为什么会ambiguous redirect

338378WN8

bash-2.03$ while read filename filesize
> do
> echo $filename $filesize
> done < `ls -l |awk '{print $9,$5}'`
bash: `ls -l |awk '{print $9,$5}'`: ambiguous redirect
bash-2.03$ ls -l |awk '{print $9,$5}'

100607 96
100611 96
100614 96
100615 96
100617 96
100618 96

为什么直接执行可以,但是加到while do的判断中不行,
难道一定要把ls -l |awk '{print $9,$5}' 指定到一个文件,然后再用while do done调用文件才可以吗.

expert1

好像没见过这个用法。awk的输入来自重定向？

其实可以这样
while read filename filesize
do awk .....
done.
或是ls -l|xags |awk ;然后while read......

ccjsj1

把awk放前面，通过管道给while

1. ls -l |awk '{print $9,$5}' | while read filename filesize; do echo $filename $filesize; done

复制代码

338378WN8

谢谢,可以了
但是不清楚为什么while read 可以从文件读取,不能从命令替换中读取

bbgg1983

回复 4# 338378WN8

要这样写：
while read filename filesize;do echo $filename $filesize ;done <<< $(ls -l|awk '{print $9,$5}')

找本书搜索herestring的用法就直到了

338378WN8

尝试了一下
while read filename filesize; do echo $filename $filesize; done << end
> `ls -l |awk '{print $9,$5}'`
> end
这下可以了

expert1

<<< here string
<< here document

?

keymanlu

我遇到过这个问题，我当时的处理是把shell换成了ksh就可以执行成功了，不知道为什么。呵呵

spacehaibin

回复 6# 338378WN8


    也可以这样：
while read line;do
           。。。。。。。。。。                     
   done < <(  )

waker

1.
The words that the parser has marked as variable assignments (those preceding the command name) and redirections are saved for later processing.

2.

The words that are not variable assignments or redirections are expanded. If any words remain after expansion, the first word is taken to be the name of the command and the remaining words are the arguments.

3.

Redirections are performed as described above under REDIRECTION.

4.

The text after the = in each variable assignment undergoes tilde expansion, parameter expansion, command substitution, arithmetic expansion, and quote removal before being assigned to the variable.

If no command name results, the variable assignments affect the current shell environment. Otherwise, the variables are added to the environment of the executed command and do not affect the current shell environment. If any of the assignments attempts to assign a value to a readonly variable, an error occurs, and the command exits with a non-zero status.
If no command name results, redirections are performed, but do not affect the current shell environment. A redirection error causes the command to exit with a non-zero status.

If there is a command name left after expansion, execution proceeds as described below. Otherwise, the command exits. If one of the expansions contained a command substitution, the exit status of the command is the exit status of the last command substitution performed. If there were no command substitutions, the command exits with a status of zero.