Un-CGI version 1.11


This documentation, along with the most recent version of the software, is available via the World-Wide Web at <http://www.midwinter.com/~koreth/uncgi.html>.


This is uncgi, a frontend for processing queries and forms from the Web on UNIX systems. (It can also be run under Windows in some cases; see below.) You can get it here.

Without this program, if you wanted to process a form, you'd have to either write or dig up routines to translate the values of the form's fields from "URL encoding" to whatever your program required. This was a hassle in C, and a real pain in the shell, and you had do things differently for GET and POST queries.

Which is where uncgi comes in. It decodes all the form fields and sticks them into environment variables for easy perusal by a shell script, a C program, a Perl script, or whatever you like, then executes whatever other program you specify.

(Actually, "uncgi" is something of a misnomer, as the weird URL syntax is from the HTML forms specification, not from CGI itself.)


This section assumes you have at least a passing familiarity with compiling and installing programs from the net (e.g., that you know how to unpack a compressed tarfile and use "make") and with the operation of your HTTP server. If you don't, you're probably not the right person to install Un-CGI; point your system administrators to this page and ask them to set it up for you.

To install, edit the Makefile. Change the following settings:

The name of your system's C compiler. Typically "cc" or "gcc".

The directory where you want Un-CGI to look for your programs. This doesn't have to be the same as your server's CGI directory, if any. Note that you cannot use "~" here to signify a home directory; you have to use the entire path, beginning with "/". You can put your programs in subdirectories of SCRIPT_BIN if you like.

The directory where you want the Un-CGI executable to be installed. If your server has a "cgi-bin" directory, that's generally what you'll need to put here, since the server needs to know to run Un-CGI as a CGI program, and often that can only happen for programs that are located in the server's cgi-bin directory.

Note that you cannot just make a directory called "cgi-bin" in your account and expect uncgi to be run from it. The HTTP server needs to be configured to know where to look for executable programs. If you don't manage the HTTP server on your system, you probably cannot install uncgi in the right place. (On some servers, you can put CGI programs anywhere if you give them a certain file extension; talk to your system administrator to find out if this is the case on your system, and if so, see the next item.)

If your server allows CGI programs to be run from anywhere as long as they have a particular filename extension (typically .cgi,) you should set this to that extension, including the ".". In that case you can set DESTDIR to point into any directory that the server has access to (e.g. your public_html directory.)

You don't need to follow any particular naming rules for the programs you're going to ask Un-CGI to run; as long as they're in the SCRIPT_BIN directory, you can name them any way you see fit.

Security-related options may be set at compile time:

Once you're done editing the Makefile, run "make install" and your system will build and install Un-CGI into whatever directory you specified as DESTDIR. If you get an error message like "make: Command not found" or "cc: Command not found," talk to your system administrator. I have no way of magically knowing where your system happens to put its compiler tools, so asking me where to look will do you no good.

Also, make sure the file permissions on Un-CGI (and the directory it's in) are set so the HTTP server can execute it. On most systems the HTTP server runs as user "nobody" or "www". You may want to make Un-CGI a setuid program if you want to manipulate private files with your backend scripts, since they will ordinarily be run under the same user ID as the HTTP server. Consult your system administrator to find out your site's policy on setuid programs; they are frowned on in some places.


An example is the easiest way to demonstrate uncgi's use. Suppose you have the following in an HTML file:

<form method=POST action="/cgi-bin/uncgi/myscript">
What's your name?
<input type=text size=30 name=name>
Type some comments.
<textarea name=_comments rows=10 cols=60></textarea>
What problem are you having? <select name=problem multiple>
<option> Sleeplessness
<option> Unruly goat
<option> Limousine overcrowding
<input type=submit value="  Send 'em in!  ">
When the user selects the "Send 'em in!" button, the HTTP server will run uncgi. Uncgi will set three environment variables, WWW_name, WWW_comments and WWW_problem, to the values of the "name", "comments", and "problem" fields in the form, respectively. Then it will execute myscript in the SCRIPT_BIN directory.

