Print  the  value  of  EXPRESSION to standard output.  A blank line below separates increasing precedence

       groups.  EXPRESSION may be:

       ARG1 | ARG2

              ARG1 if it is neither null nor , otherwise ARG2

       ARG1 & ARG2

              ARG1 if neither argument is null or , otherwise 

       ARG1 < ARG2

              ARG1 is less than ARG2

。。。等等

跟字符串相关的

  match STRING REGEXP

              same as STRING : REGEXP

       substr STRING POS LENGTH

              substring of STRING, POS counted from 

       index STRING CHARS

              index in STRING where any CHARS is found, or 

       length STRING

              length of STRING

[devtac@test_1 ~]$ expr length "how can i just let you walk away"

32

[devtac@test_1 ~]$ expr length "how "
4

[devtac@test_1 ~]$ expr substr "how can i just" 2 6
ow can
[devtac@test_1 ~]$ expr index "how can i how" h
1