Strange result

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# 1  
Strange result


I have following codes which looks ok:

$ string1="123456789 abc2"
$ string2="abc"
$ position_of_string2=`expr index "$string1" "$string2"`
$ echo $position_of_string2

however, when string2="abc2", it gives me the following result:

$ string1="123456789 abc2"
$ string2="abc2"
$ position_of_string2=`expr index "$string1" "$string2"`
$ echo $position_of_string2

It seems the program searches ANY part of string2 (instead of the whole string2) and returns the position if finds first.

How can I find the position of a WHOLE substring within a string?

Last edited by littlewenwen; 04-23-2013 at 02:23 PM..
# 2  
From man expr:

       index STRING CHARS
              index in STRING where any CHARS is found, or 0

I don't think expr can do that.

You can do that with replacement and some arithmetic:

# Strip everything up to and including string2 from front of string1

if [ "${#string3}" -eq "${#string1}" ]
        echo "${string2} not found in ${string1}"
        # Remaining length of string3 is length of string1 - (offset+length of string2)
        echo "offset is $(( 1+(${#string1}-${#string3})-${#string2}))"

This User Gave Thanks to Corona688 For This Post:
# 3  
Playing around a bit to show some things

$ string1="123456789 abc2"

$ echo $string1
123456789 abc2

$ echo $string1 | awk -v v1="abc2" '{print $0,"___",v1,"___",index($0,v1)}'
123456789 abc2 ___ abc2 ___ 11

set your string to a variable
echo to screen, so you can see
echo string, also set a variable v1, print initial variable (note $0 as $1 would separate at the space character), show underline, print my search string, show another underline, finally display the location of the substring
You obviously don't need all that display, but wanted to show the logic behind the approach.
# 4  
Thank you very much for help.

Corona688, could you explain a bit more about string3,


what does the ##* do here? where can I find an introduction of their use? thanks a lot.
# 5  
See Advanced Bash Scripting Guide -- String Operations

It matches the longest match of "*abc2" from the start of the string, and replaces it with nothing, so you end up with a blank string, "".

If it wasn't at the end, if you were matching against "123456789 abc2 qwerty", you'd end up with " qwerty".

You can do math on the resulting string lengths to figure out where, or if, the string was found. If it wasn't found, the resulting string would be identical.
This User Gave Thanks to Corona688 For This Post:
# 6  
Thank you very much for spending time helping me.
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