Problems
in this case required to write a script sumx.sh , and from seeking, requirements 1-x as follows:
the x value is read from the keyboard
when the user does not enter any values, a calculation according to the default
program
steps
implemented This case requires the following steps get on.
Step one: know the most common treatment method for the initial value of the string
1) values only, $ {var: -word}
If the variable var already exists and not Null, then the $ var; otherwise, returns the string "word", the value of the original variable var unaffected.
Circumstances existing variable values:
[root@svr5 ~]# echo $SCHOOL //查看原变量值
Tarena IT Group.
[root@svr5 ~]# echo ${SCHOOL:-Tarena} //因SCHOOL已存在，输出变量SCHOOL的值
Tarena IT Group.
[root@svr5 ~]# echo $SCHOOL //原变量的值并不改变
Tarena IT Group.
Variable Value absence:
[root@svr5 ~]# unset SCHOOL //清除SCHOOL变量
[root@svr5 ~]# echo ${SCHOOL:-Tarena} //因SCHOOL已不存在，输出“Tarena”
Tarena
[root@svr5 ~]# echo $SCHOOL //变量SCHOOL仍然不存在
[root@svr5 ~]#
2) + value assignment, $ {var: = word}
If the variable var already exists and is non-Null, the $ var is returned, the old variable values unchanged; otherwise, the string "word", and this string assignment to the variable var.
Variable Value absence:
[root@svr5 ~]# echo $SCHOOL //前面已将此变量清除
[root@svr5 ~]# echo ${SCHOOL:=Tarena} //因SCHOOL不存在，输出“Tarena”
Tarena
[root@svr5 ~]# echo $SCHOOL //且将“Tarena”赋值给变量SCHOOL
Tarena
Circumstances existing variable values:
[root@svr5 ~]# echo $SCHOOL //确认当前的变量值
Tarena
[root@svr5 ~]# echo ${SCHOOL:=Tarena IT Group.} //变量已存在，输出其值
Tarena
[root@svr5 ~]# echo $SCHOOL //原变量的值也不受影响
Tarena