the input lines sequentially is sufficient (as opposed to extracting lines by specific line numbers, then a single, simple while read -r line; do ... # fold and output, then sleep ... done < "$filename" is enough.
# Determine the input filename.
filename='slash'
# Count its number of lines.
lineCount=$(wc -l < "$filename")
# Loop over the line numbers of the file.
for (( lineNum = 1; lineNum <= lineCount; ++lineNum )); do
# Use `sed` to extract the line with the line number at hand,
# reformat it, and output to the target file.
fold -w 21 -s <(sed -n "$lineNum {p;q;}" "$filename") > 'news1'
sleep 5
done
A simplified version of what I think you're trying to achieve:
#!/bin/bash
# Split fields by newlines on input,
# and separate array items by newlines on output.
IFS=$'\n'
# Read all input lines up front, into array ${lines[#]}
# In terms of your code, you'd use
# read -d '' -ra lines < "$filename"
read -d '' -ra lines <<<$'line 1\nline 2\nline 3\nline 4\nline 5\nline 6\nline 7\nline 8\nline 9\nline 10\nline 11\nline 12\nline 13\nline 14\nline 15'
# Define the arrays specifying the line ranges to select.
firstline=(0 1 2 3 4 5 6 7 8 9 10 11 12 13 14)
lastline=(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15)
# Loop over the ranges and select a range of lines in each iteration.
for ((i=0; i<${#firstline[#]}; i++)); do
extractedLines="${lines[*]: ${firstline[i]}: 1 + ${lastline[i]} - ${firstline[i]}}"
# Process the extracted lines.
# In terms of your code, the `> slash1` and `fold ...` commands would go here.
echo "$extractedLines"
echo '------'
done
Note:
The name of the array variable filled with read -ra is lines; ${lines[#]} is Bash syntax for returning all array elements as separate words (${lines[*]} also refers to all elements, but with slightly different semantics), and this syntax is used in the comments to illustrate that lines is indeed an array variable (note that if you were to use simply $lines to reference the variable, you'd implicitly get only the item with index 0, which is the same as: ${lines[0]}.
<<<$'line 1\n...' uses a here-string (<<<) to read an ad-hoc sample document (expressed as an ANSI C-quoted string ($'...')) in the interest of making my example code self-contained.
As stated in the comment, you'd read from $filename instead:
read -d '' -ra lines <"$filename"
extractedLines="${lines[*]: ${firstline[i]}: 1 + ${lastline[i]} - ${firstline[i]}}" extracts the lines of interest; ${firstline[i]} references the current element (index i) from array ${firstline[#]}; since the last token in Bash's array-slicing syntax
(${lines[*]: <startIndex>: <elementCount>}) is the count of elements to return, we must perform a calculation to determine the count, which is what 1 + ${lastline[i]} - ${firstline[i]} does.
By virtue of using "${lines[*]...}" rather than "${lines[#]...}", the extracted array elements are joined by the first character in $IFS, which in our case is a newline ($'\n') (when extracting a single line, that doesn't really matter).