NAME

       catch - Evaluate script and trap exceptional returns

SYNOPSIS

       catch script ?resultVarName? ?optionsVarName?
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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 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, four additional entries are
       defined in  the  dictionary  of  return  options  stored  in  optionsVarName:  -errorinfo,
       -errorcode, -errorline, and -errorstack.  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 value of the -errorstack entry is an even-sized │
       list made of token-parameter pairs accumulated while unwinding the stack. The token may be │
       “CALL”,  in  which case the parameter is a list made of the proc name and arguments at the │
       corresponding level; or it may be “UP”, in which case the parameter is the relative  level │
       (as  in  uplevel) of the previous CALL. The salient differences with respect to -errorinfo │
       are that:                                                                                  │

       [1]                                                                                        │
              it is a machine-readable form that is amenable to  processing  with  [foreach  {tok │
              prm} ...],                                                                          │

       [2]                                                                                        │
              it  contains  the true (substituted) values passed to the functions, instead of the │
              static text of the calling sites, and                                               │

       [3]                                                                                        │
              it is coarser-grained, with only one  element  per  stack  frame  (like  procs;  no │
              separate elements for foreach constructs for example).

       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.  The │
       value of the -errorstack entry surfaces as info errorstack.

       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.

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
              }

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

SEE ALSO

       break(3tcl), continue(3tcl), dict(3tcl),  error(3tcl),  errorCode(3tcl),  errorInfo(3tcl),
       info(3tcl), return(3tcl)

KEYWORDS

       catch, error, exception