NAME

       catch - Evaluate script and trap exceptional returns

SYNOPSIS

       catch script ?varName?
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DESCRIPTION

       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 varName 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 varName 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  script  does  not raise an error, catch will return 0 (TCL_OK) and set the variable to
       the value returned from script.

       Note that catch catches all exceptions, including those generated by break and continue as
       well  as  errors.   The  only  errors that are not caught are syntax errors found when the
       script is compiled.  This is because the catch command only catches errors during runtime.
       When the catch statement is compiled, the script is compiled as well and any syntax errors
       will generate a Tcl error.

EXAMPLES

       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
              }

       The catch command will not catch compiled syntax errors.   The  first  time  proc  foo  is
       called, the body will be compiled and a Tcl error will be generated.
              proc foo {} {
                  catch {expr {1 +- }}
              }

SEE ALSO

       break(3tcl), continue(3tcl), error(3tcl), return(3tcl), tclvars(3tcl)

KEYWORDS

       catch, error