Linux – Command to insert/prefix characters in lines after searching for a phrase from a file

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Similar to a question posted by me here earlier, am looking for a help in Linux command that performs/does the following that is subtle different from my original question:

Searches for a particular word/phrase case-insensitively in a given file, and then insert/prefix with the character -- (in this case, this is an SQL comment) in the immediate next 'n' lines including the line where the word/phrase was matched in the given file.

EXAMPLE:
If I try to search for the phrase "CREATE FUNCTION plpgsql_call_handler" case-insensitively and if it matches at line no.102644, then I expect to insert/prefix the character -- in line no.102644 and the immediate next 2 lines in the given file. In this case, I expect to insert/prefix line nos. 102644,102645,102646 with the character --.

Example file:

102644 CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler
102645     AS '/usr/lib/pgsql/plpgsql.so', 'plpgsql_call_handler'
102646     LANGUAGE c;

I expect/want it to be changed into (in this case, it is an SQL comment):

102644 -- CREATE FUNCTION plpgsql_call_handler() RETURNS language_handler
102645 --    AS '/usr/lib/pgsql/plpgsql.so', 'plpgsql_call_handler'
102646 --    LANGUAGE c;

Best Answer

This is how you'd do it in-place with sed:

sed -r -i -e '/CREATE FUNCTION plpgsql_call_handler/I,+2 s/^/-- /'

(May require GNU sed.)

Replace 2 with the number of lines you want (or use any other sed address forms).

This is much simpler than that perl monstrosity. Also, perl could do it in place by using the -i as well:

perl -i -n -e horrible_perl_code /path/to/file

In both cases if you wish to keep a backup change -i to -i.bak.

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1|2||4

$ echo "1|2|  |4" | awk  -F'|' '$3 ~ /^[ \t]*$/   {print $0}'

1|2|  |4

$ echo "1|2|  3|4" | awk  -F'|' '$3 ~ /^[ \t]*$/   {print $0}'

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sed -n '10000000,10000020p' filename

You might be able to speed that up a little like this:

sed -n '10000000,10000020p; 10000021q' filename

In those commands, the option -n causes sed to "suppress automatic printing of pattern space". The p command "print[s] the current pattern space" and the q command "Immediately quit[s] the sed script without processing any more input..." The quotes are from the sed man page.

By the way, your command

tail -n 10000000 filename | head 10

starts at the ten millionth line from the end of the file, while your "middle" command would seem to start at the ten millionth from the beginning which would be equivalent to:

head -n 10000010 filename | tail 10

The problem is that for unsorted files with variable length lines any process is going to have to go through the file counting newlines. There's no way to shortcut that.

If, however, the file is sorted (a log file with timestamps, for example) or has fixed length lines, then you can seek into the file based on a byte position. In the log file example, you could do a binary search for a range of times as my Python script here* does. In the case of the fixed record length file, it's really easy. You just seek linelength * linecount characters into the file.

^{* I keep meaning to post yet another update to that script. Maybe I'll get around to it one of these days.}

Best Answer

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