How to set default values for Tcl variables?

Question

Hi,

I have some Tcl scripts that are executed by defining variables in the command-line invocation:

$ tclsh84 -cmd <script>.tcl -DEF<var1>=<value1> -DEF<var2>=<value2>

Is there a way to check if var1 and var2 are NOT defined at the command line and then assign them with a set of default values?

I tried the keywords global, variable, and set, but they all give me this error when I say "if {$<var1>==""}": "can't read <var1>: no such variable"

Thanks!

Arjun

Answer 1

You can catch your command to prevent error from aborting the script.

if { [ catch { set foo $<var1> } ] } {
    set <var1> defaultValue
}

(Warning: I didn't check the exact syntax with a TCL interpreter, the above script is just to give the idea).

Answer 2

I'm not familiar with the -def option on tclsh.

However, to check if a variable is set, instead of using 'catch', you can also use 'info exist ':

if { ![info exists blah] } {
  set blah default_value
}

Answer 3

Alternatively you can use something like the cmdline package from tcllib. This allows you to set up defaults for binary flags and name/value arguments, and give them descriptions so that a formatted help message can be displayed. For example, if you have a program that requires an input filename, and optionally an output filename and a binary option to compress the output, you might use something like:

package require cmdline
set sUsage "Here you put a description of what your program does"
set sOptions {
    {inputfile.arg ""  "Input file name - this is required"}
    {outputfile.arg "out.txt"     "Output file name, if not given, out.txt will be used"}
    {compressoutput       "0"      "Binary flag to indicate whether the output file will be compressed"}
}

array set options [::cmdline::getoptions argv $sOptions $sUsage]
if {$options(inputfile) == ""} {puts "[::cmdline::usage $sOptions $sUsage]";exit}

The .arg suffix indicates this is a name/value pair argument, if that is not listed, it will assume it is a binary flag.