Linux Bash Scripting: How to remove a blank line after a particular line using sed

Removing a blank line after a particular line can be achieved using the single-line next command (n) of the sed editor.

The lowercase n command tells the sed editor to move to the next line of text in the data stream, without going back to the beginning of the sed commands. Normally the sed editor processes all of the defined commands on a line before moving to the next line of text in the data stream. The single‐line next command alters this flow.

In the below example, the data file that contains five lines, two of them empty. The goal is to remove the blank line after the header line but leave the blank line before the last line intact. If you write a sed script to just remove blank lines, you will remove both blank lines.

$ cat data This is the header line. This is a data line. This is the last line. $ $ sed ‘/ˆ$/d’ data This is the header line. This is a data line. This is the last line. $  

This happened because there was no specific way to find the unique blank line that you wanted to delete. The same can be achieved using the n command as shown in the below script.

$ sed ‘/header/{n ; d}’ data This is the header line. This is a data line. This is the last line. $

The above script searches for the keyword "header" and then moves to the next line when it executes the n command and then applies the d command to delete the blank line below it.