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 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 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(3tcl), concat(3tcl), error(3tcl),  errorCode(3tcl),  errorInfo(3tcl),  interp(3tcl),
       list(3tcl), namespace(3tcl), subst(3tcl), uplevel(3tcl)

KEYWORDS

       concatenate, evaluate, script