Sed to operate on line index based matching pattern and do operations

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How sed is working on line match and occurrence based match patterns like below, where deleting or replacing a particular line ..

Below are some test, i could not understand why it only works on first matched and not on second.

Initial 10 line of the log file:

$ head smbd.log.old
[2020/02/16 03:53:55.112796,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 52391 -- ignoring
[2020/02/16 04:08:55.126564,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 52891 -- ignoring
[2020/02/16 04:23:55.209096,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 53648 -- ignoring
[2020/02/16 04:38:55.301910,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 54165 -- ignoring
[2020/02/16 04:53:55.382553,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 54920 -- ignoring

Changing the first matched string to "Change_to_new":

$ sed  '0,/2020/s//Change_to_new /' smbd.log.old | head
[Change_to_new /02/16 03:53:55.112796,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 52391 -- ignoring
[2020/02/16 04:08:55.126564,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 52891 -- ignoring
[2020/02/16 04:23:55.209096,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 53648 -- ignoring
[2020/02/16 04:38:55.301910,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 54165 -- ignoring
[2020/02/16 04:53:55.382553,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 54920 -- ignoring

deleting the First match string:

$ sed  '0,/2020/{//d;}' smbd.log.old | head
  Could not find child 52391 -- ignoring
[2020/02/16 04:08:55.126564,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 52891 -- ignoring
[2020/02/16 04:23:55.209096,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 53648 -- ignoring
[2020/02/16 04:38:55.301910,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 54165 -- ignoring
[2020/02/16 04:53:55.382553,  2] ../source3/smbd/server.c:576(remove_child_pid)
  Could not find child 54920 -- ignoring
[2020/02/16 05:08:55.427916,  2] ../source3/smbd/server.c:576(remove_child_pid)

Why it did not work for Second match.

$ sed  '1,/2020/{//d;}' smbd.log.old | head
sed: -e expression #1, char 0: no previous regular expression
sed linux
1 Answers

Line address 0 is special. From GNU sed manual:

0,/regexp/

A line number of 0 can be used in an address specification like 0,/regexp/ so that sed will try to match regexp in the first input line too.....

Because sed uses /regexp/ on the first line of input in 0,/regexp/, the 0,/regexp/{//d} works fine. But 1,/regexp/{//d} does not work, because regexp is unused on first line, so there were no previous matches.

user_90727530
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