Linux伊甸园论坛 » Shell编程 » eval的疑问！

TUDOU01 发表于 2007-8-27 08:35

eval的疑问！

[dell@scsm dell]$ unamer=`uname -r`
[dell@scsm dell]$ eval version=`echo $unamer | awk -F '.' '{ print "(" $1 " " $2 ")" }'`
[dell@scsm dell]$ echo $version
2
[dell@scsm dell]$ version=`echo $unamer | awk -F '.' '{ print "(" $1 " " $2 ")" }'`
[dell@scsm dell]$ echo $version
(2 4)
[dell@scsm dell]$ version="(2 4)"
[dell@scsm dell]$ echo $version
(2 4)
[dell@scsm dell]$ eval version="(2 4)"
[dell@scsm dell]$ echo $version
2
为什么加入eval后version值发生了变化，eval起到什么作用？

dearvoid 发表于 2007-8-27 09:02

version="(2 4)" 是将字符串赋值给 version; eval version="(2 4)" 执行时变成了 version=(2 4), 是把数组 (2 4) 赋值给 version:
[quote][font=Courier][color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[16167 0][/color] [color=red]; eval v="(2 4)"
[/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[16167 0][/color] [color=red]; echo ${v[0]}
[/color]2
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[16167 0][/color] [color=red]; echo ${v[1]}
[/color]4
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[16167 0][/color] [color=red]; echo ${v[@]}
[/color]2 4
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[16167 0][/color] [color=red]; bye
[/color][/font][/quote]
eval 的功能是将其参数作为输入传给当前的 shell

dearvoid 发表于 2007-8-27 09:07

常见的 scripting languages (如 perl, python, tcl, php) 都有提供一个 eval 函数/指令, 用以动态构造并执行语句, 这也是 scripting languages 区别于 C/C++ 之类语言的一大特点/优点 :)

TUDOU01 发表于 2007-8-27 09:48

:victory: 明白了！Thank You Very Much!

TUDOU01 发表于 2007-8-27 10:43

dearvoid能举出一些eval的例子吗？

dearvoid 发表于 2007-8-27 11:14

FYI:
[quote][font=Courier][color=magenta][/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/void/bash)-
[30470 0][/color] [color=red]; cat random.sh
[/color][color=magenta]#! /bin/bash

cmd="hexdump -e '\"%02x\" \"\\n\"' -n 4 /dev/random"
echo "\$ $cmd"
eval $cmd

cmd="echo \$RANDOM"
echo "\$ $cmd"
eval $cmd
[/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/void/bash)-
[30470 0][/color] [color=red]; ./random.sh
[/color][color=magenta]$ hexdump -e '"%02x" "\n"' -n 4 /dev/random
43dccb1f
$ echo $RANDOM
5486
[/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/void/bash)-
[30470 0][/color] [color=red]; bye
[/color][color=magenta][/color][/font][/quote]

[[i] 本帖最后由 dearvoid 于 2007-8-27 11:27 编辑 [/i]]

dearvoid 发表于 2007-8-27 11:19

FYI:
[quote][font=Courier][color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[21645 0][/color] [color=red]; cat foo.sh
[/color]#!/bin/sh

eval "v=\$(( $@ ))"
echo "$@ = $v"
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[21645 0][/color] [color=red]; ./foo.sh 1 + 2
[/color]1 + 2 = 3
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[21645 0][/color] [color=red]; ./foo.sh 3 '**' 3
[/color]3 ** 3 = 27
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[21645 0][/color] [color=red]; ./foo.sh 7 '*' 11 '*' 13
[/color]7 * 11 * 13 = 1001
[color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[21645 0][/color] [color=red]; bye
[/color][/font][/quote]

TUDOU01 发表于 2007-8-27 13:57

非常感谢！回去仔细研究研究！

dearvoid 发表于 2007-8-27 15:23

前几天 jiahuan 发的把 ASCII 码转成 char 的例子也可以借鉴一下:
[quote][font=Courier][color=black][/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[30505 0][/color] [color=red]; cat ascii2chr.sh
[/color][color=black]#!/bin/bash

ascii2chr()
{
    printf -v ascii '%x' $1
    eval echo -e "'\\x$ascii'"
}

for i in 97 98 99; do
    ascii2chr $i
done
[/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[30505 0][/color] [color=red]; ./ascii2chr.sh
[/color][color=black]a
b
c
[/color][color=blue]-(dearvoid@LinuxEden:Forum)-(~/tmp)-
[30505 0][/color] [color=red]; bye
[/color][color=black][/color][/font][/quote]

TUDOU01 发表于 2007-8-27 20:20

例子非常的好！！！

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