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I'm using GNU sed 4.5, per sed --version.

I'm new to sed and I created below sed's workflow based on my understanding (correct me if you find anything wrong).

So it seems the default auto printing of the pattern space will always include a newline at the end. My question is, will p include a newline, too? I have below examples.

# seq 3 | sed -rn 'p'

1

2

3

Here the newline at the end of each number is added by sed itself (see the diagram "adds back newline to pattern space"). So it seems p will not append a newline. However, see below example.

# seq 3 | sed -rn 'x;p;x;p'



1



2



3

Here x exchange pattern space with hold space, which will result in an empty pattern space. Now p applies to the pattern space (nothing in it) should print nothing. But based on the result, it seems here p prints a newline. To me it seems this is inconsistent behavior. Can anyone help to explain, please?

There is an error in the diagram. Adds back newline to pattern space only happens when sed is run without the -n option. So the correct diagram should be:

Below is an excerpt from sed's documentation at How sed Works

When the end of the script is reached, unless the -n option is in use, the contents of pattern space are printed out to the output stream, adding back the trailing newline if it was removed. Then the next cycle starts for the next input line.

So if we run sed with -n, there will be no Adds newline back step.

sed's documentation states that the p command simply prints the pattern space. See Often-Used Commands for details.

p

Print out the pattern space (to the standard output). This command is usually only used in conjunction with the -n command-line option.

But it seems that p will always append a newline at the end of the pattern space (please correct me if you think this is wrong).

Based on the above-mentioned facts, let take a look the examples.

# seq 3 | sed -rn 'p'

1

2

3

The process for this example is:

1. Read text 1<newline>, strip <newline> and put 1 in pattern space.


2. 
   p prints 1 and then prints a <newline>
   


3. Read text 2<newline>, strip <newline> and put 2 in pattern space.


4. 
   p prints 2 and then prints a <newline>
   


5. Read text 3<newline>, strip <newline> and put 3 in pattern space.


6. 
   p prints 3 and then prints a <newline>
   

For the second example:

# seq 3 | sed -rn 'x;p;x;p'



1



2



3

The process is:

1. Read text 1<newline>, strip <newline> and put 1 in pattern space.


2. Put 1 in hold space and put empty string in the pattern space.


3. Print empty string and then print a <newline>.


4. Put 1 back to the pattern space and put empty string in hold space.


5. Print 1 and then print a <newline>
   


6. Repeat step 1~5 for other input lines

So there is no inconsistency in these examples.

BTW, I'm still learning sed and this answer is based on my understanding. Please correct me if you find anything wrong. Thanks.