exec eval source

来源：互联网发布：淘宝店最高等级是什么编辑：程序博客网时间：2024/04/30 15:04

转自：http://zhgw01.blog.163.com/blog/static/10414812200912025735381/

eval

eval "translates" a value buried inside a variable, and then runs the command that was buried in there
eval arg1 [arg2 arg3]就是将参数组合起来，并替换其中的变量，这样得到的结果作为一条命令进行运行

Code:

for i in 1 2 3do   eval myvar="$i"   echo "$myvar"done# this gives 123# why? because there is no metavalue or special meaning to 1 or 2 or 3

Code:

for i in ls dfdo    eval myvar="$i"    echo "$myvar"done# here you get output from the ls command and the df command如果不使用eval的话，结果就是ls跟df了

另外一个用eval要注意的是：

比如 nn=aa; aa=final; eval dn=\$$nn; echo $dn 会给出final
但是 eval dn=$"$nn";echo $dn给出 aa
eval dn="$"$nn; echo$dn给出final
原因据说是eval对\后面或者“”里面的字符串不进行扩展，而仅仅只是在其它需要扩展的扩展完成后将“”或者\去掉，最后再作为命令执行
所以eval dn="$"$nn先扩展出dn="$"aa，再去点""得到dn=$aa，然后将这个作为命令执行
eval dn=$"$nn"由于“”不进行扩展，而$"xxx"得到的结果是xxx，所以得到dn=$nn作为命令执行

exec

exec starts another process - BUT - it exits the current process when you do this kind of thing
使用exec，shell就不会调用fork，直接以后面的命令的内容代替当前shell的代码

Code:

#!/bin/bashexec echo "leaving this script forever  $0"   # Exit from script here.# ----------------------------------# The following line never happensecho "This echo will never echo."

source

When you run a command in the shell - like another script or a command like ls -
the shell creates a subprocess (called child process). Any environmentvariable that got defined or changed down in the child is LOST FOREVERto the parent process.

However if you source a script (there are two ways) you force thescript to run in the current process. That means environment variablesin the script you ran are NOT LOST.

Code:

# script1MYVAR="HI there!"# end script1

Code:

# script2# first, run script 1script1# now is the MYVAR variable set?  No -why? it got set in the child, when the child exited it GOT LOSTecho "MYVAR= $MYVAR"# now source script1 :: putting a lonesome dot or the command  "source" #  in front of the script name have the same effect. script1#  . script1 == source script1  these two are the SAME THINGsource script1# now is the MYVAR variable set?  This time we see that MYVAR has a valueecho "MYVAR= $MYVAR"

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