All the usual CGI environment variables (PATH_INFO, QUERY_STRING, etc.) are available to the script or program you tell uncgi to run. A couple of them (PATH_INFO and PATH_TRANSLATED) are tweaked by uncgi to the values they'd have if your program were being executed directly by the server. PATH_INFO is, in case you haven't read up on CGI, set to all the path elements after the script name in your URL, if any. This is an easy way to specify additional parameters to your script without resorting to hidden fields.

Myscript might be as simple as this:

echo 'Content-type: text/html'
echo ''
cat /my/home/directory/htmlfiles/thanks.html

	echo "$WWW_name is having $WWW_problem problems and said:"
	echo "$WWW_comments"
) | mail webmaster

With uncgi, that's all you need to do to write a script to send you mail from a form and print a prewritten file as a response. And it's the same whether you want to use GET or POST queries.

If you're using Perl, $ENV{"WWW_xyz"} will look up the value of the "xyz" form field. (You're on your own beyond that; Perl isn't my native language.)

If more than one "problem" is selected in the example above, the values will all be placed in WWW_problem, separated by hash marks ('#'). You can use the library function strtok() to separate the values in a C program; in a shell script, your best bet is probably to replace the hash marks with newlines using "tr", something like:

echo $WWW_problem | tr '#' \\012 | while read value; do
        echo $value 'selected.'

A useful learning tool is to point your form at a script that just prints the contents of the environment. On most systems there is a program to do that, called either env or printenv; you can write a little script that runs it:

echo 'Content-type: text/plain'
echo ''

That'll show you exactly what your script can expect from a form.

Special Processing

Eagle-eyed readers noticed that the text area in the above example had an underscore ("_") at the beginning of its name. When Un-CGI sees a form field whose name begins with an underscore, it strips whitespace from the beginning and end of the value and makes sure that all end-of-line characters in the value are UNIX-style newlines (as opposed to DOS-style CR-LF pairs, or Macintosh-style CRs, both of which are sent by some browsers.) This makes processing text easier, since your program doesn't have to worry about the browser's end-of-line conventions. Note that if a form field is named _xyz, you still get an environment variable called WWW_xyz (i.e., the extra underscore doesn't show up in the environment variable name.)

Un-CGI also modifies variable names containing periods, the major source of which is <input type=image>. Shell scripts have trouble coping with periods in environment variable names, so Un-CGI converts periods to underscores. This is only done to variable names -- periods in values will remain untouched.

Other Features

Extra feature: If you compile with -DLIBRARY, you can use uncgi as a library function in a C program of your own. Just call uncgi() at the start of your program. See this example.

Uncgi will handle hybrid GET/POST requests. Specify a method of POST in the form, and add a GET-style query string to the action, for example <form method=POST action="/cgi-bin/uncgi/myscript?form_id=feedback">. When your script is run, WWW_form_id will be set to "feedback". This will only work if your HTTP server supports it (NCSA's does, for now anyway.)

Un-CGI is freeware; if you want to include it in a commercial product, please send me mail. I'll probably just ask you to include a pointer to this page with your product.


There should be a way to specify a list of directories that uncgi will search for backend scripts.

If you want to report a problem...

If you find problems with Un-CGI, I'll take a look at them. Since Un-CGI is getting fairly popular, though, I have to lay down a few rules:

If you think you've found a genuine problem with Un-CGI, I'll most effectively be able to help you if you give me the following information. The more of this you leave out, the less likely I'll be able to do anything useful.

I won't guarantee to fix every problem, but if you give me a complete report, I'll often be able to help.

Frequently Asked Questions

  1. I get an error about strerror when I compile.
  2. How do I put forms in my documents?
  3. I'm having trouble getting my output to look right.
  4. Where do I put everything?
  5. Why does uncgi tell me it can't run my script?
  6. What do I do in my forms?
  7. I get an error 403 when I submit my form.
  8. I get an error code 500 from the server.
  9. My script doesn't see the form variables.
  10. The browser just sits there when I submit my form.
  11. How do I parse checkbox results?
  12. Do I need to make a directory called "uncgi"?
  13. Does uncgi strip out special shell characters that might cause security problems?
  14. Do you have a Windows version?
  15. What about security?

