I'm writing a small shell script that needs to reverse the lines of a text file. Is there a standard filter command to do this sort of thing?
My specific application is that I'm getting a list of Git commit identifiers, and I want to process them in reverse order:
git log --pretty=oneline work...master | grep -v DEBUG: | cut -d' ' -f1 | reverse
The best I've come up with is to implement reverse like this:
... | cat -b | sort -rn | cut -f2-
This uses cat to number every line, then sort to sort them in descending numeric order (which ends up reversing the whole file), then cut to remove the unneeded line number.
The above works for my application, but may fail in the general case because cat -b only numbers nonblank lines.
Is there a better, more general way to do this?
In GNU coreutils, there's tac(1)
There is a standard command for your purpose:
tail -r file.txt
prints the lines of file.txt in reverse order!
Answer is not 42 but tac.
Edit: Slower but more memory consuming using sed
sed 'x;1!H;$!d;x'
and even longer
perl -e'print reverse<>'
cat -b only numbers nonblank lines"
If that's the only issue you want to avoid, then why not use "cat -n" to number all the lines?
: "@(#)$Id: reverse.sh,v 1.2 1997/06/02 21:45:00 johnl Exp $"
#
# Reverse the order of the lines in each file
awk ' { printf("%d:%s\n", NR, $0);}' $* |
sort -t: +0nr -1 |
sed 's/^[0-9][0-9]*://'
Works like a charm for me...
Similar to the sed example above, using perl - maybe more memorable (depending on how your brain is wired):
perl -e 'print reverse <>'
In this case, just use --reverse:
$ git log --reverse --pretty=oneline work...master | grep -v DEBUG: | cut -d' ' -f1
rev <name of your text file.txt>
You can even do this:
echo <whatever you want to type>|rev
awk '{a[i++]=$0}END{for(;i-->0;)print a[i]}'
More faster than sed and compatible for embed devices like openwrt.
来源：https://stackoverflow.com/questions/418280/how-to-reverse-lines-of-a-text-file