grep two lines from a file


 
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# 1  
grep two lines from a file

Sample File
Code:
abc
xyz
def
abc
ggh
abc
xyz

I just created a sample file above to show what I need. I need to grep two lines. e.g abc and xyz(only if they are one after the other) so output would be
Code:
abc
xyz
abc
xyz

(note abc followed by ggh line would not come out in the output). I may have to extend the same for three line grep. Would appreciate if some one can help .

Thanks

Moderator's Comments:
Mod Comment Please use next time code tags for your code and data

Last edited by vbe; 01-19-2012 at 10:54 AM..
# 2  
sed could do it:
Code:
sed -n '/abc/{N;/xyz/p;}' infile

# 3  
Thank you very much for the quick response. I am sorry, my bad, I have lot of files and want to know the file name that has this two consecutive pattern. Can sed be used for the same as well ?

Thanks
# 4  
look at the -e option of grep command, in the man pages
# 5  
You could use awk for that, for instance
Code:
awk '/abc/ && getline && /xyz/{print FILENAME}' infile

This prints the filename for every occurrence of the two-line pattern.
You could run it through sort -u to get a list of filenames:
Code:
awk '/abc/ && getline && /xyz/{print FILENAME}' infile | sort -u

# 6  
grep has no memory, variables, conditionals; it's not really a language. It only matches lines. It can't do logic like "if one line, do something to another line".

That's just the sort of thing awk was made for, though.
Code:
awk '/abc/{C=-2;}; ((++C)==0) && /xyz/ && (LF!=FILENAME) { print LF=FILENAME; }' *.txt

Whenever a line matches the regex /abc/, it sets the variable C to -2.

It increments the value of C every single line, including lines matching /abc/, so matching /abc/ effectively sets C to -1.

Whenever the value of C(after increment) is 0, and the line matches /xyz/, and the special FILENAME variable isn't the same value it was last time the program printed, it prints the filename. The value of C would have to be -1 for (++C)==0 to be true, so this only ever happens when the last line matched /abc/ and the current line matches /xyz/.

---------- Post updated at 09:23 AM ---------- Previous update was at 09:21 AM ----------

Quote:
Originally Posted by Scrutinizer
You could use awk for that, for instance
Code:
awk '/abc/ && getline && /xyz/{print FILENAME}' infile

This prints the filename for every occurrence of the two-line pattern.
You could run it through sort -u to get a list of filenames:
Code:
awk '/abc/ && getline && /xyz/{print FILENAME}' infile | sort -u

Neat idea, and much simpler than mine, but I think I see a minor flaw in that; what happens when you read the first line from one file, and the second line from another?

Come to think of it, that could be an error condition in mine too.

---------- Post updated at 09:30 AM ---------- Previous update was at 09:23 AM ----------

This should be more resilient:

Code:
$ cat 2line.awk

# If we've switched to a new file, ignore matches from old file
LF!=FILENAME { C=0; LF=FILENAME }

# If the line matches /abc/, set C=-2.  This will immediately become -1
# from the (++C) below it.
/abc/ { C=-2; }

# If ((++C)==0), i.e. C was -1 before increment, and the line matches /xyz/,
# and we haven't printed this filename before, print this filename.
((++C)==0) && /xyz/ && F!=FILENAME      {       print F=FILENAME;       }

$ cat data

abc
xyz
qwer
abc
xyz
qwer

$ cat data2

abc
qwer
xyz

$ awk -f 2line.awk data*
data

$

These 2 Users Gave Thanks to Corona688 For This Post:
# 7  
Quote:
Originally Posted by Corona688
Neat idea, and much simpler than mine, but I think I see a minor flaw in that; what happens when you read the first line from one file, and the second line from another?
[..]
Hey, good catch !!

This should fix it, no?
Code:
awk '/abc/ && getline && /xyz/ && FNR>1 {print FILENAME}' *.txt | sort -u

These 2 Users Gave Thanks to Scrutinizer For This Post:
 

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