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BUILTIN(1)             DragonFly General Commands Manual            BUILTIN(1)

NAME

builtin, !, %, ., :, @, [, {, }, alias, alloc, bg, bind, bindkey, break, breaksw, builtins, case, cd, chdir, command, complete, continue, default, dirs, do, done, echo, echotc, elif, else, end, endif, endsw, esac, eval, exec, exit, export, false, fc, fg, filetest, fi, for, foreach, getopts, glob, goto, hash, hashstat, history, hup, if, jobid, jobs, kill, let, limit, local, log, login, logout, ls-F, nice, nohup, notify, onintr, popd, printenv, printf, pushd, pwd, read, readonly, rehash, repeat, return, sched, set, setenv, settc, setty, setvar, shift, source, stop, suspend, switch, telltc, termname, test, then, time, times, trap, true, type, ulimit, umask, unalias, uncomplete, unhash, unlimit, unset, unsetenv, until, wait, where, which, while, wordexp -- shell built-in commands

SYNOPSIS

See the built-in command description in the appropriate shell manual page.

DESCRIPTION

Shell builtin commands are commands that can be executed within the run- ning shell's process. Note that, in the case of csh(1) builtin commands, the command is executed in a subshell if it occurs as any component of a pipeline except the last. If a command specified to the shell contains a slash `/', the shell will not execute a builtin command, even if the last component of the speci- fied command matches the name of a builtin command. Thus, while specify- ing ``echo'' causes a builtin command to be executed under shells that support the echo builtin command, specifying ``/bin/echo'' or ``./echo'' does not. While some builtin commands may exist in more than one shell, their oper- ation may be different under each shell which supports them. Below is a table which lists shell builtin commands, the standard shells that sup- port them and whether they exist as standalone utilities. Only builtin commands for the csh(1) and sh(1) shells are listed here. Consult a shell's manual page for details on the operation its builtin commands. Beware that the sh(1) manual page, at least, calls some of these commands ``built-in commands'' and some of them ``reserved words''. Users of other shells may need to consult an info(1) page or other sources of documentation. Commands marked ``No**'' under External do exist externally, but are implemented as scripts using a builtin command of the same name. Command External csh(1) sh(1) ! No No Yes % No Yes No . No No Yes : No Yes Yes @ No Yes No [ Yes No Yes { No No Yes } No No Yes alias No** Yes Yes alloc No Yes No bg No** Yes Yes bind No No Yes bindkey No Yes No break No Yes Yes breaksw No Yes No builtin No No Yes builtins No Yes No case No Yes Yes cd No** Yes Yes chdir No Yes Yes command No** No Yes complete No Yes No continue No Yes Yes default No Yes No dirs No Yes No do No No Yes done No No Yes echo Yes Yes Yes echotc No Yes No elif No No Yes else No Yes Yes end No Yes No endif No Yes No endsw No Yes No esac No No Yes eval No Yes Yes exec No Yes Yes exit No Yes Yes export No No Yes false Yes No Yes fc No** No Yes fg No** Yes Yes filetest No Yes No fi No No Yes for No No Yes foreach No Yes No getopts No** No Yes glob No Yes No goto No Yes No hash No No Yes hashstat No Yes No history No Yes No hup No Yes No if No Yes Yes jobid No No Yes jobs No** Yes Yes kill Yes Yes Yes let No No Yes limit No Yes No local No No Yes log No Yes No login Yes Yes No logout No Yes No ls-F No Yes No nice Yes Yes No nohup Yes Yes No notify No Yes No onintr No Yes No popd No Yes No printenv Yes Yes No printf Yes No Yes pushd No Yes No pwd Yes No Yes read No** No Yes readonly No No Yes rehash No Yes No repeat No Yes No return No No Yes sched No Yes No set No Yes Yes setenv No Yes No settc No Yes No setty No Yes No setvar No No Yes shift No Yes Yes source No Yes No stop No Yes No suspend No Yes No switch No Yes No telltc No Yes No termname No Yes No test Yes No Yes then No No Yes time Yes Yes No times No No Yes trap No No Yes true Yes No Yes type No No Yes ulimit No No Yes umask No** Yes Yes unalias No** Yes Yes uncomplete No Yes No unhash No Yes No unlimit No Yes No unset No Yes Yes unsetenv No Yes No until No No Yes wait No** Yes Yes where No Yes No which Yes Yes No while No Yes Yes wordexp No No Yes

SEE ALSO

csh(1), echo(1), false(1), info(1), kill(1), login(1), nice(1), nohup(1), printenv(1), printf(1), pwd(1), sh(1), test(1), time(1), true(1), which(1)

HISTORY

The builtin manual page first appeared in FreeBSD 3.4.

