Using nested for loop bash

Question

I am writing bash script for first time . I have to use nested for loop. I have an array like this:

(foo,bar a,b)

There are two elements in the array and each element in array is separated by commas. I want each element in array to split and print each element. The final result I want is

foo
bar
a
b

I am using nested for loop as below but I am not getting any response

    IFS=','
    echo "class names is::${classNames[@]}" // prints foo,bar a,b
    for ((n=0;n<${#classNames[@]};n++)){
    
    classes=${classNames[n]}
    
    for ((i=0;n<${#classes[@]};i++)){
    
    echo "${eachClass[i]}"
    
    }
    }

IFS=',' aaaand where does IFS count?

KamilCuk
– KamilCuk

2021-02-10 10:21:24 +00:00
Commented Feb 10, 2021 at 10:21 — KamilCuk
– KamilCuk, Commented Feb 10, 2021 at 10:21
So IFS should be a space or a comma?

Raman Sailopal
– Raman Sailopal

2021-02-10 10:25:33 +00:00
Commented Feb 10, 2021 at 10:25 — Raman Sailopal
– Raman Sailopal, Commented Feb 10, 2021 at 10:25

KamilCuk · Accepted Answer · 2021-02-10 10:58:41Z

You could:

arr=(foo,bar a,b)
for i in "${arr[@]}"; do
   while IFS=, read -ra arr_in; do
       for j in "${arr_in[@]}"; do
           echo "$j"
       done
   done <<<"$i"
done

or like:

for i in "${arr[@]}"; do
   readarray -d, -t arr_in < <(printf "%s" "$i")
   for j in "${arr_in[@]}"; do
       echo "$j"
   done
done

or just:

while IFS=, read -ra arr_in; do
    for j in "${arr_in[@]}"; do
       echo "$j"
    done
done < <(printf "%s\n" "${arr[@]}"

or even:

while IFS=, read -ra arr_in; do
    printf "%s\n" "${arr_in[@]}"
done < <(printf "%s\n" "${arr[@]}"

but that said:

printf "%s\n" "${arr[@]}" | tr ',' '\n'

will also print the same.

Do not use { } as loop terminators - it's undocumented very old bash/shell syntax. Use do..done.

classes=... is a normal variable, not an array, and in such assignment IFS does not matter - here the right part of = is not word splitted. To assign an array, use braces classes=(${classNames[n]}).

You could just split the values on spaces and comma with IFS:

IFS=', ' read -ra arr <<<"${arr[*]}"
for i in "${arr[@]}"; do
    echo "$i"
done

or even like:

while IFS= read -r i; do
    echo "$i"
done < <(IFS=', '; printf "%s\n" ${arr[*]})

Ivan · Accepted Answer · 2021-02-10 11:20:38Z

1

... or like this

for item in "${arr[@]}"; {
    sub=(${item//,/ })
    echo "item1=${sub[0]}"
    echo "item2=${sub[1]}"
}

with read

for item in "${arr[@]}"; {
    read i1 i2 <<< ${item//,/ }
    echo "item1=$i1"
    echo "item2=$i2"
}

The trick is that we switch ',' to ' '(space) via this bash syntax ${item//,/ }(${var_name//pattern/replacement}) this is called variable substitution. And this one <<< is used to read directly from var.

edited Feb 10, 2021 at 11:20

answered Feb 10, 2021 at 10:55

Ivan

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1 Comment

srinivas chaitanya Over a year ago

can you please explain what is role of //,/ and <<< in bash and what are they called in bash.

M. Nejat Aydin · Accepted Answer · 2021-02-10 17:29:49Z

0

A practical one-liner solution without using an apparent loop is

(IFS=', '; printf '%s\n' ${classNames[*]})

It runs in a subshell not to mess up with current shell's IFS.

edited Feb 10, 2021 at 17:29

answered Feb 10, 2021 at 17:24

M. Nejat Aydin

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