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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 com-mands. commands.
       mands.  Eval concatenates all its arguments 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 pro-
       cedure 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)