Bash iterate array, detect if empty

Given this file:

$ cat test.json
{"alias": ["foo", "bar"], "whatever": "xyzzy"}

at least my version of jq gives this output for jq --raw-output '.alias[]' < test.json

$ jq --raw-output '.alias[]' < test.json
foo
bar

i.e. the entries are on separate lines, which is important, since we can use that to separate them from each other. For example, by reading them to an array with readarray. (<(...) is a process substitution, it makes the output of the command available like it was a file, so < <(...) makes it available in stdin. A bit like a pipe, actually, except that pipes run subshells so the read values would not be available after the pipe.)

#!/bin/bash
readarray -t entries < <(jq  '.alias[]' < test.json)
if [ "${#entries[@]}" = 0 ]; then
    echo empty array...
fi

# this will not do anything if the array is empty
for entry in "${entries[@]}"; do
    echo "processing entry $entry..."
done

To deal with a possibly missing alias field, use .alias[]? in jq instead. Note however, that will deal a non-array string value (like {"alias": "foo"}) as empty, so if that's a possibility, we'd need to do something else.

Also, note that if the entries contain newlines, --raw-output will print them as-is, so entries with them will show up split to multiple lines, as if they were multiple distinct entries.

Alternatively, without process substitution, so this should work with a standard shell, not only with Bash.

#!/bin/sh
jq --raw-output '.alias[]' < json.txt | 
(
any=
while IFS= read -r line; do 
    echo "doing something with '$line'..."
    any=1
done
if [ "$any" != 1 ]; then
    echo "empty input..."
fi
)

See Why is my variable local in one 'while read' loop, but not in another seemingly similar loop? as to why the parenthesis are necessary.

Now, as to your code:

ZEIT_DEPLOYMENT_ALIASES=['test.sh']

This would assign the string [test.sh] to the variable. That's not the same as assigning the string ["test.sh"] like you'd get from jq, since here, the shell processes the quotes you gave it. They're not processed similarly from the output of a command substitution. That's also a single scalar variable, not an array.

if [ -z "$ZEIT_DEPLOYMENT_ALIASES" ]

This tests if the string is the empty string, which probably isn't what you meant. Anyway, jq's .alias could give the string null, which is not the same as an empty string.

${#ZEIT_DEPLOYMENT_ALIASES[@]}

This will always be 1, since it's not an array. And for the same reason, the for loop doesn't do what you wanted.

Note that Bash doesn't process JSON itself, if it gets a string like ["foo", "bar"] from a command substitution, it's just that, a string. You need to split it individual entries yourself...