String Operations in Shell

Function index return the position of substring in string counting from one and 0 if substring is not found.

这个很容易误解，indeX 指的是 substring中的字符最早在 string 出现的位置

expr index $string $substring
  Numerical position in $string of first character in $substring that matches.

stringZ=abcABC123ABCabc echo `expr index "$stringZ" C12` # 6 # C position. echo `expr index "$stringZ" c` # 3 # 'c' (in #3 position)

This is the near equivalent of strchr()  in C.

expr substr $string $position $length Extracts $length characters from $string starting at $position.. The first character has index one.
 

stringZ=abcABC123ABCabc # 123456789...... # 1-based indexing. echo `expr substr $stringZ 1 2` # ab echo `expr substr $stringZ 4 3` # ABC

Determining the Length of Matching Substring at Beginning of String This is pretty rarely used capability of expr built-in but still sometimes it can be useful:

expr match "$string" '$substring'

where:

• String is any variable of literal string.
• $substring  is a .

stringZ=abcABC123ABCabc # |------| echo `expr match "$stringZ" 'abc[A-Z]*.2'` # 8 echo `expr "$stringZ" : 'abc[A-Z]*.2'` # 8

There are several ways to get length of the string.

• The simplest one is ${#varname}, which returns the length of the value of the variable as a character string. For example, if filename has the value fred.c, then ${#filename} would have the value 6.
   

• The second is to use built in function  expr, for example

  expr length $string or expr "$string" : '.*'
   

• Additional examples from stringZ=abcABC123ABCabc echo ${#stringZ} # 15 echo `expr length $stringZ` # 15 echo `expr "$stringZ" : '.*'` # 15
• More complex example. Here's the function for validating that that string is within a given max length. It requires two parameters, the actual string and the maximum length the string should be.check_length() # check_length # to call: check_length string max_length_of_string { # check we have the right params if (( $# != 2 )) ; then echo "check_length need two parameters: a string and max_length" return 1 fi if (( ${#1} > $2 )) ; then return 1 fi return 0 }

  You could call the function check_length like this:

  #!/usr/bin/bash # test_name while : do echo -n "Enter customer name :" read NAME [ check_length $NAME 10 ] && break echo "The string $NAME is longer then 10 characters" done echo $NAME