问题
Can anyone tell me what the big difference here is and why the latter doesn't work?
test="ls -l"
Both now work fine:
eval $test
echo `$test`
But in this case:
test="ls -l >> test.log"
eval $test
echo `$test`
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?
回答1:
When you're using backticks to execute your command, the command being sent to the shell is:
ls -l '>>' test.log
which makes both >> and test.log arguments to ls (note the quotes around >>).
While using eval, the command being executed is:
ls -l >> test.log
(Execute your script by saying bash -vx scriptname to see what's happening.)
回答2:
eval is 'expression value' i.e.
test="ls -l >> test.log"
eval $test
is execute in same way in terminal as
ls -l >> test.log
whether
echo is for display purpose only.
来源：https://stackoverflow.com/questions/18849567/difference-between-eval-and-backticks-reverse-apostrophe