What is the G Option in Sed

“The term sed is an abbreviation of stream editor. In UNIX/Linux, the sed command parses and transforms streams (texts, in most cases). It’s a command-line tool that comes pre-installed in every UNIX/Linux system.”

In this guide, we will explore the G option in sed.

Basic Usage of Sed

The most common usage of the sed command is finding and replacing contents in a given stream. The command structure is as follows:

 
Have a look at the following example:

 

Here,

• • The echo command prints the given string on the STDOUT. In Bash, STDOUT, STDIN, and STDERR are built-in data streams.
  • The sed command uses STDOUT as the stream, operates as specified, and prints the output on the console.

If you’re trying to edit a text file, sed won’t change the file contents by default. This is to avoid accidental data loss. To change the contents of a file directly, you need to run sed with in-place editing.

The g Option in Sed

When finding and replacing, the default behavior of sed is to terminate its actions upon finding the first instance. The following example demonstrates this behavior perfectly:

 

Here,

• • Using the echo command, we’re passing the same word “the” multiple times to the sed command.
  • However, in the output, only the first instance of “the” is transformed into “THE”.

To instruct sed to transform all the matching instances of the given pattern, we have to add the g option. Update the command as follows:

 

Now, all the instances of “the” are changed to “THE” properly.

The G Option in Sed

We finally arrive at the focus point of this guide. While the g option is applicable for text substitution, the G option is used for a completely different purpose.

Using the G option, we can append hold space to pattern space. In simple terms, we can add new lines before/after the matching pattern.

For demonstration, I’ve created a sample text file test.txt:

 

Additional Spacing

We can now perform various operations on the spacing of the file. For example, to double space the file, use the following command:

 

Similarly, we can triple space the file:

 

Here,

• • In both cases, because we’re not providing any pattern to match for, sed is matching for all the texts in the file.
  • The sed command works line-by-line on a given text stream. After matching each line (because no specific pattern was provided), a new line is added.

Removing Additional Spacing

We can also use the G option to delete all blank lines and double space:

 

Inserting Blank Lines Before/After the Matches

We can specify a pattern using regular expression and tell sed to add a blank line before/after it.

The first example will be adding a blank line below the matching pattern:

 

The next example will be adding a blank line above the matching pattern:

 

Finally, we can combine both of them together to insert blank lines around the matching pattern:

 

Final Thoughts

In this guide, we successfully demonstrated various ways to use the G option with sed to add spaces in the text. This technique can be useful in various situations, for example, singling out specific lines in a debug log.

A huge advantage of sed is its regular expression support. Learn more about sed with regex. The sed command can also easily be integrated into shell scripting.

Happy computing!