[root@localhost blue]# a="ls |more"
[root@localhost blue]# $a                                           #ls

ls: cannot access |more: No such file or directory
[root@localhost blue]# eval $a                                    #取出

1
1.cpp
2
2.cpp
3
3.cpp
[root@localhost test]# a="123"
[root@localhost test]# echo '${'"a"'}'                        
${a}
[root@localhost test]# eval echo '${'"a"'}'                #再次執行${a}命令
123 

[root@localhost blue]# echo 'eval echo /$$#' > last
[root@localhost blue]# cat last
eval echo /$$#
[root@localhost blue]# ./last 1 2 3 4
4