NAMEcatch - Evaluate script and trap exceptional returnsSYNOPSIScatch script ?varName?
DESCRIPTIONThe 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 thereturn, 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 fromscript.

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.

EXAMPLESThe 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 +- }} }載自：http://www.tcl.tk/man/tcl8.4/TclCmd/catch.htm

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