Resolving Shell Quoting Issues in curl POST Requests with JSON Data

Introduction

When working with curl in bash scripts to send POST requests with JSON data, developers often encounter quoting issues that lead to unexpected errors. The original problem demonstrates this clearly: while a manually entered curl command works perfectly in the terminal, the same command fails when executed from a script file with variables. The error messages—Could not resolve host for various words and unmatched close brace/bracket—indicate improper shell interpretation of the command structure.

Understanding the Problem

The core issue lies in how bash handles quotation marks and variable expansion within complex command structures. In the failing script example, the excessive quoting around headers and data creates parsing confusion. The -H "'Accept: application/json'" syntax actually passes the single quotes as part of the header value, causing curl to interpret application as a hostname rather than part of the header content.

Similarly, the complex nesting of quotes in the --data argument disrupts proper JSON structure interpretation. When bash processes --data "'"'{...}"'"", the variable expansion and quote escaping create a malformed JSON string that curl cannot properly parse, leading to the unmatched brace error at position 158.

The Heredoc Function Solution

The most robust approach involves separating JSON generation from the curl command using a bash function with heredoc syntax:

generate_post_data() {
  cat <<EOF
{
  "account": {
    "email": "$email",
    "screenName": "$screenName",
    "type": "$theType",
    "passwordSettings": {
      "password": "$password",
      "passwordConfirm": "$password"
    }
  },
  "firstName": "$firstName",
  "lastName": "$lastName",
  "middleName": "$middleName",
  "locale": "$locale",
  "registrationSiteId": "$registrationSiteId",
  "receiveEmail": "$receiveEmail",
  "dateOfBirth": "$dob",
  "mobileNumber": "$mobileNumber",
  "gender": "$gender",
  "fuelActivationDate": "$fuelActivationDate",
  "postalCode": "$postalCode",
  "country": "$country",
  "city": "$city",
  "state": "$state",
  "bio": "$bio",
  "jpFirstNameKana": "$jpFirstNameKana",
  "jpLastNameKana": "$jpLastNameKana",
  "height": "$height",
  "weight": "$weight",
  "distanceUnit": "MILES",
  "weightUnit": "POUNDS",
  "heightUnit": "FT/INCHES"
}
EOF
}

This function uses a heredoc (<<EOF) to create a clean JSON template where variables are properly interpolated without complex quoting. The curl command then becomes significantly simpler:

curl -i \
-H "Accept: application/json" \
-H "Content-Type:application/json" \
-X POST --data "$(generate_post_data)" "https://xxx:xxxxx@xxxx-www.xxxxx.com/xxxxx/xxxx/xxxx"

Shell Quoting Rules Explained

Understanding bash quoting behavior is crucial for avoiding similar issues:

Double Quotes ("): Preserve whitespace and allow variable expansion. In -H "Accept: application/json", the entire string becomes a single argument to curl, with application/json properly grouped.

Single Quotes ('): Preserve all characters literally, preventing variable expansion and special character interpretation. This is useful for static JSON strings but problematic when variables need interpolation.

Variable Interpolation: When combining variables with quoted strings, proper concatenation is essential. The pattern 'static text'"$variable"'more static text' allows precise control over what gets expanded and what remains literal.

Alternative Approaches and Considerations

For simpler cases with variables containing no spaces, direct interpolation within single quotes can work:

curl -X POST -H "Content-Type: application/json" -d '{"number":"'$i'"}' "https://httpbin.org/post"

However, this approach becomes unreliable with variables containing spaces or special characters. For such cases, additional quoting is necessary:

curl -X POST -H "Content-Type: application/json" -d '{"elem":"'"$i"'"}' "https://httpbin.org/post"

In CI/CD environments, similar issues arise with pipeline variables. The reference article shows attempts using ${CI_PROJECT_NAME} syntax, but the heredoc approach remains the most reliable solution across different execution contexts.

Best Practices and Recommendations

When working with curl and JSON in bash scripts:

1. Separate Data Generation: Use functions or external files for complex JSON structures to maintain readability and avoid quoting complexity.
2. Validate JSON Structure: Test your JSON generation independently before integrating with curl commands.
3. Use Appropriate Tools: For very complex JSON manipulation, consider using jq or dedicated JSON libraries rather than manual string construction.
4. Test in Target Environment: Always test your scripts in the actual execution environment (CI/CD, cron jobs, etc.) where quoting behavior might differ from interactive shells.

Conclusion

Proper shell quoting is essential for successful curl POST requests with JSON data. The heredoc function approach provides a clean, maintainable solution that avoids the pitfalls of complex nested quoting. By understanding bash's quoting rules and adopting structured approaches to JSON generation, developers can create robust scripts that work reliably across different execution environments.