Bash - 为什么循环中的这两个命令是连续运行而不是并行运行？

Question

I'm trying to pick up some bash, and I can't figure out the following behavior. I have a for loop, and it seems like one command is running through the whole loop before the other command starts even though they're within the same do and done structure. See my example:

Here's the shell script:

for i in "$(ls .)"
do
    printf "$i\n"
    printf "$i\n"
done

If this was run in a directory with the files a , b , c I would expect the output to be:

a
a
b
b
c
c

Instead it is:

a
b
c
a
b
c

Can anyone explain to me what's going on?

Answer 1

Because you're expanding the expression in $(ls .) before you pass it to the loop, it's evaluating all the files in the directory first and then printing it twice. In essence, your loop only contains one element which you print twice.

The behaviour you want can be obtained by using the glob * operator instead:

for i in *
do
    printf "$i\n"
    printf "$i\n"
done

This way * represents a list to iterate over rather than a string that's already been evaluated.