Question

dpkg --list |grep linux-image |grep "ii  " | while read line
do
  arr=(${line})
  let i=i+1
  _constr+="${arr[2]} "
done
echo $i
echo ${_constr}

The echo statements outside of the loop do not display the expected variables.

How should I make the contents of the variable propagate outside the loop?

Answer 1

The problem is the pipe, not the loop. Try it this way

let i=0
arr=()
_constr=

while read -r line ; do
    arr=("${line}")
    let i=i+1
    _constr+="${arr[2]} "
done < <(dpkg --list | grep linux-image | grep 'ii  ')

echo "$i"
echo "${_constr}"

Pipes are executed in a subshell, as noted by Blagovest in his comment. Using process substitution instead (this is the < <(commands) syntax) keeps everything in the same process, so changes to global variables are possible.

Incidentally, your pipeline could be improved as well

dpkg --list | grep '^ii.*linux-image'

One less invocation of grep to worry about.