AUTHORS

This manual page was written by Sheldon Hearn <sheldonh@FreeBSD.org>. DragonFly 3.5 May 18, 2012 DragonFly 3.5 return(n) Tcl Built-In Commands return(n) ______________________________________________________________________________

NAME

return - Return from a procedure, or set return code of a script

SYNOPSIS

return ?result? return ?-code code? ?result? return ?option value ...? ?result? ______________________________________________________________________________

DESCRIPTION

In its simplest usage, the return command is used without options in the body of a procedure to immediately return control to the caller of the procedure. If a result argument is provided, its value becomes the result of the procedure passed back to the caller. If result is not specified then an empty string will be returned to the caller as the result of the procedure. The return command serves a similar function within script files that are evaluated by the source command. When source evaluates the contents of a file as a script, an invocation of the return command will cause script evaluation to immediately cease, and the value result (or an empty string) will be returned as the result of the source command.

EXCEPTIONAL RETURN CODES

In addition to the result of a procedure, the return code of a procedure may also be set by return through use of the -code option. In the usual case where the -code option is not specified the procedure will return normally. However, the -code option may be used to generate an exceptional return from the procedure. Code may have any of the following values: ok (or 0) Normal return: same as if the option is omitted. The return code of the procedure is 0 (TCL_OK). error (or 1) Error return: the return code of the procedure is 1 (TCL_ERROR). The procedure command behaves in its calling context as if it were the command error result. See below for additional options. return (or 2) The return code of the procedure is 2 (TCL_RETURN). The procedure command behaves in its calling context as if it were the command return (with no arguments). break (or 3) The return code of the procedure is 3 (TCL_BREAK). The procedure command behaves in its calling context as if it were the command break. continue (or 4) The return code of the procedure is 4 (TCL_CONTINUE). The procedure command behaves in its calling context as if it were the command continue. value Value must be an integer; it will be returned as the return code for the current procedure. When a procedure wants to signal that it has received invalid arguments from its caller, it may use return -code error with result set to a suitable error message. Otherwise usage of the return -code option is mostly limited to procedures that implement a new control structure. The return -code command acts similarly within script files that are evaluated by the source command. During the evaluation of the contents of a file as a script by source, an invocation of the return -code code command will cause the return code of source to be code.

RETURN OPTIONS

In addition to a result and a return code, evaluation of a command in Tcl also produces a dictionary of return options. In general usage, all option value pairs given as arguments to return become entries in the return options dictionary, and any values at all are acceptable except as noted below. The catch command may be used to capture all of this information -- the return code, the result, and the return options dictionary -- that arise from evaluation of a script. As documented above, the -code entry in the return options dictionary receives special treatment by Tcl. There are other return options also recognized and treated specially by Tcl. They are: -errorcode list The -errorcode option receives special treatment only when the value of the -code option is TCL_ERROR. Then the list value is meant to be additional information about the error, presented as a Tcl list for further processing by programs. If no -errorcode option is provided to return when the -code error option is provided, Tcl will set the value of the -errorcode entry in the return options dictionary to the default value of NONE. The -errorcode return option will also be stored in the global variable errorCode. -errorinfo info The -errorinfo option receives special treatment only when the value of the -code option is TCL_ERROR. Then info is the initial stack trace, meant to provide to a human reader additional information about the context in which the error occurred. The stack trace will also be stored in the global variable errorInfo. If no -errorinfo option is provided to return when the -code error option is provided, Tcl will provide its own initial stack trace value in the entry for -errorinfo. Tcl's initial stack trace will include only the call to the procedure, and stack unwinding will append information about higher stack levels, but there will be no information about the context of the error within the procedure. Typically the info value is supplied from the value of -errorinfo in a return options dictionary captured by the catch command (or from the copy of that information stored in the global variable errorInfo). -errorstack list The -errorstack option receives special treatment only when the | value of the -code option is TCL_ERROR. Then list is the | initial error stack, recording actual argument values passed to | each proc level. The error stack will also be reachable through | info errorstack. If no -errorstack option is provided to return | when the -code error option is provided, Tcl will provide its | own initial error stack in the entry for -errorstack. Tcl's | initial error stack will include only the call to the procedure, | and stack unwinding will append information about higher stack | levels, but there will be no information about the context of | the error within the procedure. Typically the list value is | supplied from the value of -errorstack in a return options | dictionary captured by the catch command (or from the copy of | that information from info errorstack). -level level The -level and -code options work together to set the return code to be returned by one of the commands currently being evaluated. The level value must be a non-negative integer representing a number of levels on the call stack. It defines the number of levels up the stack at which the return code of a command currently being evaluated should be code. If no -level option is provided, the default value of level is 1, so that return sets the return code that the current procedure returns to its caller, 1 level up the call stack. The mechanism by which these options work is described in more detail below. -options options The value options must be a valid dictionary. The entries of that dictionary are treated as additional option value pairs for the return command.

