Next: , Up: sed Programs  3.1 How sed Works  sed maintains two
data buffers: the active pattern space, and the auxiliary hold
space. Both are initially empty.  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. Then commands are executed; each
command can have an address associated to it: addresses are a
kind of condition code, and a command is only executed if the
condition is verified before the command is to be executed.
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.1 Then the next cycle starts for the next input line.
Unless special commands (like ‘D’) are used, the pattern space
is deleted between two cycles. The hold space, on the other
hand, keeps its data between cycles (see commands ‘h’, ‘H’, ‘x’,
‘g’, ‘G’ to move data betwee