TCL exec echo does not work with enviornment varaibles - environment-variables

Question

Let us say that I have environment variable PO, with value 1.If I use the LINUX echo command I get:
>echo $PO
1
However, if I use TCL and exec, I do not get interpolation:
>exec echo "\$PO"
$PO
Now, if I do something more elaborate, by using regsub to replace every ${varname} with [ lindex array get env varname ] 0 ], and use substr, it works:
>subst [ regsub -all {\$\{(\S+?)\}} "\${PO}/1" "\[ lindex \[ array get env \\1 \] 1 \]" ]
1/1
I have some corner cases, sure. But why is the exec not giving back what the shell would do?

Answer 1

why is the exec not giving back what the shell would do?
Because exec is not a shell.
When you do echo $PO from a shell, echo is not responsible for resolving the value. It is the shell that converts $PO to the value 1 before calling echo. echo never sees $PO when calling it from the shell.
If you are trying to emulate what the shell does, then you need to do the same work as the shell (or, invoke an actual shell to do the work for you).

Answer 2

Tcl is a lot more careful about where it does interpolation than Unix shells normally are. It keeps environment variables out of the way so that you don't trip over them by accident, and does far less processing when it invokes a subprocess. This is totally by design!
As much as possible (with a few exceptions) Tcl passes the arguments to exec through to the subprocesses it creates. It also has standard mechanisms for quoting strings so that you can control exactly what substitutions happen before the arguments are actually passed to exec. This means that when you do:
exec echo "\$PO"
Tcl is going to do its normal substitution rules and get these exact arguments to the command dispatch: exec, echo, and $PO. This then calls into the exec command, which launches the echo program with one argument, $PO, which does exactly that. (Shells usually substitute the value first.) If you'd instead done:
exec echo {$PO}
you would have got the same effect. Or even if you'd done:
exec {*}{echo $PO}
You still end up feeding the exact same characters into exec as its arguments. If you want to run the shell on it, you should explicitly ask for it:
exec /bin/sh -c {echo $PO}
The bit in the braces there is a full (small) shell script, and will be evaluated as such. And you could do this even:
exec /bin/sh -c {exec echo '$PO'}
It's a bit of a useless thing to do but it works.
You can also do whatever substitutions you want from your own code. My current favourite from Tcl 8.7 (in development) is this:
exec echo [regsub -all -command {\$(\w+)} "\$PO" {apply {- name} {
global env
return $env($name)
}}]
OK, total overkill for this but since you can use any old complex RE and script to do the substitutions, it's a major power tool. (You can do similar things with string map, regsub and subst in older Tcl, but that's quite a bit harder to do.) The sky and your imagination are the only limits.