loop with more than one item at a time

Question

We can iterate over a set of items, considering one at a time, like this:

#!/bin/bash
for i in $( ls ); do
    echo item: $i
done

How can we process several items at a time in a similar loop? Something like:

#!/bin/bash
for i,j,k in $( ls ); do
    echo item i: $i
    echo item j: $j
    echo item k: $k
done

That second shell script is incorrect but should give an accurate illustration of what I am trying to achieve.

Answer 1

Assuming you don't have too many items (although the shell should be able to handle quite a few positional arguments.

# Save the original positional arguments, if you need them
original_pp=( "$@" )
set -- *
while (( $# > 0 )); do
    i=$1 j=$2 k=$3     # Optional; you can use $1, $2, $3 directly
    ...
    shift 3 || shift $#   # In case there are fewer than 3 arguments left
done

# Restore positional arguments, if necessary/desired
set -- "${original_pp[@]}"

For POSIX compatibility, use [ "$#" -gt 0 ] instead of the ((...)) expression. There's no easy way to save and restore all the positional parameters in a POSIX-compatible way. (Unless there is a character you can use to concatenate them unambiguously into a single string.)

Here is the subshell jm666 mentions:

(
    set -- *
    while [ "$#" -gt 0 ]; do
        i=$1 j=$2 k=$3
        ...
        shift 3 || shift $#
    done
)

Any changes to parameters you set inside the subshell will be lost once the subshell exits, but the above code is otherwise POSIX-compatible.

Answer 2

If filenames does not contain spaces:

find . -maxdepth 1 | xargs -L 3 | while read i j k; do
  echo item i: $i
  echo item j: $j
  echo item k: $k
done

Edit:

I removed -print0 and -0.

Answer 3

Bit late answer, I would do this with non-spectacular way :), like:

while read -r -d $'\0' f1
do
        read -r -d $'\0' f2
        read -r -d $'\0' f3

        echo "==$f1==$f2==$f3=="

done < <(find test/ ... findargs... -print0)

Answer 4

To get to get n items a time from the list I think you want to get n items from an array.

Use it like this:

n=3
arr=(a b c d e)

echo "${arr[@]:0:$n}"
a b c