---
name: "eval"
section: "n"
platform: "macos"
package: "tcl"
version: "152"
release: "macOS 26.5"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/eval/n"
---

# eval(n)

Evaluate a Tcl script

## NAME

eval - Evaluate a Tcl script

## SYNOPSIS

**eval** *arg* ?*arg ...*?

## 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(n)](/catch/n.md), [concat(n)](/concat/n.md), [error(n)](/error/n.md), [interp(n)](/interp/n.md), [list(n)](/list/n.md), [namespace(n)](/namespace/n.md), [subst(n)](/subst/n.md), [tclvars(n)](/tclvars/n.md), [uplevel(n)](/uplevel/n.md)

## KEYWORDS

concatenate, evaluate, script