bash: read stdin lines to array

23rd Jun, 2026 - programming bash

Whilst working on a script for work, I was getting a confusing error.

./some-script.sh: line 57: IDS: unbound variable

This might not seem confusing at first but this was the code (changed for obvious reasons) on line 56 and 57:

local ids=$(curl -sSL https://jsonplaceholder.typicode.com/todos | jq -r .[].id)
echo "Found ${#ids[@]} ids" # Error here

After a little searching, it turns out the shell expansion on line 56 stores the value as a plain string but on line 57, I’m accessing it as an array which causes the unbound error!

The fix is to use mapfile instead to store it as an array.

local ids
mapfile -t ids < <(curl -sSL https://jsonplaceholder.typicode.com/todos | jq -r .[].id)
echo "Found ${#ids[@]} ids" # Prints "Found 200 ids"

Note

It’s worth saying that I only got this error because I had set -u (actually the classic set -euo pipefail) at the start of the script. If you don’t have it, it prints “Found 1 ids” instead and treats it like an array with one item containing the long output string.