Using sed to change all contents of a file except a particular line number (the header)?

Question

I want to use sed on Linux to change all the second fields in a CSV file for all rows except the first row (which is the header). I'm aware that I can just loop this and go line by line using sed -i '{$LINE_NUMBER ....' and not using this on that line, but I'm wondering if there is a way to do this without a loop which is slower.

I guess I could also just change all contents. Then in the next line change it back for this one particular line.

| Q1 | Q2 | Q3 |
| Name    | Resource_ID | Key |
| Joseph | 343433| AGFD |
| Mary | 242355 | AREW |

Answer 1

We can't give you a definitive answer without seeing your data, but from what you describe, you're looking for 2,${}. For example:

$ seq 5 | sed '2,${s/$/foo/}'
1
2foo
3foo
4foo
5foo

That means run the code in {} from the 2nd line until the end of the file. But chances are you'd be better off using awk or any other tool that can natively handle "fields".

Answer 2

Awk is almost certainly a better choice than sed for what you're trying to do. I'm guessing that when you say you want to modify the 2nd field, it's actually the 3rd |-separated field you're referring to in which case:

$ awk 'BEGIN{FS=OFS="|"} NR>1{$3=" whatever "} 1' file
| Q1 | Q2 | Q3 |
| Name    | whatever | Key |
| Joseph | whatever | AGFD |
| Mary | whatever | AREW |