Unix `sed` examples: how to insert text before and after existing lines

If you ever need to use the Unix/Linux sed command to insert text before or after a line of text in an existing file, here's how I just ran several sed commands to update my old Function Point Analysis tutorial to have a format that doesn't look like it was created in the 1990s.

This tutorial consists of over 40 files, and I had eight changes I wanted to make each file. So I had two choices: modify each file by hand over the next six hours, or run a series of sed commands and be done in 30 minutes. (I chose the sed commands.)

sed example - Step 1

Because you can easily screw up a lot of files with one sed command, Step 1 is to make a backup of whatever files you're thinking about modifying with sed. Then move that backup to a different directory.

Skip this step at your own risk. You've been warned.

sed example: insert a line after another line

To demonstrate a simple example, here's how I used sed to insert an HTML break tag after a line that contained the text string "header.shtml". After looking at a few files I knew this string (a) was in every file and (b) was unique in those files, so I wrote this sed command:

/header.shtml /a\

I saved that sed command in a file named changes.sed.

Next, to make sure I remembered the sed syntax for inserting new text after an existing line, I ran a test sed command from the Linux command line like this:

sed -f changes.sed < node2.shtml | more

This command doesn’t actually change the file node2.shtml, it just reads the file, and then makes the changes as it writes the file contents to standard output. I piped that sed command output into the Linux more command so I could make sure my change worked as expected.

A Linux shell script to change all the files with sed

Satisfied that my change looked correct on this one file, I wrote a simple Linux shell script to run my sed command on every file in the current directory, like this:

# change all the "node*.html" files in the current directory

for i in `ls node*html`
  echo "working on $i ..."
  sed -f changes.sed < $i > ${i}.tmp
  mv ${i}.tmp $i

I saved this shell script to a file named go.sh, and then ran it like this:

sh go.sh

After it ran, I checked some of my files, and this simple “sed insert after” example worked just fine.

A sed script to insert text before and after a line

That sed example demonstrates how to insert text after a given line in a text file. Next, here's a sed script I used to insert two HTML "div" tags, the first one after the opening body tag, and the second one before the closing body tag.

# insert a div tag *after* the opening body tag
/<body /a\
<div id="wrap_body">

# insert the closing div tag *before* the closing body tag

Again, I tested this sed script like this:

sed -f changes.sed < node2.shtml | more

and once I was sure everything looked right, I again ran my Linux shell script to modify all the HTML files in the current directory.

sed example - inserting text before and after lines

I hope this short example of how to use the sed command to insert text before and after lines in many text files has been helpful.