数据重定向 redirection

cat file1 file2 >file3

cat file1 >file4
cat file2 >>file4

>file4
>file5
ls >output.txt 2>&1
ls &>output.txt

#!/usr/bin/env bash
>output.txt
for dir in /bin/usr /usr/bin
do
    ls $dir &>output.txt
done

ls /bin|grep ed