Eval (idea)

In dynamic languages, this is a method (or keyword of some type) that evaluates given string argument(s) as they were code. programming languages (scripting languages) that support this include any language which is worth its salt (and some that aren't), such as Ruby, Tcl, Perl, Bourne shell, C shell, and JavaScript.

Admittedly, eval can be useful, but is mostly a hack. Most common uses for eval are hacks in shell scripts. For example:

_print_dir_stack () {
        local _bar
        _bar=$_dir_stack_size
        while  $_bar -gt -1 ; do
                eval echo -n \"\$_dir_stack$_bar \" >&2	# use eval to dereference a variable
                _bar=`expr $_bar - 1`
        done
        echo >&2
}

Using eval is a form of dereferencing. In this case, eval has been used to simulate an array in Bourne. After the eval, the line is interpreted and run as, e.g. echo -n "$_dir_stack1 ", simulating the Korn shell line, echo -n "${_dir_stack[$((_bar--))\} " >&2

Another usage is to stick into the environment variables that certain utilities spit out. For example, SSH's ssh-agent(1) will give output in the form:

SSH_AUTH_SOCK=/tmp/ssh-Zig94521/agent.94521; export SSH_AUTH_SOCK;
SSH_AGENT_PID=94522; export SSH_AGENT_PID;
echo Agent pid 94522;

If I "eval $(ssh-agent)" in the shell, it will define these variables to make the agent useful.

Ruby goes farther than other languages wrt its support of eval. Using methods like "eval", "instance_eval", "class_eval", the evaluation time and context of an eval statement can be set arbitrarily.