catch(n) Tcl Built-In Commands catch(n) 







catch - Evaluate script and trap exceptional returns








catch script ?resultVarName?
    ?optionsVarName?












The catch command may be used to prevent errors from
    aborting command interpretation. The catch command calls the Tcl
    interpreter recursively to execute script, and always returns without
    raising an error, regardless of any errors that might occur while executing
    script.


If script raises an error, catch will return a
    non-zero integer value corresponding to the exceptional return code returned
    by evaluation of script. Tcl defines the normal return code from
    script evaluation to be zero (0), or TCL_OK. Tcl also defines four
    exceptional return codes: 1 (TCL_ERROR), 2 (TCL_RETURN), 3
    (TCL_BREAK), and 4 (TCL_CONTINUE). Errors during evaluation of
    a script are indicated by a return code of TCL_ERROR. The other
    exceptional return codes are returned by the return, break,
    and continue commands and in other special situations as documented.
    Tcl packages can define new commands that return other integer values as
    return codes as well, and scripts that make use of the return -code
    command can also have return codes other than the five defined by Tcl.


If the resultVarName argument is given, then the variable
    it names is set to the result of the script evaluation. When the return code
    from the script is 1 (TCL_ERROR), the value stored in
    resultVarName is an error message. When the return code from the
    script is 0 (TCL_OK), the value stored in resultVarName is the
    value returned from script.


If the optionsVarName argument is given, then the variable
    it names is set to a dictionary of return options returned by evaluation of
    script. Tcl specifies two entries that are always defined in the
    dictionary: -code and -level. When the return code from
    evaluation of script is not TCL_RETURN, the value of the
    -level entry will be 0, and the value of the -code entry will
    be the same as the return code. Only when the return code is
    TCL_RETURN will the values of the -level and -code
    entries be something else, as further described in the documentation for the
    return command.


When the return code from evaluation of script is
    TCL_ERROR, three additional entries are defined in the dictionary of
    return options stored in optionsVarName: -errorinfo,
    -errorcode, and -errorline. The value of the -errorinfo
    entry is a formatted stack trace containing more information about the
    context in which the error happened. The formatted stack trace is meant to
    be read by a person. The value of the -errorcode entry is additional
    information about the error stored as a list. The -errorcode value is
    meant to be further processed by programs, and may not be particularly
    readable by people. The value of the -errorline entry is an integer
    indicating which line of script was being evaluated when the error
    occurred. The values of the -errorinfo and -errorcode entries
    of the most recent error are also available as values of the global
    variables ::errorInfo and ::errorCode respectively.


Tcl packages may provide commands that set other entries in the
    dictionary of return options, and the return command may be used by
    scripts to set return options in addition to those defined above.








The catch command may be used in an if to branch
    based on the success of a script.




if { [catch {open $someFile w} fid] } {


    puts stderr "Could not open $someFile for writing\n$fid"


    exit 1
}




There are more complex examples of catch usage in the
    documentation for the return command.










break(n), continue(n), dict(n), error(n), return(n),
  tclvars(n)










catch, error







  

    8.5
    Tcl