Insert Lines With Sed

Intro¶

Some of these examples and solutions are from real situations at work where I had to perform some surgery on one or more files to help automate some processes.

Add a header to multiple .csv files¶

Consider several files containing content similar to this:

1,Aayla Secura,Lightsaber
2,Ahsoka Tano,Lightsaber
3,Master Yoda,The Force
...

And then we want to insert a CSV header “ID,Name,Skill” row to all the files.

To achieve it, we can use the -i option to replace in place, and the 1i address + + command to insert a line at line 1.

For this example, let’s suppose we have three such files:

$ ls -1
file-1.txt
file-2.txt
file-3.txt

Then we loop over each .txt file and apply our desired sed command to each of them:

$ for file in *.txt
> do
> sed -i.bkp '1i\
> ID,Name,Skill
> ' "$file"
> done

The sed command uses -i.bkp so we save a .bkp file for each file we are processing (in case we mess up and need to restore the original files before trying again).

Then, the sed script itself starts with a single quote followed by 1i\. We need to say, “at the address 1 (line 1), apply the insert command”. Then we add a \ (backslash) and press Enter. The backslash must be followed by a newline (no spaces between the backslash and the newline). Then on the next line we type the content we want for the header and press Enter again. Finally, close the single quote and provide the current file of the loop as input to the sed command.

If we list the files again, we have three .txt.bkp files. One for each of the original .txt file:

$ ls -1
file-1.txt
file-1.txt.bkp
file-2.txt
file-2.txt.bkp
file-3.txt
file-3.txt.bkp

Finally, if we inspect the files, they should show that the first line now contains our header row:

$ head -n 4 *.txt
==> file-1.txt <==
ID,Name,Skill
1,Aayla Secura,Lightsaber
2,Ahsoka Tano,Lightsaber
3,Master Yoda,The Force

... same for the other files

And just for kicks, let’s visualize the contents through the lens of the column command:

$ column -ts, ./file-1.txt
ID  Name          Skill
1   Aayla Secura  Lightsaber
2   Ahsoka Tano   Lightsaber
3   Master Yoda   The Force