Bash iterate array, detect if empty

Question

I'm trying to iterate over a variable that might be either null or an array of strings.

ZEIT_DEPLOYMENT_ALIASES=null or ZEIT_DEPLOYMENT_ALIASES=['domain.sh]

I'm a beginner in bash, I read bash iterate file list, except when empty but I couldn't figure it out.

I tried two different approaches.

The values of ZEIT_DEPLOYMENT_ALIASES actually comes from jq library, which reads JSON.

ZEIT_DEPLOYMENT_ALIASES=$(cat now.$CUSTOMER_REF_TO_DEPLOY.staging.json | jq --raw-output '.alias') 

Approach 1

 ZEIT_DEPLOYMENT_ALIASES=['test.sh'] # Check if there are no aliases configured if [ -z "$ZEIT_DEPLOYMENT_ALIASES" ] then ZEIT_DEPLOYMENT_ALIASES_COUNT=${#ZEIT_DEPLOYMENT_ALIASES[@]} echo "$ZEIT_DEPLOYMENT_ALIASES_COUNT alias(es) found. Aliasing them now..." # For each alias configured, then alias it to the deployed domain for DEPLOYMENT_ALIAS in "${ZEIT_DEPLOYMENT_ALIASES_COUNT[@]}" do echo "npx now alias "$ZEIT_DEPLOYMENT_URL $DEPLOYMENT_ALIAS npx now alias $ZEIT_DEPLOYMENT_URL $DEPLOYMENT_ALIAS --token $ZEIT_TOKEN || echo "Aliasing failed for '$DEPLOYMENT_ALIAS', but the build will continue regardless." done else # $ZEIT_DEPLOYMENT_ALIASES is null, this happens when it was not defined in the now.json file echo "There are no more aliases to configure. You can add more aliases from your now.json 'alias' property. See https://vercel.com/docs/configuration?query=alias%20domain#project/alias" echo "$ZEIT_DEPLOYMENT_ALIASES" fi 

But with this, even when ZEIT_DEPLOYMENT_ALIASES=['something'] it doesn't go into the then clause.

Approach 2

ZEIT_DEPLOYMENT_ALIASES=['test.sh'] echo "Alias(es) for current project:" $ZEIT_DEPLOYMENT_ALIASES for DEPLOYMENT_ALIAS in $ZEIT_DEPLOYMENT_ALIASES; do [ -z "$DEPLOYMENT_ALIAS" ] || continue echo "npx now alias "$ZEIT_DEPLOYMENT_URL $DEPLOYMENT_ALIAS npx now alias $ZEIT_DEPLOYMENT_URL $DEPLOYMENT_ALIAS --token $ZEIT_TOKEN || echo "Aliasing failed for '$DEPLOYMENT_ALIAS', but the build will continue regardless." done 

Similarly, it seems like [ -z "$DEPLOYMENT_ALIAS" ] always evaluate to true.

Here is a playground if you'd like:

1. https://www.jdoodle.com/iembed/v0/3bs
2. https://www.jdoodle.com/iembed/v0/3bo

Answer 1

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 stringnull, 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...

Answer 2

Here is the solution for Attempt #1

It was tricky because of the null value, which is not empty unlike I first thought.

And the hardest thing was to convert the jq JSON array into a bash array using:

readarray -t ZEIT_DEPLOYMENT_ALIASES < <(jq --raw-output '.alias[]' < now.$CUSTOMER_REF_TO_DEPLOY.staging.json) 

Big thanks to https://unix.stackexchange.com/a/615717/60329

 ZEIT_DEPLOYMENT_ALIASES_JSON=$(cat now.$CUSTOMER_REF_TO_DEPLOY.staging.json | jq --raw-output '.alias') echo "Custom aliases: " $ZEIT_DEPLOYMENT_ALIASES_JSON # Convert the JSON array into a bash array - See https://unix.stackexchange.com/a/615717/60329 readarray -t ZEIT_DEPLOYMENT_ALIASES < <(jq --raw-output '.alias[]' < now.$CUSTOMER_REF_TO_DEPLOY.staging.json) # Check if there are no aliases configured (it will return "null" in such case, which is not the same as bash "empty") if [ "$ZEIT_DEPLOYMENT_ALIASES" != null ] then ZEIT_DEPLOYMENT_ALIASES_COUNT=${#ZEIT_DEPLOYMENT_ALIASES[@]} echo "$ZEIT_DEPLOYMENT_ALIASES_COUNT alias(es) found. Aliasing them now..." # For each alias configured, then assign it to the deployed domain for DEPLOYMENT_ALIAS in "${ZEIT_DEPLOYMENT_ALIASES[@]}"; do echo "npx now alias "$ZEIT_DEPLOYMENT_URL $DEPLOYMENT_ALIAS npx now alias $ZEIT_DEPLOYMENT_URL $DEPLOYMENT_ALIAS --token $ZEIT_TOKEN || echo "Aliasing failed for '$DEPLOYMENT_ALIAS', but the build will continue regardless." done else # $ZEIT_DEPLOYMENT_ALIASES is null, this happens when it was not defined in the now.json file echo "There are no more aliases to configure. You can add more aliases from your now.json 'alias' property. See https://vercel.com/docs/configuration?query=alias%20domain#project/alias" echo "$ZEIT_DEPLOYMENT_ALIASES" fi 

Thank you!