评估号和反引号之间的差异(反撇号) [英] Difference between eval and backticks (reverse apostrophe)

问题描述

有人可以告诉我这里的最大区别是什么，为什么后者不起作用?

Can anyone tell me what the big difference here is and why the latter doesn't work?

test="ls -l"

两者现在都可以正常工作:

Both now work fine:

eval $test
echo `$test`

但是在这种情况下:

test="ls -l >> test.log"

eval $test
echo `$test`

后者将不起作用.这是为什么?我知道eval只是在执行脚本，而撇号正在执行脚本并将结果作为字符串返回.是什么使得无法在命令中使用>>或类似内容来执行?也许有一种方法可以使它与撇号一起使用，但我做错了什么?

The latter will not work. Why is that? I know that eval is just executing a script while the apostrophes are executing it and return the result as a string. What makes it not possible to use >> or simmilar stuff inside the command to execute? Maybe is there a way to make it work with apostrophes and I'm doing something wrong?

推荐答案

在使用反引号执行命令时，发送到shell的命令是:

When you're using backticks to execute your command, the command being sent to the shell is:

ls -l '>>' test.log

使得ls的>>和test.log自变量(请注意>>周围的引号).

which makes both >> and test.log arguments to ls (note the quotes around >>).

使用eval时，正在执行的命令是:

While using eval, the command being executed is:

ls -l >> test.log

(通过说出bash -vx scriptname来执行脚本，以查看发生了什么.)

(Execute your script by saying bash -vx scriptname to see what's happening.)

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