In [Tcl], There are many ways to evaluate a string as a script.  This page, by [Andreas Leitgeb], [dgp], [ms], [pyk], [rs], et al, enumerates them.

As implied by its name, `[eval]` is the most explicit way to evaluate a string
as a script.  In the following example, every command (except the first, of
course, does the same thing:  evaluates `$cmd` as a script.

======
set cmd {set name {Zaphod Beeblebrox}}

#these examples are not byte-compiled, so slower
eval $cmd
uplevel 0 $cmd
source nameoffile

#these examples are byte-compiled, so faster
if 1 $cmd
interp eval {} $cmd
namespace eval [namespace current] $cmd
proc {} {} [list uplevel $cmd]; {}
apply [list {} [list uplevel $cmd] [namespace current]]
apply [list argv {tailcall {*}$argv} [namespace current]] $argv
======

[MS] explained: "... eval does not bytecompile the script as it is "usually" used for one-time scripts.

The above examples all act like `[eval]` in the sense that they return the
result of evaluating the script.  The commands in the following example
evaluate the string as a script, but don't return the result of the script:

======
foreach don't care $cmd
while 1 "$cmd; break"
interp alias {} breakable {} foreach _ _
breakable $cmd
catch $cmd
time $cmd
try $cmd
======

Taking it one step further, the following evaluation tactics not only don't
return the result of evaluating the script, but don't evaluate it in the
namespace they are called from:

======
after idle $cmd
======

`[subst]` is in a category of its own, as it results in the evaluation in
arbitrary text of substrings that can be interpreted as command or variable
substitutions:

======
subst {Why do {[set a {my eyes}]} ; hurt?}
======

In the example above, braces did not escape the command-substitution meaning
of brackets.  `[subst]` is probably the most mis-used command in Tcl.



** See Also **

   [Injection Attack]:   Any command that results in evaluation of arbitrary strings is potentially vulnerable to injection attacks.  Well-written code always uses such commands in a way that precludes this class of attack.

   [Tcl chatroom], 2002-12-18:   

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