Add a line in a specific position with Linux

If you need to add a line to a file in Linux, and you need to add that line in a specific position of the file there is an easy solution, even if you need to do it to hundreds of thousands of files.

Consider this file:

line 1
line 2
line 4

As you can see we missed line 3, so to add it just execute this command:

sed '3iline 3' filename.txt

Parts of the command

• sed: is the command itself
• 3: is the line where you want the new line inserted
• i: is the parameter that says sed to insert the line.
• line 3: is the text to be added in that position.
• filename: is the file where sed is going to act.

That will just put the result in the screen but the file will remain the same, you can redirect the output to a new file:

sed '3iline 3' input.txt > output.txt

Shell script add a line to multiple files

Now, if you need to do this with hundreds of files:

for i in *; do sed '3iline 3"' $i > /tmp/$i ; done

You can then copy from /tmp/ to your actual folder and you have been changed all files in current folder.