Forum Discussion

ichalis_37981's avatar
ichalis_37981
Historic F5 Account
Dec 13, 2011

The EVAL command

Hi all,

 

 

I am testing the eval command and want to generate an error if my input script contains an error. My test code is as follows:

 

 

set cmd "if { \[HTTP::uri\] starts_with \"/hello\" }{ log local0.alert \"match!!!\" }"

 

 

if { [catch {

 

[eval $cmd]

 

} myresult] != 0 } {

 

log local0. "My rule failed: $myresult"

 

}

 

 

The output to this is as follows:

 

 

Rule : match!!!

 

Rule : My rule failed: invalid command name ""

 

 

So the "match!!!" output clearly shows the command was run successfully, but the catch command is returning an error.

 

 

Any ideas as to why?

 

 

 

 

application delivery
dev
devops
iRules

9 Replies

Sort By
  • hooleylist's avatar
    hooleylist
    Icon for Cirrostratus rankCirrostratus
    Dec 13, 2011
    Hi,

    The catch command already executes the wrapped expression. So you could use [catch { eval $cmd } myresult]. Wrapping eval $cmd in square braces executes the output of eval again resulting in the invalid command name. Here's a version which should work and not require escaping the characters in the command: 

    
when HTTP_REQUEST {

set cmd {if { [HTTP:uri] starts_with "/hello" }{ log local0.alert "match!!!" }}
if { [catch {
eval $cmd
} myresult] != 0 } {
log local0. "My rule failed: $myresult, \$cmd: $cmd"
} else {
log local0. "My rule succeeded: $myresult, \$cmd: $cmd"
}
}

    That said, why do you want to use catch for this? catch handles errors, but it's not too likely that you'd get a runtime error with that anyhow.

    Aaron
  • ichalis_37981's avatar
    ichalis_37981
    Historic F5 Account
    Dec 13, 2011
    Thanks!

     

     

    My intention is for a set of TCL commands to be stored in a data group as a set of signatures to be eval'd against each request. I want to use catch in case the admin inputs a syntax error in the datagroup..

     

     

    CHeers..
  • brad_11480's avatar
    brad_11480
    Icon for Nimbostratus rankNimbostratus
    Apr 19, 2012
    From what I understand the EVAL command is an old, old command and continues to exist because it is used widely. Further I understand that the preferred method is to use the {*} notation to allow safe argument expansion. That is what I wanted to do.. I had the list of arguments in a string. But the use of {*}$string resulted in a syntax error.

     

     

    Wondering if it will be supported in the near future?!

     

     

    ( {*} - The artist formerly known as {expand}. )
  • hooleylist's avatar
    hooleylist
    Icon for Cirrostratus rankCirrostratus
    Apr 19, 2012
    Hi Brad,

     

     

    I haven't heard of the {*} syntax before and couldn't find anything on the TCL wiki pages about it. Do you have any references for it?

     

     

    Also in 8.4, the eval command has the following:

     

     

    http://www.tcl.tk/man/tcl8.4/TclCmd/eval.htm

     

    Note that the list command quotes sequences of words in such a way that they are not further expanded by the eval command.

     

     

    Aaron
  • spark_86682's avatar
    spark_86682
    Historic F5 Account
    Apr 19, 2012
    It's more that eval and {*} do different things. Eval takes a string and executes it as a command. {*} is used to expand a Tcl list into separate arguments ("stringifying" it, essentially), typically for use in calling a command.

    For example, if you had a variable called $headers which was a Tcl list of header names and values, which looked like: 

    { Set-Cookie "foo=bar" Set-Cookie "baz=quux" X-Server "BIG-IP iRule" }

    you might want to do something like: 

    HTTP::respond 302 Location http://new.example.com $headers

    to craft an HTTP response with all of those headers. But that wouldn't work, because Tcl would pass in those headers as one big parameter, confusing the HTTP::respond command. With {*}, you could make it work: 

    HTTP::respond 302 Location http://new.example.com {*}$headers

    which would be the same as: 

    HTTP::respond 302 Location http://new.example.com Set-Cookie "foo=bar" Set-Cookie "baz=quux" X-Server "BIG-IP iRule"

    However, the {*} syntax was introduced in Tcl 8.5, and BIG-IP is still using Tcl 8.4. So you need to use eval as a workaround: 

    eval "HTTP::respond 302 Location http://new.example.com $headers"

    This will expand the $headers variable as a string, and parse the contents as separate arguments to the HTTP::respond command.

    So eval is an old command, but no more than the rest of Tcl. 🙂
  • Simon_Mittelber's avatar
    Simon_Mittelber
    Icon for Nimbostratus rankNimbostratus
    Jul 19, 2017

    is there any news on this one?

    using eval for this, as spark suggests, is dangerous in my eyes:

    eval "HTTP::respond 302 Location http://new.example.com $headers"

    what if headers contain user input? someone could inject commands...

    or what if anyone would use it for a response including content:

    eval "HTTP::respond 200 content $content"

    what if the content contains user input?

    i think introducing the {*}-command is the only way to enable safe argument expansion.

    i did a 

    [info patchlevel]
    on big ip 13.0.0. tcl version is still 8.4.6.

    is big ip expected to move to 8.5 in the near future?