bash - shell脚本中的 "for i"和 "for i in 1 2 3 4"有什么区别？

Question

我必须在不同的行上打印在 shell 脚本中解析的所有参数。我写了一个脚本

for i in 1 2 3 4 5
do
   echo $i
done

但这打印

1
2
3
4
5

即使我将参数解析为“10 20 30 40 50”

和互联网上的一个代码

for i
do
   echo $i
done

此代码正确打印参数。

有人可以解释一下为什么该代码有效但我的无效吗？

另外，如何使用一个变量 ($i) 的值作为变量名来打印其他内容。喜欢

i=1
$($i)

应该打印 $1 的值。

Answer 1

for i相当于for i in "$@"来自 Bash help for :

for: for NAME [in WORDS ... ] ; do COMMANDS; done
   Execute commands for each member in a list.

   The 'for' loop executes a sequence of commands for each member in a
   list of items.  If 'in WORDS ...;' is not present, then 'in "$@"' is
   assumed.  For each element in WORDS, NAME is set to that element, and
   the COMMANDS are executed.

If in WORDS ...; is not present, then in "$@" is assumed

如果要从变量中获取变量，请使用间接扩展:

set -- arg1 arg2 arg3 foo
for i in 3 4 1 2
do
    echo "${!i}"
done
# Output: arg3 foo arg2 arg1