Bash Request Method: GET


Example

It is quite easy to call a CGI-Script via GET.
First you will need the encoded url of the script.

Then you add a question mark ? followed by variables.

  • Every variable should have two sections seperated by =.
    First section should be always a unique name for each variable,
    while the second part has values in it only
  • Variables are seperated by &
  • Total length of the string should not rise above 255 characters
  • Names and values needs to be html-encoded (replace: </ , / ? : @ & = + $ )
    Hint:
    When using html-forms the request method can be generated by it self.
    With Ajax you can encode all via encodeURI and encodeURIComponent

Example:

http://www.example.com/cgi-bin/script.sh?var1=Hello%20World!&var2=This%20is%20a%20Test.&

The server should communicate via Cross-Origin Resource Sharing (CORS) only, to make request more secure. In this showcase we use CORS to determine the Data-Type we want to use.

There are many Data-Types we can choose from, the most common are...

  • text/html
  • text/plain
  • application/json

When sending a request, the server will also create many environment variables. For now the most important environment variables are $REQUEST_METHOD and $QUERY_STRING.

The Request Method has to be GET nothing else!
The Query String includes all the html-endoded data.

The Script

#!/bin/bash
    
# CORS is the way to communicate, so lets response to the server first
echo "Content-type: text/html"    # set the data-type we want to use
echo ""    # we dont need more rules, the empty line initiate this.

# CORS are set in stone and any communication from now on will be like reading a html-document.
# Therefor we need to create any stdout in html format!
    
# create html scructure and send it to stdout
echo "<!DOCTYPE html>"
echo "<html><head>"
    
# The content will be created depending on the Request Method 
if [ "$REQUEST_METHOD" = "GET" ]; then
   
    # Note that the environment variables $REQUEST_METHOD and $QUERY_STRING can be processed by the shell directly. 
    # One must filter the input to avoid cross site scripting.
    
    Var1=$(echo "$QUERY_STRING" | sed -n 's/^.*var1=\([^&]*\).*$/\1/p')    # read value of "var1"
    Var1_Dec=$(echo -e $(echo "$Var1" | sed 's/+/ /g;s/%\(..\)/\\x\1/g;'))    # html decode
    
    Var2=$(echo "$QUERY_STRING" | sed -n 's/^.*var2=\([^&]*\).*$/\1/p')
    Var2_Dec=$(echo -e $(echo "$Var2" | sed 's/+/ /g;s/%\(..\)/\\x\1/g;'))
    
    # create content for stdout
    echo "<title>Bash-CGI Example 1</title>"
    echo "</head><body>"
    echo "<h1>Bash-CGI Example 1</h1>"
    echo "<p>QUERY_STRING: ${QUERY_STRING}<br>var1=${Var1_Dec}<br>var2=${Var2_Dec}</p>"    # print the values to stdout

else

    echo "<title>456 Wrong Request Method</title>"
    echo "</head><body>"
    echo "<h1>456</h1>"
    echo "<p>Requesting data went wrong.<br>The Request method has to be \"GET\" only!</p>"

fi

echo "<hr>"
echo "$SERVER_SIGNATURE"    # an other environment variable
echo "</body></html>"    # close html
    
exit 0

The html-document will look like this ...

<html><head>
<title>Bash-CGI Example 1</title>
</head><body>
<h1>Bash-CGI Example 1</h1>
<p>QUERY_STRING: var1=Hello%20World!&amp;var2=This%20is%20a%20Test.&amp;<br>var1=Hello World!<br>var2=This is a Test.</p>
<hr>
<address>Apache/2.4.10 (Debian) Server at example.com Port 80</address>


</body></html>

The output of the variables will look like this ...

var1=Hello%20World!&var2=This%20is%20a%20Test.&
Hello World!
This is a Test.
Apache/2.4.10 (Debian) Server at example.com Port 80

Negative side effects...

  • All the encoding and decoding dont look nice, but is needed
  • The Request will be public readable and leave a tray behind
  • The size of a request is limited
  • Needs protection against Cross-Side-Scripting (XSS)