bash/sh -c不能使用多个管道

Question

所以,命令:

dmesg | grep 'Attached SCSI disk' | awk '{ print $5}'

当直接从shell执行时,就像应该的那样工作,输出:

[sdb]
[sdc]
[sda]

但是,当我启动它时:

sh -c "dmesg | grep 'Attached SCSI disk' | awk '{ print $5}'"
# or
bash -c "dmesg | grep 'Attached SCSI disk' | awk '{ print $5}'"

我明白了:

[ 2.460353] sd 1:0:0:0: [sdb] Attached SCSI disk
[ 2.461348] sd 2:0:0:0: [sdc] Attached SCSI disk
[ 2.464181] sd 0:0:0:0: [sda] Attached SCSI disk

这清楚地表明最后一个管道没有执行.

我究竟做错了什么？

Answer 1

您$5的评估过早.将其更改为\$5.

如果要替换bash为echo,则会看到$5正在替换为空字符串:

 % echo "dmesg | grep 'Attached SCSI disk' | awk '{ print $5}'"
 dmesg | grep 'Attached SCSI disk' | awk '{ print }'

因此,当bash评估命令时,awk将打印整行,而不是第五个字段.

当你逃避美元符号时(通过预先用反斜杠预先添加),变量将$5被保留:

% echo "dmesg | grep 'Attached SCSI disk' | awk '{ print \$5}'"
dmesg | grep 'Attached SCSI disk' | awk '{ print $5}'