substitute a letter at a specific position in the file itself using bash

Question

I am trying to do this:

I have a file with content like below;

file: abcdefgh

I am looking for a way to do this;

file: aBCdefgh

So,make the 2nd and 3rd letter "capital/uppercase" in the file itself, because I have to do multiple conversions at different positions in a string in the file. Can someone please help me to know how to do this?

I came to know something like this below, but it does only for a single first character of the string in the file:

sed -i 's/^./\U&/' file

output: Abcdefgh

Thanks much!

Answer 1

Change your sed approach to the following:

sed -i 's/\(.\)\(..\)/\1\U\2/' file

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$ cat file
aBCdefgh

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matching section:

• \(.\) - match the 1st char of the string into the 1st captured group

• \(..\) - match the next 2 chars placing into the 2nd captured group

replacement section:

• \1 - points to the 1st parenthesized group \1 i.e. the 1st char

• \U\2 - uppercase the characters from the 2nd captured group \2

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Bonus approach for I want to capitalize "105th & 106th" characters:

sed -Ei 's/(.{104})(..)/\1\U\2/' file