1eval(n)                      Tcl Built-In Commands                     eval(n)
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NAME

8       eval - Evaluate a Tcl script
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SYNOPSIS

11       eval arg ?arg ...?
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DESCRIPTION

15       Eval  takes one or more arguments, which together comprise a Tcl script
16       containing one or more commands.  Eval concatenates all  its  arguments
17       in  the  same  fashion  as  the concat command, passes the concatenated
18       string to the Tcl interpreter recursively, and returns  the  result  of
19       that  evaluation  (or  any  error generated by it).  Note that the list
20       command quotes sequences of words in such a way that they are not  fur‐
21       ther expanded by the eval command.
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EXAMPLES

24       Often,  it  is useful to store a fragment of a script in a variable and
25       execute it later on with extra values appended. This technique is  used
26       in a number of places throughout the Tcl core (e.g. in fcopy, lsort and
27       trace command callbacks). This example shows how to do this using  core
28       Tcl commands:
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30              set script {
31                  puts "logging now"
32                  lappend $myCurrentLogVar
33              }
34              set myCurrentLogVar log1
35              # Set up a switch of logging variable part way through!
36              after 20000 set myCurrentLogVar log2
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38              for {set i 0} {$i<10} {incr i} {
39                  # Introduce a random delay
40                  after [expr {int(5000 * rand())}]
41                  update    ;# Check for the asynch log switch
42                  eval $script $i [clock clicks]
43              }
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45       Note  that  in the most common case (where the script fragment is actu‐
46       ally just a list of words forming a command prefix), it  is  better  to
47       use  {*}$script when doing this sort of invocation pattern.  It is less
48       general than the eval command, and hence easier to make robust in prac‐
49       tice.   The  following procedure acts in a way that is analogous to the
50       lappend command, except it inserts the argument values at the start  of
51       the list in the variable:
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53              proc lprepend {varName args} {
54                  upvar 1 $varName var
55                  # Ensure that the variable exists and contains a list
56                  lappend var
57                  # Now we insert all the arguments in one go
58                  set var [eval [list linsert $var 0] $args]
59              }
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61       However, the last line would now normally be written without eval, like
62       this:
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64              set var [linsert $var 0 {*}$args]
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SEE ALSO

67       catch(n), concat(n), error(n), errorCode(n),  errorInfo(n),  interp(n),
68       list(n), namespace(n), subst(n), uplevel(n)
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KEYWORDS

71       concatenate, evaluate, script
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75Tcl                                                                    eval(n)
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