Which is the correct way to pass variable params in tcl exec?

Question

i work in Linux and i normally execute the following command in the shell:

SomeCommand <Standard params> <params of type A> <params of type B> > Out.log

Now i want to call this in a TCL script like so:

set ParamsA "-some params of type A"
set ParamsB "-some params of type B"
set AllParams "-moreStandardParams $ParamsA $ParamsB"
exec SomeCommand $AllParams > Out.log

i am having some hard time making this work, because no matter what i try i cant seem to make all the params visible to SomeCommand. what is the correct way of doing this? i am obviously new to TCL. what is the difference between "" [] {} and are there any other ways of enclosing the params? What is the rule with the white spaces pre and post parenthesis?

Possible duplicate of How to add a variable amount of arguments to exec in tcl? — mrcalvin
– mrcalvin, Commented Aug 21, 2018 at 14:56

Brad Lanam · Accepted Answer · 2018-08-21 14:48:33Z

The basic problem is that exec takes a set of words as arguments and it is getting passed only one.

I like to use a list to build up the command line. Then for the actual exec, the list must be expanded into separate words. lists have an advantage that when they are converted to a string (e.g. $mylist), the canonicalized (string-ified, serialized) form will always convert back to the same list.

set cmd mycommand
lappend cmd -param1 p1
lappend cmd -param2 p2
if { $someoption } {
  lappend cmd -param3 p3
}
lappend cmd -debug

exec {*}$cmd > $outfile

If you are using an older version of Tcl, you will need to use the eval statement:

eval exec $cmd > $outfile

Edit: missed the other half of your question.

Double quotes enclose a string and allow interpolation of escaped characters and substitution of variables and commands:

 set cmd command
 "here is [string toupper my] $cmd\r\n"

Braces are similar to single quotes in algol-derived languages. They specify a string. No command and variable substitution is done. Backslash interpolation is not done:

 {here is [string toupper my] $cmd\r\n}

Brackets specify that the first word is a command and should be processed as a command.

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