Python下执行stdin,stdout,stderr = work.exec_command('cd /;ls -l')可以正常执行，
但是stdin,stdout,stderr = work.exec_command('cd /;ll')就无法执行

>>> stdin,stdout,stderr = work.exec_command('cd /;ls -l') >>> print stdout.read() total 110 dr-xr-xr-x. 2 root root 4096 Jul 10 2015 bin dr-xr-xr-x. 5 root root 1024 Sep 22 12:52 boot drwxr-xr-x. 18 root root 3900 Sep 25 16:01 dev drwxr-xr-x. 102 root root 12288 Sep 27 03:26 etc drwxr-xr-x. 2 root root 4096 Sep 27 11:08 file drwxr-xr-x. 4 root root 4096 Jul 10 2015 home drwxrwxrwx. 4 root root 4096 Sep 26 19:54 i2test -rw-r--r--. 1 root root 3874 Sep 27 11:18 iawork_2016-09-27.log dr-xr-xr-x. 10 root root 4096 Jul 10 2015 lib dr-xr-xr-x. 9 root root 12288 Jul 10 2015 lib64 drwx------. 2 root root 16384 Jul 4 2015 lost+found drwxr-xr-x. 2 root root 4096 Sep 23 2011 media drwxr-xr-x. 3 root root 4096 Jul 4 2015 mnt drwxr-xr-x. 5 root root 4096 Jul 10 2015 opt dr-xr-xr-x. 151 root root 0 Sep 25 16:00 proc dr-xr-x---. 24 root root 4096 Sep 26 09:24 root dr-xr-xr-x. 2 root root 12288 Jul 10 2015 sbin drwxr-xr-x. 7 root root 0 Sep 25 16:00 selinux drwxr-xr-x. 2 root root 4096 Sep 23 2011 srv drwxr-xr-x. 13 root root 0 Sep 25 16:00 sys drwxrwxrwt. 20 root root 4096 Sep 27 09:43 tmp drwxr-xr-x. 13 root root 4096 Jul 4 2015 usr drwxr-xr-x. 21 root root 4096 Jul 4 2015 var >>> stdin,stdout,stderr = work.exec_command('cd /;ll') >>> print stdout.read() >>>

这是什么原因导致的呢，请大神赐教哈