sed

sed is a Linux utility that can be used to manipulate files, one line at a time.

The sed “execution cycle” consists of:

1. Read a line from stdin or the file
2. Remove trailing new line character
3. Place the line in a pattern buffer
4. Modify the pattern buffer according to the specified command
5. Print the pattern buffer to stdout

The pattern buffer is a special place in memory that sed copies each line of the file to so that it can perform the operations specified on it.

There is another special buffer called the hold buffer. This buffer can be used to save the pattern space and then retrieve it later.

No operations can be performed on the hold buffer. It is used only to store data. All operations are performed on the pattern buffer.

sed provides commands that you can use to move or copy the pattern buffer into the hold buffer, and vice versa:

Command	Description
h	overwrite the hold buffer with contents of the pattern buffer
H	append a new line, and then the contents of the pattern buffer to the existing contents of the hold buffer
g	overwrite the pattern buffer with the contents of the hold buffer
G	append a new line, and then the contents of the hold buffer to the existing contents of the pattern buffer
x	exchange the contents of the hold and pattern buffers