eval 的含義 （copied）

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eval is part of POSIX. Its an interface which can be a shell built-in.

Its described in the "POSIX Programmer's Manual": http://www.unix.com/man-page/posix/1posix/eval/

eval - construct command by concatenating arguments

It will take an argument and construct a command of it, which will be executed by the shell. This is the example of the manpage:

1) foo=10 x=foo
2) y='$'$x
3) echo $y
4) $foo
5) eval y='$'$x
6) echo $y
7) 10

1. In the first line you define $foo with the value '10' and $x with the value 'foo'.
2. Now define $y, which consists of the string '$foo'. The dollar sign must be escaped with '$'.
3. To check the result, echo $y.
4. The result will be the string '$foo'
5. Now we repeat the assignment with eval. It will first evaluate $x to the string 'foo'. Now we have the statement y=$foo which will get evaluated to y=10.
6. The result of echo $y is now the value '10'.

This is a common function in many languages, e.g. Perl and JavaScript. Have a look at perldoc eval for more examples: http://perldoc.perl.org/functions/eval.html

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edited Oct 25 '11 at 14:13

answered Oct 22 '11 at 20:37 echox 5,6911534

In shell terminology, eval is a built-in, not a function. In practice, built-ins behave a lot like functions that don't have an in-language definition, but not quite (as becomes apparent if you are twisted enough to define a function called eval). –  Gilles Oct 23 '11 at 1:17

thx, fixed that :-) –  echox Oct 25 '11 at 14:14




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