eval(n) 						       Tcl Built-In Commands							   eval(n)

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NAME

       eval - Evaluate a Tcl script

SYNOPSIS

       eval arg ?arg ...?
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DESCRIPTION

       Eval  takes  one  or more arguments, which together comprise a Tcl script containing one or more commands.  Eval concatenates all its argu-
       ments in the same fashion as the concat command, passes the concatenated string to the Tcl interpreter recursively, and returns the  result
       of  that  evaluation  (or any error generated by it).  Note that the list command quotes sequences of words in such a way that they are not
       further expanded by the eval command.

EXAMPLES

       Often, it is useful to store a fragment of a script in a variable and execute it later on with extra values  appended.  This  technique	is
       used  in  a  number of places throughout the Tcl core (e.g. in fcopy, lsort and trace command callbacks). This example shows how to do this
       using core Tcl commands:
	      set script {
		  puts "logging now"
		  lappend $myCurrentLogVar
	      }
	      set myCurrentLogVar log1
	      # Set up a switch of logging variable part way through!
	      after 20000 set myCurrentLogVar log2

	      for {set i 0} {$i<10} {incr i} {
		  # Introduce a random delay
		  after [expr {int(5000 * rand())}]
		  update    ;# Check for the asynch log switch
		  eval $script $i [clock clicks]
	      }

       Note that in the most common case (where the script fragment is actually just a list of words forming a command prefix), it  is	better	to |
       use  {*}$script	when  doing this sort of invocation pattern.  It is less general than the eval command, and hence easier to make robust in |
       practice.  The following procedure acts in a way that is analogous to the lappend command, except it inserts the  argument  values  at  the
       start of the list in the variable:
	      proc lprepend {varName args} {
		 upvar 1 $varName var
		 # Ensure that the variable exists and contains a list
		 lappend var
		 # Now we insert all the arguments in one go
		 set var [eval [list linsert $var 0] $args]
	      }
       However, the last line would now normally be written without eval, like this:								   |
	      set var [linsert $var 0 {*}$args] 												   |

SEE ALSO

       catch(n), concat(n), error(n), interp(n), list(n), namespace(n), subst(n), tclvars(n), uplevel(n)

KEYWORDS

       concatenate, evaluate, script

Tcl																	   eval(n)