I get an error about sys_errlist when I compile.

Some UNIX variants' standard include files contain a declaration for the list of system error codes, and some don't; for the latter, I need to include the declaration in uncgi.c. If you get an error about the "strerror" function not being defined, edit the Makefile, search for "SYS_ERRLIST", and remove the "#" from the beginning of the line.

How do I put forms in my documents?

See the NCSA documentation on forms. It includes several examples.

I'm having trouble getting my output to look right.

I'm afraid I don't have the time to help people with HTML. Pick up any of the numerous HTML books on the market. Many of them discuss putting together forms as well as regular documents.

Where do I put everything?

Short answer: Look in the Makefile.

Long answer: When you edit uncgi's Makefile, you'll see two macro definitions relating to paths. The first, SCRIPT_BIN, tells uncgi where to look for your scripts or programs. It must be set to the name of an existing directory; you can set it to any directory you like. Sometimes it's set to the location of your server's cgi-bin directory, though that's not necessary.

The second, DESTDIR, tells the Makefile where to install uncgi. It must point to your server's cgi-bin directory, if your server has one.

Note that you can't just create a directory called "cgi-bin" in your account and expect it to work. The server has configuration files that tell it where to look for scripts and programs. Talk to your system administrator if you aren't the person in charge of the server on your machine.

An additional Makefile setting, EXTENSION, should be of help if your server allows you to put CGI programs anywhere on the system. Again, consult your system administrator to see if this is the case.

Why does uncgi tell me it can't run my script?

First, make sure your script is in the right place; see the preceding section.

Second, make sure your script can be executed by the server. Remember, Un-CGI is run by the server, which is probably using a system user ID, not your user ID. You need to set the permissions on your script such that it can be run by any user. Usually, you can say "chmod 755 scriptname" (replacing scriptname with the name of your script) to set the permissions properly.

If you're still having trouble, edit the Makefile, uncomment the DEBUG_PATH line, and rebuild. This will instruct Un-CGI to include the full path to where it thinks your script should be in its error message.

What do I do in my forms?

The <form> tag has two attributes, METHOD and ACTION. METHOD must be set to either GET or POST; uncgi will handle either one, but POST is preferred if you have textarea fields or are expecting a lot of information from the client. Note that the attribute name (METHOD) can be upper or lower case, but the value must be all caps.

ACTION should have the following components:

So, if you wanted to tell uncgi to run sendmemail with no additional parameters, you'd put sendmemail in the directory you specified as the value of CGI_BIN in uncgi's makefile, and use the following tag to begin your form:

<form method=POST action="/cgi-bin/uncgi/sendmemail">

I get an error 403 when I submit my form.

The usual cause of this is that your server requires a special filename extension on CGI programs. (Most often it's ".cgi".) You need to give uncgi a name that will identify it as a CGI program to the Web server. There is nothing special about uncgi from the Web server's point of view; it's just another program that gets run like any other, and your installation of it has to obey the same rules as other programs'.

I get an error code 500 from the server.

That code almost always results from a CGI script spitting out something invalid when the server is expecting an HTTP header. There are a few common reasons that can happen.

In general, if your HTTP server outputs CGI scripts' standard error streams to a logfile (often called "error_log") you should look there to see if your script, or its shell, is outputting any diagnostic information. Most Web servers will log any illegal header lines they get from CGI programs, and once you see what it is that's confusing your server, it should be straightforward to fix.

If that's no help, try removing the # from the start of the "DEBUG" line in the Makefile and rebuilding Un-CGI. It will spit out diagnostic information that might help.

My script doesn't see the form variables.

There are a few common causes for this:

The browser just sits there when I submit my form.

The most common cause of hangs like that is trying to convert an old CGI-compliant program or script to run under Un-CGI. Your script is detecting that the form was submitted with the POST method, and is trying to read form results from the browser. Unfortunately, by the time your script is run, Un-CGI has already read the results, so your program sits there waiting forever for bytes that've already been consumed.

Remove the part of your code that reads a query from the standard input -- it'll generally be located somewhere around the word "POST", which is easy to search for -- and replace it with code that fills your program's internal variables via getenv() calls (assuming your program is in C) on the WWW_ variables defined by Un-CGI.