RETURN CODE HANDLING MECHANISMS

Return codes are used in Tcl to control program flow. A Tcl script is a sequence of Tcl commands. So long as each command evaluation returns a return code of TCL_OK, evaluation will continue to the next command in the script. Any exceptional return code (non-TCL_OK) returned by a command evaluation causes the flow on to the next command to be interrupted. Script evaluation ceases, and the exceptional return code from the command becomes the return code of the full script evaluation. This is the mechanism by which errors during script evaluation cause an interruption and unwinding of the call stack. It is also the mechanism by which commands like break, continue, and return cause script evaluation to terminate without evaluating all commands in sequence. Some of Tcl's built-in commands evaluate scripts as part of their functioning. These commands can make use of exceptional return codes to enable special features. For example, the built-in Tcl commands that provide loops -- such as while, for, and foreach -- evaluate a script that is the body of the loop. If evaluation of the loop body returns the return code of TCL_BREAK or TCL_CONTINUE, the loop command can react in such a way as to give the break and continue commands their documented interpretation in loops. Procedure invocation also involves evaluation of a script, the body of the procedure. Procedure invocation provides special treatment when evaluation of the procedure body returns the return code TCL_RETURN. In that circumstance, the -level entry in the return options dictionary is decremented. If after decrementing, the value of the -level entry is 0, then the value of the -code entry becomes the return code of the procedure. If after decrementing, the value of the -level entry is greater than zero, then the return code of the procedure is TCL_RETURN. If the procedure invocation occurred during the evaluation of the body of another procedure, the process will repeat itself up the call stack, decrementing the value of the -level entry at each level, so that the code will be the return code of the current command level levels up the call stack. The source command performs the same handling of the TCL_RETURN return code, which explains the similarity of return invocation during a source to return invocation within a procedure. The return code of the return command itself triggers this special handling by procedure invocation. If return is provided the option -level 0, then the return code of the return command itself will be the value code of the -code option (or TCL_OK by default). Any other value for the -level option (including the default value of 1) will cause the return code of the return command itself to be TCL_RETURN, triggering a return from the enclosing procedure.

EXAMPLES

First, a simple example of using return to return from a procedure, interrupting the procedure body. proc printOneLine {} { puts "line 1" ;# This line will be printed. return puts "line 2" ;# This line will not be printed. } Next, an example of using return to set the value returned by the procedure. proc returnX {} {return X} puts [returnX] ;# prints "X" Next, a more complete example, using return -code error to report invalid arguments. proc factorial {n} { if {![string is integer $n] || ($n < 0)} { return -code error \ "expected non-negative integer,\ but got \"$n\"" } if {$n < 2} { return 1 } set m [expr {$n - 1}] set code [catch {factorial $m} factor] if {$code != 0} { return -code $code $factor } set product [expr {$n * $factor}] if {$product < 0} { return -code error \ "overflow computing factorial of $n" } return $product } Next, a procedure replacement for break. proc myBreak {} { return -code break } With the -level 0 option, return itself can serve as a replacement for break, with the help of interp alias. interp alias {} Break {} return -level 0 -code break An example of using catch and return -options to re-raise a caught error: proc doSomething {} { set resource [allocate] catch { # Long script of operations # that might raise an error } result options deallocate $resource return -options $options $result } Finally an example of advanced use of the return options to create a procedure replacement for return itself: proc myReturn {args} { set result "" if {[llength $args] % 2} { set result [lindex $args end] set args [lrange $args 0 end-1] } set options [dict merge {-level 1} $args] dict incr options -level return -options $options $result }

SEE ALSO

break(n), catch(n), continue(n), dict(n), error(n), errorCode(n), errorInfo(n), proc(n), source(n), throw(n), try(n)

KEYWORDS

break, catch, continue, error, exception, procedure, result, return Tcl 8.5 return(n)

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