Unix one-liner to swap/transpose two lines in multiple text files?

Question

I wish to swap or transpose pairs of lines according to their line-numbers (e.g., switching the positions of lines 10 and 15) in multiple text files using a UNIX tool such a

Answer 1

If you want to edit a file, you can use ed, the standard editor. Your task is rather easy in ed:

printf '%s\n' 14m26 26-m14- w q | ed -s file

How does it work?

• 14m26 tells ed to take line #14 and move it after line #26
• 26-m14- tells ed to take the line before line #26 (which is your original line #26) and move it after line preceding line #14 (which is where your line #14 originally was)
• w tells ed to write the file
• q tells ed to quit.

If your numbers are in a variable, you can do:

linea=14
lineb=26
{
    printf '%dm%d\n' "$linea" "$lineb"
    printf '%d-m%d-\n' "$lineb" "$linea"
    printf '%s\n' w q
} | ed -s file

or something similar. Make sure that linea.