Propagate value of variable to outside of the loop [duplicate]

Answer 1

Alternatively, you can move the echo statements inside the subshell:

dpkg --list |grep linux-image |grep "ii  " | (
  let i=0
  declare -a arr

  while read line
  do
    arr=(${line})
    let i=i+1
    _constr+="${arr[2]} "
  done
  echo $i
  echo ${_constr}
)

Note the insertion of the parenthesis to explicitly define where the subshell begins and ends.

Answer 2

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

let i=0
declare -a arr

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}

You should also pre-declare globals for clarity, as shown above.

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.

Answer 3

This somewhat by-passes your question (and it's a good question), but you can achieve the same results using simply:

 _constr=($(dpkg --list | awk '/^ii.*linux-image/{print $2}'))

The ($(cmd)) construct initialises a bash array using the output of the command within.

[me@home]$ echo ${_constr[*]}
linux-image-2.6.35-22-generic linux-image-2.6.35-28-generic linux-image-generic
[me@home]$ echo ${_constr[2]}
linux-image-generic

and you can get the number of elements using ${#_constr[*]}.

[me@home]$ echo ${#_constr[*]}
3