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

Issue

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

Solution

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.

Answered By - KamilCuk

Answer Checked By - Mary Flores (WPSolving Volunteer)