catch(n)

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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  recur-
       sively  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_CON-
       TINUE).  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 situa-
       tions 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 mes-
       sage.  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(n), continue(n), error(n), return(n), tclvars(n)

KEYWORDS

       catch, error

Tcl                                   8.0                             catch(n)