sed next & Next command

Question

Wonder if someone can explain the difference between Next (N) and the next(n) command in sed. i was trying following.

this is my input file

Consult Section 3.1 in the Owner and Operator Guide
Consult Section 3.1 in the Owner and Operator Guide
for a description of the tape drives
available on your system.

i was using following (next command)

/Operator/{
n
s/Owner and Operator Guide/Installation Guide/
}

result is

Consult Section 3.1 in the Owner and Operator Guide
Consult Section 3.1 in the Installation Guide
for a description of the tape drives
available on your system.

then i was using following

 /Operator/{
 N
 s/Owner and Operator Guide/Installation Guide/
 }

result is

Consult Section 3.1 in the Installation Guide
Consult Section 3.1 in the Owner and Operator Guide
for a description of the tape drives
available on your system.

can you please explain why I'm getting two different results ? do i need to use Next command (N) only in multiline pattern space ? and where to use next (n) command.

If i use sed 's/Owner and Operator Guide/Installation Guide/' input.txt will change everything as expected.

Answer 1

The n command prints out the pattern space (unless the "-n" flag is used), clears the pattern space, and reads in the next line. In your example,

/Operator/{
n
s/Owner and Operator Guide/Installation Guide/
}

 /Operator/ is matched for the first line. After performing the n command, we print the pattern space so your first line is print as is. Second line is loaded in to pattern space and the substitution is performed. As a result, you see the changes on the second line.

The N command, unlike n command, does not print the current pattern space, and does not clear it. Instead it appends the next line to the pattern space separated by a newline. As a result your substitution is performed on the first line since you are not using a global flag.