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Full Discussion: Want to remove all lines but not latest 50 lines from a file
Top Forums UNIX for Dummies Questions & Answers Want to remove all lines but not latest 50 lines from a file Post 302868277 by drl on Sunday 27th of October 2013 12:30:28 AM
Old 10-27-2013
Hi.

For the version of tail that I used, as noted above:
Code:
       If the first character of N (the number of bytes or lines) is a `+',
       print beginning with the Nth item from the start of each file,
       otherwise, print the last N items in the file.  N may have a multiplier
       suffix: b 512, kB 1000, K 1024, MB 1000*1000, M 1024*1024, GB
       1000*1000*1000, G 1024*1024*1024, and so on for T, P, E, Z, Y.

so it looks like the GNU folks have made provisions for some large quantities of data. I have not looked at the source, however.

For the tests that I ran, I numbered all the lines, counted the final tail (50,000 of 14,000,000), and printed the first and last line of the output file. All seemed correct and intact. The system tail and the version in perl at http://cpansearch.perl.org/src/CWEST/ppt-0.14/bin/tail both return essentially immediately when asked for the last 5 lines of the 14M line file. I noticed some seeks in the perl version, so I'd guess that both those versions are using such performance techniques.

I don't disagree that some utilities will produce wrong results without warning, but that's not the right thing to do, and those systems should be avoided if possible, and especially if one will normally try to solve problems such as we are addressing here. Another possibility is to use the perl version, although I didn't rigorously test it (and the calling sequence is slightly different, but easily changed).

I might worry more about the shell capacity, although I didn't have any trouble, and I recall a test I did on an early version of SunOS, and was pleasantly surprised at its capacity for variable storage. Of course, the MMV for specific cases.

Best wishes ... cheers, drl
Last edited by drl; 10-27-2013 at 09:48 AM.. Reason: Minor typo.
 

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