虽说现在rhel已经到7版本，5版本也已淘汰，但不可否认仍有相当一部分的系统仍使用5版本在坚挺的提供服务，所以有必要搞清楚一下一些基础知识。
1、首先看问题
在rhel5下直接执行ifconfig命令会提示命令无法找到，如：
[reed@localhost ~]$ ifconfig
-bash: ifconfig: command not found
而这样执行则可以，如：
[reed@localhost ~]$ /sbin/ifconfig
eth0 Link encap:Ethernet HWaddr 00:0C:29:F1:35:E1
inet addr:192.168.209.133 Bcast:192.168.209.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:8413 errors:0 dropped:0 overruns:0 frame:0
TX packets:7240 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:5215094 (4.9 MiB) TX bytes:1271295 (1.2 MiB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:1635 errors:0 dropped:0 overruns:0 frame:0
TX packets:1635 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:4508168 (4.2 MiB) TX bytes:4508168 (4.2 MiB)
在rhel6版本下却没有这样的问题，如下：
[reed@reed ~]$ ifconfig
eth1 Link encap:Ethernet HWaddr 00:0C:29:A7:4F:64
inet addr:192.168.209.142 Bcast:192.168.209.255 Mask:255.255.255.0
inet6 addr: fe80::20c:29ff:fea7:4f64/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1608 errors:0 dropped:0 overruns:0 frame:0
TX packets:869 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:584674 (570.9 KiB) TX bytes:124717 (121.7 KiB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:0 (0.0 b)
virbr0 Link encap:Ethernet HWaddr 52:54:00:62:53:D7
inet addr:192.168.122.1 Bcast:192.168.122.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:0 errors:0 dropped:0 overruns:0 frame:0
TX packets:11 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:0 (0.0 b) TX bytes:2595 (2.5 KiB)
2、原因：区别在于，rhel5下的普通用户PATH变量不包括/sbin，而rhel6则包括，两者异同如下：
rhel5
[reed@localhost ~]$ echo $PATH
/usr/kerberos/bin:/usr/local/bin:/bin:/usr/bin:/home/reed/bin
rhel6
[reed@reed ~]$ echo $PATH
/usr/lib64/qt-3.3/bin:/usr/local/bin:/bin:/usr/bin:/usr/local/sbin:/usr/sbin:/sbin:/home/reed/bin
3、解决办法
1）直接输入绝对路径命令即可，如/sbin/ifconfig
2）修改环境变量：
2.1）临时生效
[reed@localhost ~]$ PATH=$PATH:/sbin
[reed@localhost ~]$ ifconfig
eth0 Link encap:Ethernet HWaddr 00:0C:29:F1:35:E1
inet addr:192.168.209.133 Bcast:192.168.209.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:8935 errors:0 dropped:0 overruns:0 frame:0
TX packets:7552 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:5258006 (5.0 MiB) TX bytes:1319923 (1.2 MiB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:1635 errors:0 dropped:0 overruns:0 frame:0
TX packets:1635 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:4508168 (4.2 MiB) TX bytes:4508168 (4.2 MiB)
2.2）永久生效，修改.bash_profile