Source grep expression from array

Question

I am passing input to grep from previously declared variable that contains multiple lines. My goal is to extract only certain lines. As I increase the argument count in grep, the readability goes down.

var1="
_id=1234
_type=document
date_found=988657890
whateverelse=1211121212"


echo "$var1"

_id=1234
_type=document
date_found=988657890
whateverelse=1211121212


grep -e 'file1\|^_id=\|_type\|date_found\|whateverelse' <<< $var1
_id=1234
_type=document
date_found=988657890
whateverelse=1211121212

My idea was to pass parameters from array and it will increase readibility:

declare -a grep_array=(
"^_id=\|"
"_type\|"
"date_found\|"
"whateverelse"
)

echo ${grep_array[@]}
^_id=\| _type\| date_found\| whateverelse


grep -e '${grep_array[@]}' <<<$var1

---- no results

How can I do it with grep to pass parameters with multiple OR conditions from somewhere else not one line? As I have more arguments the readibility and manageability goes down.

if the lines are cotngiuous and are using GNU grep, look at Context control in grep --help to get n lines context, maybe more suitable — Nahuel Fouilleul
– Nahuel Fouilleul, Commented Mar 22, 2019 at 12:09

Inian · Accepted Answer · 2019-03-22 12:17:51Z

Your idea is right but you've got couple of issues in the logic. The array expansion of type ${array[@]} puts the contents of the array as separate words, split by the white space character. While you wanted to pass a single regexp string to grep, the shell has expanded the array into its constituents and tries it to evaluate as

grep -e '^_id=\|' '_type\|' 'date_found\|' whateverelse

which means each of your regexp strings are now evaluated as a file content instead of a regexp string.

So to let grep treat your whole array content as a single string use the ${array[*]} expansion. Since this particular type of expansion uses the IFS character for joining the array content, you get a default space (default IFS value) between the words if it is not reset. The syntax below resets the IFS value in a sub-shell and prints out the expanded array content

grep -e "$(IFS=; printf '%s' "${grep_array[*]}")" <<<"$str1"

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