Calling the current and next item in a bash for loop - linux

Question

I'm conducting a reiterative analysis and having to submit more than 5000 jobs to a batch system on a large computer cluster.
I'm wanting to run a bash for loop but call both the current list item and the next item in my script. I'm not sure the best way to do this using this format:
#! /bin/bash
for i in `cat list.txt`;
do
# run a bunch of code on $i (ex. Data file for 'Apples')
# compare input of $i with next in list {$i+1} (ex. Compare 'Apples' to 'Oranges', save output)
# take output of this comparison and use it as an input for the next analysis of $i (ex. analyze 'Apples' some more, save output for the next step, analyze data on 'Oranges')
# save this output as the input for next script which analyses the next item in the list {$i+1} (Analysis of 'Oranges' with input data from 'Apples', and comparing to 'Grapes' in the middle of the loop, etc., etc.)
done
Would it be easiest for me to provide a tabular input list in a while loop? I would really prefer not to do this as I would have to do some code editing, albeit minor.
Thanks for helping a novice -- I've looked all over the interwebs and ran through a bunch of books and haven't found a good way to do this.
EDIT: For some reason I was thinking there might have been a for loop trick to do this but I guess not; it's probably easier for me to do a while loop with a tabular input. I was prepared to do this, but I didn't want to re-write the code I had already.
UPDATE: Thank you all so much for your time and input! Greatly appreciated.

Answer 1

Another solution is to use bash arrays. For example, given a file list.txt with content:
1
2
3
4
4
5
You can create an array variable with the lines of the file as elements as:
$ myarray=(1 2 3 4 4 5)
While you could also do myarray=( $(echo list.txt) ) this may split on whitespace and handle other output inappropriately, the better method is:
$ IFS=$'\n' read -r -d '' -a myarray < list.txt
Then you can access elements as:
$ echo "${myarray[2]}"
3
To length of the array is given by ${#myarray[#]}. A list of all indices is given by ${!myarray[#]}, and you can loop over this list of indices:
for i in "${!myarray[#]}"; do
echo "${myarray[$i]} ${myarray[$(( $i + 1))]}"
done
Output:
1 2
2 3
3 4
4 4
4 5
5
While there are likely simpler solutions to your particular use case, this would allow you to access arbitrary combinations of array elements in the loop.

Answer 2

This answer assumes that you want your values to overlap -- meaning that a value given as next then becomes curr on the following iteration.
Assuming you encapsulate your code in a function that takes two arguments (current and next) when a next item exists, or one argument when on the last item:
# a "higher-order function"; it takes another function as its argument
# and calls that argument with current/next input pairs.
invokeWithNext() {
local funcName=$1
local curr next
read -r curr
while read -r next; do
"$funcName" "$curr" "$next"
curr=$next
done
"$funcName" "$curr"
}
# replace this with your own logic
yourProcess() {
local curr=$1 next=$2
if (( $# > 1 )); then
printf 'Current value is %q, and next item is %q\n' "$curr" "$next"
else
printf 'Current value is %q; no next item exists\n' "$curr"
fi
}
These definitions done, you can run:
invokeWithNext yourProcess <list.txt
...yield output such as:
Current value is 1, and next item is 2
Current value is 2, and next item is 3
Current value is 3, and next item is 4
Current value is 4, and next item is 5
Current value is 5; no next item exists

Answer 3

$ printf '%d\n' {0..10} | paste - -
0 1
2 3
4 5
6 7
8 9
10
So if you just want to interpolate lines so that you can read two variables per line...
while read -r odd even; do
…
done < <(paste - - < inputfile)
You will need to do additional work if your lines contain whitespace.

Answer 4

I would replace the for loop with a while read xx loop.
Something along the lines of
cat list.txt | while read line; do
if read nextline; then
# You have $line and $nextline
else
# You have garbage in $nextline and the last line of list.txt in $line
fi
done