Using grep in conditional statement in bash

You're almost there. Just omit the exclamation mark:

OUTPUT='blah blah (Status: 200)'
if echo "$OUTPUT" | grep -q "(Status:\s200)"; then
    echo "MATCH"
fi

Result:

MATCH

The if condition is fulfilled if grep returns with exit code 0 (which means a match). The ! exclamation mark will negate this.

Since you're already using bash, you could keep it internal to bash:

if [[ $OUTPUT =~ (Status:[[:space:]]200) ]]; then
  echo match
fi

Sample runs:

OUTPUT='something bogus'
[[ $OUTPUT =~ (Status:[[:space:]]200) ]] && echo match


OUTPUT='something good (Status: 200)'
[[ $OUTPUT =~ (Status:[[:space:]]200) ]] && echo match
match

This is not an answer to your question, but few suggestions from a fellow scripter:

• Use $() instead of backticks, don't use them both
• Indent conditional if statements
• Remove unnecessary usage of $()

Consistentecy and simple rules will help you debug and maintain scripts in a long run ...

#!/bin/bash

LOGIN="email"
TOKEN="token"
DOMAIN_ID="domain"
RECORD_ID="record"
IP=$(curl -s http://icanhazip.com/)

OUTPUT=$(
curl -H "Accept: application/json" \
    -H "Content-Type: application/json" \
    -H "X-DNSimple-Domain-Token: $TOKEN" \
    -X "PUT" \
    -i "https://api.dnsimple.com/v1/domains/$DOMAIN_ID/records/$RECORD_ID" \
    -d "{\"record\":{\"content\":\"$IP\"}}"
)

if ! echo "$OUTPUT" | grep -q "(Status:\s200)"; then
    echo "match"
    echo "$OUTPUT" | grep -oP '(?<="message":")(.[^"]*)' >> /home/ddns/ddns.log
    echo "$OUTPUT"| grep -P '(Status:\s[0-9]{3}\s)' >> /home/ddns/ddns.log
fi