Perl+deletion of multiple lines in a file

[Alain Yoshida]

Hi

Scenario
========
I have a file with the word root appearing randomly in it. Ie on line 7 , 13 , 27 etc.
Id like to delete the line containing "root" and 4 lines above it and 2 lines below it (in a script). Any ideas? If Perl can do it?
thks

[Dennis Handly]

For ex(1) you can do:
ex </root/-4,/root/+2d
wq
EOF

Unfortunately I don't know how to repeat that delete.

With awk, you can save the last 4 lines and and if you find "root", delete those, the current and the next two.
If not, you can write the oldest, and reread.

[Hein van den Heuvel]

For problems like those I like to set up a little circular 'lag buffer' using a MOD function on the line number.
Store the lines as $line[$. % 10]
Here I used 10, but any number larger then the look-back range will do.

Now loop through the data, filling the buffer with the current line but deciding whether to print (or modify or whatever) 'lag' lines back.

In your example.
Let's create test data first:

# perl -le "for (1..40) { $_ = q(root) unless $_%11; print }" > test.txt
# cat test.txt

The script

# cat test.pl
while (<>) {
$line[$. % 10] = $_;
$skip = 7 if /root/;
print $line[($. - 4) % 10] unless ($skip-- > 0 or $.<5);
}
# deal with rest of lines in buffer.
for ($.+1 .. $.+4) {
print $line[($_ - 4) % 10] unless $skip-- > 0;
}

Now execute:

# perl test.pl test.txt

This will work for any size input file, at only 10 (or less) records are ever remembered.
An alternative is to suck in all lines, mark the special ones, and print all that are not marked. But now you need enough memory to hold all lines.

# cat test.pl
while (<>) {
$line[$.] = $_;
if (/root/) {
delete $line[$_] for ($.-4 .. $.);
<>;
<>;
}
}
print @line;

Enjoy,
Hein

[Dennis Handly]

Here is the awk solution:
awk '
BEGIN {
getline save[NR%4] # assume 4
getline save[NR%4]
getline save[NR%4]
getline save[NR%4]
}
/root/ {
getline # assume 2 to skip
getline
if (getline save[NR%4] != 1) {
exit
}
last=NR
if (getline save[(last+1)%4] != 1) {
print save[(last) % 4]
exit
}
last=NR
if (getline save[(last+1)%4] != 1) {
print save[(last-1) % 4]
print save[(last) % 4]
exit
}
last=NR
if (getline save[(last+1)%4] != 1) {
print save[(last-2) % 4]
print save[(last-1) % 4]
print save[(last) % 4]
exit
}
next
}
{
print save[NR % 4]
save[NR % 4] = $0
}
END {
for (i = NR-(4-1); i <= NR; ++i)
print save[(i) % 4]
} ' file