//**sed**// is a command-line utility in Linux and Unix systems that is used for text processing. It is particularly useful for performing basic text transformations on an input file, such as substitution, deletion and addition of text.

The basic syntax for **sed** is:

<code>
sed 'command' inputfile</code>
Where **//command//** is the command to be executed on the inputfile

Examples:

Replace all occurrences of the word "pc" with "mac":
<code>sed 's/pc/mac/g' inputfile</code>

Delete all lines that contain the word "mac":
<code>sed '/mac/d' inputfile</code>

Insert the text "Hello, world!" at the beginning of each line:
<code>sed 's/^/Hello, world! /' inputfile</code>

Print only the lines from 2 to 5
<code>sed -n '2,5p' inputfile</code>

Substitute the word "pc" with "mac" only on the first line
<code>sed '1 s/pc/mac/' inputfile</code>


**sed** is a very powerful command that can be used to perform various text transformations. It can also use regular expressions for more advanced text processing.