Is there a linux command or some way to look at logs from bottom up rather than from top towards bottom. I know about tail -n <number of lines>, but is there something that I can actually scroll and go from bottom up?
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Some systems have tac, which is a whimsically-named backward cat. Without that, you can still do something like
awk '{print NR ":" $0}' $file | sort -t: -k 1nr,1 | sed 's/^[0-9][0-9]*://'
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8
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cat -nis likeawk '{print NR,$0}', so slightly more lightweight. – glenn jackman Mar 16 '11 at 16:39 -
1I was assuming that systems without
tacmight also lack the BSD-derived version ofcat(USG-flavoredcatdidn't have-v,-n, etc. I think something triggered my "old ****".) – geekosaur Mar 16 '11 at 16:47
I think less +G is what you're looking for — it'll load up the file at the end, and you can use the up arrow (or k) to scroll back. Within the viewer, g will take you to the top of the file and G will take you back to the end. And F will scroll to the end and try to keep reading, like tail -f.
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n1 +1 :) If you'll be so kind: Can you explain how to search the file from bottom up when using less with +G flag? – Oleg Belousov Jan 22 '14 at 17:20
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2
The tac command is like a reverse "cat", if that's what you're after.
If you just want to view a file by starting at the bottom, why don't you use an editor or "less"?
answered Mar 7 '11 at 16:59
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1Two problems I see with using editors are paginators: a) not all editors behave properly to network disconnects, meaning that the editor stays around even when the shell sends SIGHUP; b) many editors touch the directory (by creating a swap/temp file), which is disadvantageous if you want to keep the mtime of the directory stable. – Arcege Mar 7 '11 at 17:47
You can run less and then use M-> (that's the meta key, usually alt, and the '>' at the same time) to go to the bottom of the file.
less supports scrolling.
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1The “go to last line” function is also usually bound to
>(without Meta or ESC) andG. – Chris Johnsen Mar 8 '11 at 6:13 -
@Chris, I didn't realize Meta wasn't required in less. But that is was you would use in Emacs. – Carlos Rendon Mar 8 '11 at 17:37
If you are writing a nagios check the perl module File::ReadBackwards is useful
I use this script to read a file from bottom upward
#!/bin/bash
echo -n elpmas.file # resulting file
ctr_line=0
while read line; do
let ctr_line++
tail -n $ctr_line | head -n 1 >> elpmas.file
done <sample.file
if sample.file contains
1
2
3
the result elpmas.file will be
3
2
1
Piping into tac also works, eg:
tail -n 3 /var/log/dmesg | tac
Soluction: Combine tac with less
tac $@ | less
Install
sudo bash -c 'echo "tac \$@ | less" > /usr/local/bin/tacless'
sudo chmod +x /usr/local/bin/tacless
Usage
tacless /var/log/auth.log
