how to replace a specific char in a file

Question

I want to replace a specific char in a file. for example, I have a 20 line file and I want to replace the letter a on the 5th line and 12th column. example: what I have on line 5 hello world! and I want to replace the ! with ?

This cannot be done with a search and replace method because the actual text in the file is more or less random or changes frequently. I need to specify the unknown char at line 5 column 12, then replace

mattb · Accepted Answer · 2021-06-05 20:20:53Z

Edit - better answers than mine:

From @Cyrus:

sed -E '5 s/(.{11})./\1?/' your_file

From @potong:

sed '5s/./?/12' your_file

My original answer (just to contrast with the cleaner approaches)

This will do it:

sed '5 s/\(.\{11\}\).\(.*\)/\1?\2/' your_file

The ways it's working is as follows:

On line 5, perform a substitution
I remember the stuff in between the above using \( and \)
The first remembered part matches 11 characters of any type .\{11\}
Then a single characters of any type is matched (in your case that will be the '!')
The second remembered part matches everything after the single character\{.*\}
Then we put back the remembered content \1 and \2
But we replace the single matched character with a question mark

Once you've confirmed it does what you want, you can add the -i to do the substitution in-place:

sed -i '5 s/\(.\{11\}\).\(.*\)/\1?\2/' your_file

If we use the extended expressions flag (-E) we don't have to escape so much stuff:

sed -E '5 s/(.{11}).(.*)/\1?\2/' your_file

I think you can trim it down a bit: sed -E '5 s/(.{11})./\1?/' your_file

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how to replace a specific char in a file

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Edit - better answers than mine:

My original answer (just to contrast with the cleaner approaches)

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Edit - better answers than mine:

My original answer (just to contrast with the cleaner approaches)

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