Bash script not checking for null correctly

Question

I have a bash script:

#! /bin/bash

someId=$(curl -sk -H -X POST -d "fizzbuzz" "https://someapi.example.com/v1/orders/fire" | jq '.someId')

if [ -z "$someId" ]; then
  echo "Order Placement failed; unable to parse someId from the response"
  exit 1
fi

echo "...order $someId placed"

When I run this I get the following output:

...order null placed

So somehow $someId is null but then...shouldn't I be seeing the "Order Placement failed; unable to parse someId from the response" echo instead?

How can I modify the if [ -z "$someId" ]; then conditional to execute if $someId is null?

the string "null" is not zero-length, it's four bytes. -z checks only for zero-length strings. — Charles Duffy
– Charles Duffy, Commented Jul 28, 2020 at 18:02
Sounds like what you want is more like [ -z "$someId" ] || [ "$someId" = null ] (splitting it into two separate tests that way complies with current POSIX guidelines; see the OB markers in pubs.opengroup.org/onlinepubs/9699919799/utilities/test.html, flagging other ways to combine tests as obsolescent). — Charles Duffy
– Charles Duffy, Commented Jul 28, 2020 at 18:03

chepner · Accepted Answer · 2020-07-28 20:15:20Z

6

Use the --exit-status option, which makes jq have a non-zero exit status if the last output value is false or null.

#! /bin/bash


if ! someId=$(curl -sk -H -X POST -d "fizzbuzz" "https://someapi.example.com/v1/orders/fire" | jq -r --exit-status '.someId'); then
  echo "Order Placement failed; unable to parse someId from the response"
  exit 1
fi

echo "...order $someId placed"

edited Jul 28, 2020 at 20:15

answered Jul 28, 2020 at 18:09

chepner

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2 Comments

John Kugelman Over a year ago

Should that be if ! someId=...?

Léa Gris Over a year ago

Also jq -j or jq -r would be more appropriate to transmit the someId content, or the value returned is of the JSON format that is not appropriate in most cases for shell, like a null in JSON is the null keyword.

Charles Duffy · Accepted Answer · 2020-07-28 18:04:43Z

The "null string" is an empty string -- a zero-byte string, that is, a string with no characters contained within it. s="" assigns a null string to the variable s, just as s= or s='' do.

s=null, or s='null', or s="null", by contrast, assign a string with the four-byte string null (and then terminated by a literal NUL character, as all C strings are). These strings are not "null strings" or "empty strings" in the sense that test -z "$s" or [ -z "$s" ] check for: It doesn't have zero characters; instead, it has four (those being n, u, l and l).

To detect either an empty string or the non-empty string "null", use instead:

if [ -z "$someId" ] || [ "$someId" = null ]; then
  echo "Order Placement failed; unable to parse someId from the response"
  exit 1
fi

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