replacing characters is string

[wassssup]

hi i would like to know how to replace the 2nd/3rd... character in a string without knowing what value it holds.

[rski]

You did not give to much info. Can you use sed or similiar tools?

[wassssup]

sorry...i canot use sed or awk..
for example : if i do
$ expr index "string" n
i'll get 5, as n is the fith char, so how do i replace the 5th char?

[rski]

So you can use only expr?

[wassssup]

i can use anything..the only 2 thing i cant use is sed and awk

[rski]

You mean something like that?

echo -n `expr substr string 1 1`;echo -n TR; expr substr string 4 `expr length\ string`
 

it replaces 'tr' to 'TR' in 'string' word.

[wassssup]

when u put ; does it mean | can u give an example? i dont really understand the code..im new to unix :)

[wassssup]

i dont want to specify the char i want to replace, the the location of the char that i want changed

[rski]

I know I do not specify the char i replace but positions of them.
In my resolution I
1) print first character
2) print next two character want to appear in the string
3) print the last part of the string

[wassssup]

i see, so what is the 1 1 in your code? and does it have to be a ` ` there? what does it do

[rski]

get the first character of the string (we want to replace 2nd and 3rd character so the 1st character must rewrite)

expr substr string 1 1
 

write 'TR' as 2nd and 3rd characters

echo -n TR
 

write the rest of the string (character from 4th ). if we don't know what is the the lenght of the string we can use length as here.

expr substr string 4 `expr length\ string`
 

` ` means treat the string inside as a command and execute it.

I advise you to read manual for exec command