TCL script try catch

Question

I have a TCL script that sets a number of variables and reads several files. The last file it reads may contain sensitive information, so I need to make sure this file gets deleted even if an error occurs at the beginning of the script:

#!/usr/bin/expect -f

set timeout 5

set foo [lindex $argv 0]
set bar [lindex $argv 1]
set secret [lindex $argv 2]

if { ! ([file exists "$foo"] && [file executable "$foo"]) } {
    puts stderr "$foo does not exist or is not executable"
    exit 2
}

if { ! ([file exists "$bar"] && [file executable "$bar"]) } {
    puts stderr "$bar does not exist or is not executable"
    exit 3
}

if { ! ([file exists "$secret"] && [file readable "$secret"]) } {
    puts stderr "$secret does not exist or is not readable"
    exit 4
}

#Read secret, do foo, do bar, etc.

file delete $secret

Looking at try I think I can do this:

try {
    set foo [lindex $argv 0]
    set bar [lindex $argv 1]
    set secret [lindex $argv 2]

    if { ! ([file exists "$foo"] && [file executable "$foo"]) } {
        puts stderr "$foo does not exist or is not executable"
        exit 2
    }

    if { ! ([file exists "$bar"] && [file executable "$bar"]) } {
        puts stderr "$bar does not exist or is not executable"
        exit 3
    }

    if { ! ([file exists "$secret"] && [file readable "$secret"]) } {
        puts stderr "$secret does not exist or is not readable"
        exit 4
    }

    #Read the file, do foo, do bar, etc.

    file delete $secret
} on error {
    file delete $secret
    exit
}

But I'm unfamiliar with TCL syntax and having a hard time finding things in the documentation. Will this work, or do I need to change on error to on error {result options}? In that case, what is result and options? Or do I need to use trap instead?

Answer 1

In Tcl documentation, optional elements look like ?this?: example from http://tcl.tk/man/tcl8.6/TclCmd/try.htm showing the handlers and "finally" clauses are optional:

try body ?handler...? ?finally script?

In the on clause, variableList is not optional, but you can provide an empty list. If you provide {result options}, result is the error message (for humans) and options is a dictionary holding specific information about the error (see the return man page for details).

To reduce code duplication, use the finally block

try {
    # your code here
} on error {} {
    exit
} finally {
    file delete $secret
}