Bash: command stdout to array of lines (whitespace issue)

Question

Situation: I have to store a commands output in an array with each line being one element. These lines may contain whitespaces which seems to screw with everyting.

Normally I would do something like ARRAY=($(command)). However if I try doing something like ARRAY=($(printf '%s\n' "first line" "second line")) the array will look like this

$ for x in "${ARRAY[@]}"; do echo $x; done
first
line
second
line

I have found a way to achieve the behavior I'm looking for although it's a little ugly.

$ readarray -t ARRAY < <(printf '%s\n' "first line" "second line")
$ for x in "${ARRAY[@]}"; do echo $x; done
first line
second line

Does anyone know of a better way to do this? Am I missing something?

Answer 1

readarray/mapfile is the way to do this however this functionality was added around bash v4 so if you are using a system that doesn't have it (macos) you could do it in a much "uglier" way:

IFS=$'\n' # tune the split part
set -o noglob # disable the glob part
array=( $(printf '%s\n' "first line" "second line") ) # use split+glob

Though beware it skips the empty lines. It has the advantage of not losing the command's exit status though.

Or in an even uglier-er way:

array=()
while IFS= LC_ALL=C read -r line || [ -n "$line" ]; do 
    array+=( "$line" )
done < <(printf '%s\n' "first line" "second line")

(LC_ALL=C needed to work around a few bugs in some versions of bash; || [ -n "$line" ] to also include a non-delimited trailing line like the other approaches do)

Answer 2

My favorite way of doing this, is temporarily restricting the IFS and using Bash's array operator; some examples:

declare -a tmpArr
# Store and restrict IFS to newlines
IFS_="$IFS"; IFS=$'\n'
# Write array from STDIN, e. g. in a function (caveat, see comments below)
tmpArr=($(</dev/stdin))
# Write command output directly to array
tmpArr=($(SOME_COMMAND))
# Copy expanded and filtered array into another array
tmpArr=("${srcArray[@]//PATTERN/REPLACEMENT}")
# Restore IFS
IFS="$IFS_"

I'd rather avoid using a subshell and explicit printf, like above, due to the performance impact for larger data sets.

For input from STDIN or some arbitrary command, readarray, as noted by jesse_b, may be performing a bit better, though, and should always be a safe option:

readarray -t -u 0 tmpArr 
readarray -t array < <(SOME_COMMAND)

It may be even better, to directly redirect from a Command Substitution into a while read -r loop in these cases, similar as to shown above:

while read -r; do 
    array+=("$REPLY")
done < <(SOME_COMMAND)
#done </dev/stdin