Shell中过滤、判断字母的方法

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在Shell中过滤、判断字母方法：
方法一、

[root@SHELL-X64 scripts]# abc=1ab8ddd90      
[root@SHELL-X64 scripts]# echo ${abc//[0-9]}
abddd

[root@SHELL-X64 scripts]# abc=1ab8ddd90

[root@SHELL-X64 scripts]# echo ${abc//[0-9]}

abddd

方法二、

[root@SHELL-X64 scripts]# echo $abc | sed 's#[0-9]##g'
abddd

0 1	[root@SHELL-X64 scripts]# echo $abc \| sed 's#[0-9]##g' abddd

方法三、

[root@SHELL-X64 scripts]# abc=123
[root@SHELL-X64 scripts]# expr 1 + $abc 
124
[root@SHELL-X64 scripts]# echo $?
0
[root@SHELL-X64 scripts]# abc=23dd
[root@SHELL-X64 scripts]# expr 1 + $abc 
expr: non-numeric argument
[root@SHELL-X64 scripts]# echo $?       
2