Tcl Built-In Commands - catch manual page

### NAME

catch - Evaluate script and trap exceptional returns

### SYNOPSIS

catch script ?varName?

### DESCRIPTION

The catch command may be used to prevent errors from aborting command interpretation. Catch 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 one of the exceptional return codes (see tcl.h for the definitions of code values). If the varName argument is given, then the variable it names is set to the error message from interpreting 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 +- }}
    }

### KEYWORDS

catch, error

    Copyright © 1993-1994 The Regents of the University of California.
    Copyright © 1994-1996 Sun Microsystems, Inc.
    Copyright © 1995-1997 Roger E. Critchlow Jr.