How do I parse checkbox results?

Un-CGI puts hash marks ("#") between checkbox selections if there are several of them. How you parse that depends entirely on what language you're using. In C, use strtok(). In Python, use string.splitfields(). In Perl, use split(). In Bourne shell, do something like

echo $WWW_whatever | tr \# \\012 | while read result; do
        echo User picked $result

Do I need to make a directory called "uncgi"?

No. You're probably assuming that since your form has an ACTION like /cgi-bin/uncgi/xyz, there must be an uncgi directory under cgi-bin. That's not how it works.

The parts of a URL between the slashes are not directory names. They are symbols to tell the server where to find a particular document. In most cases, the server will look for a directory of the same name as the symbol in question, and if there is one, go into that directory and proceed to the next symbol. But that isn't always what happens. If the server sees that a particular symbol matches the name of a program, it will typically run that program and pass the rest of the URL to the program as an argument. And that's exactly what's happening here.

There's no uncgi directory. You can think of /cgi-bin/uncgi/xyz as /cgi-bin/uncgi xyz if that helps (it's not strictly speaking accurate, but that's the concept.) Uncgi sees the "xyz" argument and looks in its SCRIPT_BIN directory to find a program by that name.

If you're still not sure what all this means, just try it out and see what happens!

Does uncgi strip out special shell characters that might cause security problems?

No, for the simple reason that uncgi isn't tied to any one shell, or any shell at all, for that matter. It sits in front of C programs just as easily as Bourne shell scripts, just as easily as Perl scripts or Python scripts. (All of which are used in conjunction with uncgi at Midwinter, incidentally.)

In most of those languages, there aren't any special characters that'd inherently be security problems, so having uncgi strip characters that cause problems for a particular shell would just end up pointlessly mangling the input to other kinds of scripts and programs. And people using shells with different special characters would still have to specially handle them anyway.

On most UNIXes, you can use the "tr -d" command to strip out characters your shell processes specially. For example, to strip out dollar signs from a Bourne shell variable, you'd do:

WWW_puppycount="`echo $WWW_puppy | tr -d \$`"

Do you have a Windows version?

Thanks to a couple of contributors who prefer to remain anonymous, Un-CGI will compile under Borland's Turbo C.

I do not support Un-CGI under Windows. If you're having trouble getting Un-CGI to work under Windows, I will not help you. I know almost nothing about Windows programming and I know even less about Windows HTTP servers. It is probable that Un-CGI will work with some HTTP servers and not others. It's known to work on:

If you know of another one under which it works, send me mail and I'll list it here. There is currently a Win-CGI port underway, which should help Un-CGI work on a wider range of servers. The port will be folded into the main source base when it's available. I don't know, even roughly, when that will be.

I have no plans to make binaries available for any platform, Windows or otherwise.

If the above sounds obstructionist, that's not the intent; I want to make Un-CGI useful to a wide audience, but I'd really rather not raise people's expectations and then not be able to help them in the slightest. It's frustrating for all concerned when people send me questions I can't answer.

What about security?

Un-CGI is as secure as the SCRIPT_BIN directory. That is, if a malicious user can add files to SCRIPT_BIN, he or she can use Un-CGI to execute them under the Web server's user ID. In that sense the security level is no different than that of a typical Web server's cgi-bin directory.

Some specific security concerns:

Scripts can be setuid
If Un-CGI didn't allow setuid scripts/programs to be executed, a malicious user could simply create a trivial wrapper script to run a setuid program somewhere else. Disallowing setuid scripts would be at worst a slight inconvenience to intruders.
Symbolic links can point from SCRIPT_BIN to anywhere on the server
As is the case with setuid scripts, if Un-CGI disallowed symbolic links, a malicious user would replace a symlink with a script to run a program from somewhere else on the server. Again, disabling symlinks would prove nothing more than a slight bump in the road, meanwhile making it less convenient for a system administrator to allow specific user-supplied programs to be run using Un-CGI.

Maintained by Steven Grimm <koreth@midwinter.com>.
Send mail if you have comments or suggestions.