How do I prevent substitution before processing in TCL?

Question

I'm calling a TCL procedure like this-

::f::abstract -procname my_proc -args "-arg1 $a -arg2 $b"

Now here $a itself is something like this-

"ping -c 1 10.1.1.$ip"

When I try to run this, $a is expanded, and it becomes-

-args "-arg1 ping -c 1 10.1.1.27 -arg2 a2"

This gives an error because now it appears we have additional arguments like -c.

What I want is to take in $a as as a whole, and later on expand it inside the procedure.

How do I do that?

I can't use {} brackets because I need variable substitution. I can't remove quotes because the outer quotes take in args.

Answer 1

How about

::f::abstract -procname my_proc -args [list -arg1 $a -arg2 $b]

It allows substitution but preserves the list structure of the command line. I.e. you still have the same number of arguments on each level.

Answer 2

Welcome to quoting horrors.

If you expand it properly in the procedure, treat it like a proper list:

::f::abstract -procname my_proc -args [list -arg1 $a -arg2 $b]

This gets passed to your proc exactly like you can expect: as four things:

• the string -arg1
• the list/script ping - c1 10.1.1.$ip
• the string -arg2
• the string <whatever $b expands to>

Inside your proc, you will have to again eval your arg1's 'a' value, just as you expect (I hope, multiple evaluation is given in your structure here).

proc ... args {
    array set options <defaults>
    array set options $args
    set a [uplevel 1 $options(-arg1)]
    ...
    # $a now has a useful value...
}