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How to delete all lines before a particular pattern when the pattern is defined in a variable?

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Top Forums Shell Programming and Scripting How to delete all lines before a particular pattern when the pattern is defined in a variable?
# 15  
Old 09-26-2017
May not be the most efficient:
Code:
tac file | sed -n "/$var/q; p;" | tac
Line 4
Line 5

Last edited by RudiC; 09-26-2017 at 06:04 AM.. Reason: overread the request to use a variable.
This User Gave Thanks to RudiC For This Post:
drl
# 16  
Old 09-26-2017
Just for fun (with gnu awk) :
Code:
$ XX="Line 1"
$ awk  'BEGIN{X=0;ARGV[ARGC++]=ARGV[ARGC-1]}FNR==NR && /'"$XX"'/ {X=FNR} FNR !=NR && FNR > X' file
Line 4
Line 5

Work as Corona688's idea.
Regards.
# 17  
Old 09-26-2017
As a very different approach, how about:-
Code:
Req_Line="^Line 1 "                   # Note the leading carat to anchor to begginning of line (if that's what you want) and the trailing space to avoid matching Line 11
                                      # This is used as an Extended Regular Expression, so you can adjust this to suit your needs

IFS=":" read lastline rest < <(grep -En "$Req_Line" filename |tail -1)
((lastline=$lastline+1))

sed -n "$lastline,\$p" filename

For a large file, this has the overhead of perhaps reading the file twice, so you would have to trial it for performance.

It would be sensible to add some error checking, such as what to do if the expression does not match. As it is, this would display the whole file, which might not be what you want.



I hope that this helps, or at least gives an alternate.
Robin
This User Gave Thanks to rbatte1 For This Post:
drl
# 18  
Old 09-26-2017
Hi.
Quote:
Originally Posted by RudiC
May not be the most efficient:
Code:
tac file | sed -n "/$var/q; p;" | tac
Line 4
Line 5

Perhaps not the most efficient, but, in the absence of requirements from OP, it certainly is simple. I like simple ... cheers, drl
# 19  
Old 09-26-2017
Thanks all.

Disedorgue's 'just for fun' solution worked for me with nawk.
# 20  
Old 09-26-2017
This approach may be more efficient than others for large files and the pattern found towards the end of the file, as tac opens the file from the end (output of strace):
Code:
.
.
.
open("TMPFILE", O_RDONLY)               = 3
lseek(3, 0, SEEK_END)                   = 37790
.
.
.

and, if sed found the pattern and exits, it quits due to a broken pipe
Code:
.
.
.
write(1, "sr/bin/gpgsplit\n26696\t/usr/bin/g"..., 4096) = -1 EPIPE (Broken pipe)
--- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=14645, si_uid=1000} ---

NOT reading the entire input file.
# 21  
Old 09-26-2017
Hi.
Quote:
Originally Posted by RudiC
This approach may be more efficient than others for large files and the pattern found towards the end of the file, as tac opens the file from the end (output of strace):
Code:
.
.
.
open("TMPFILE", O_RDONLY)               = 3
lseek(3, 0, SEEK_END)                   = 37790
.
.
.

and, if sed found the pattern and exits, it quits due to a broken pipe
Code:
.
.
.
write(1, "sr/bin/gpgsplit\n26696\t/usr/bin/g"..., 4096) = -1 EPIPE (Broken pipe)
--- SIGPIPE {si_signo=SIGPIPE, si_code=SI_USER, si_pid=14645, si_uid=1000} ---

NOT reading the entire input file.
I took a quick look through tac.c\src - coreutils.git - GNU coreutils

The speed-up by seeking to end may be offset somewhat by:
1) In this situation tac is used twice,
2) There is a lot of manipulation going on, including RE matches in some cases,

If we are curious we'd benchmark it, of course Smilie

I still like the simplicity, however, regardless of what happens under the hood ... cheers, drl
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