Question

The Tcl error command writes the specified message to stderr adding to it a prefix “Error: “.

Is it possible to change that prefix with something else like “ERROR: ” or “MyError: ” ?

Answer 1

The error command itself does not write anything to anywhere other than the Tcl result. What it actually does is throw an exception (of type “error”; TCL_ERROR at the C API level). The code that prefixes it with “Error: ” is just the standard fallback handler, but you can use your own by having your main script use catch to trap any result and decide what to do with it (or try … from 8.6 onwards, which is easier to use when trapping specific problems).

if {[catch {source realscript.tcl} msg]} {
    # Caught an error!
    puts stderr "Oh noes! Teh errorz iz atakkin! $msg"
    puts stderr $::errorInfo
    exit 1
}

---

OK, if you’re using 8.5 then a slightly better way to write that is:

if {[catch {source realscript.tcl} msg resultdict]} {
    # Caught an error!
    puts stderr "Oh noes! Teh errorz iz atakkin! $msg"
    puts stderr [dict get $resultdict -errorinfo]
    exit 1
}

There’s lots of other interesting stuff in the result dictionary, which you might or might not be interested in. Have a poke around.