When working with Nginx configuration files, you might encounter situations where you need to dynamically construct parameter names by combining variables with string literals. A common use case is when handling file uploads and needing to set form field names dynamically.

Here's the basic setup that many developers start with:

upload_set_form_field $upload_field_name.name "$upload_file_name";

What we actually want is to create compound field names like "attachment[name]" by combining the variable $upload_field_name with the string literal "[name]". The intuitive approach might be:

upload_set_form_field ($upload_field_name+"[name]") "$upload_file_name";

But this syntax doesn't work in Nginx configuration files.

Nginx doesn't support the + operator for string concatenation in configurations. Instead, you can use the following approaches:

Method 1: Using Separate Directives

set $combined_field "${upload_field_name}[name]";
upload_set_form_field $combined_field "$upload_file_name";

Method 2: Direct Interpolation

upload_set_form_field "${upload_field_name}[name]" "$upload_file_name";

Here's a full example of handling file uploads with dynamic field names:

location /upload {
    upload_pass @backend;
    upload_store /tmp/uploads;
    
    upload_set_form_field "${upload_field_name}[name]" "$upload_file_name";
    upload_set_form_field "${upload_field_name}[content_type]" "$upload_content_type";
    upload_set_form_field "${upload_field_name}[path]" "$upload_tmp_path";
    
    upload_pass_args on;
}

The ${variable} syntax allows for proper variable interpolation within strings in Nginx configuration. This is the standard way to concatenate variables with string literals in Nginx config files.

For more complex scenarios where you need to build strings from multiple variables, consider using the map directive:

map $upload_field_name $dynamic_field {
    default "${upload_field_name}[data]";
    "avatar" "${upload_field_name}[profile_picture]";
}

Always remember to test your Nginx configuration after making changes:

sudo nginx -t
sudo service nginx reload

When working with Nginx file upload configurations, you might need to dynamically construct form field names by combining variables with static strings. A common use case is creating nested field names like attachment[name] where "attachment" comes from a variable.

Here's the basic setup that works for simple field names:

upload_set_form_field $upload_field_name.name \"$upload_file_name\";

The syntax becomes tricky when you need to concatenate a variable with a string to create a compound field name. The intuitive approach might be:

# This WON'T work:
upload_set_form_field ($upload_field_name+\"[name]\") \"$upload_file_name\";

Nginx's configuration language doesn't support the + operator for string concatenation in this context. Instead, you need to use variable interpolation:

upload_set_form_field ${upload_field_name}[name] \"$upload_file_name\";

Here's a full example showing how to implement this in an upload module configuration:

location /upload {
    upload_pass @backend;
    upload_store /tmp/nginx_upload;
    
    # Set the base field name
    set $field_base "attachment";
    
    # Concatenate variable with string
    upload_set_form_field ${field_base}[name] \"$upload_file_name\";
    upload_set_form_field ${field_base}[content_type] \"$upload_content_type\";
    upload_set_form_field ${field_base}[size] \"$upload_file_size\";
}

If you need more complex string manipulation, you can use the set directive to create the combined variable first:

set $combined_field "${upload_field_name}[name]";
upload_set_form_field $combined_field \"$upload_file_name\";

• The curly braces {} are crucial for proper variable interpolation
• Quotes around the entire field name aren't needed
• This syntax works in Nginx 1.11.8 and later versions