Question

From the documentation of sed:

sed maintains two data buffers: the active pattern space, and the
auxiliary hold space. Both are initially empty.

I initially think the value of pattern space and hold space is null (nothing). But from the following example, it seems that the initially value of them is a single newline character (\n).

[root@localhost ~]# cat e.txt 
aa
bb
cc
dd
[root@localhost ~]# cat e.txt | sed -r '/c/{x;p;x}'
aa
bb

cc
dd
[root@localhost ~]#

Is my understanding right?
Thanks.

Answer 1

I think the answer is that the p command, like the default print action, is actually adding a newline to the end of the empty pattern space. This is based on this little snippet from the GNU sed documentation (just below that bit you quote, by the way):

sed operates by performing the following cycle on each line of input: first, sed reads one line from the input stream, removes any trailing newline, and places it in the pattern space.

… blah, blah blah …

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.

In other words, the line being held in the pattern (and hold) space does not have the trailing newline – the aa line is held as aa rather than aa<newline>.

Of course, the hold space may still contain multiple lines but that just means that executing the H command on the first two lines of your file will give you a hold space containing aa<newline>bb, not aa<newline>bb<newline>.