How to reverse a File

[George C. Abraham_2]

hey Admins,

How can i display a file upside down.. Ie the last line fist and fist line last...

any command for this?

Thanks,
George

[A. Clay Stephenson · Accepted solution]

There are number of ways to do this (awk, Perl, ...) but here's a Perl method:

cat yourfile | perl -e '@x = ; while ($y = pop @x) {print $y;}' > newfile

Regards, Clay

[H.Merijn Brand (procura]

shorter:

# perl -e'print reverse<>' file

There is a faster way. From GNU file utils, there is a command called 'tac' (reverse of cat)

http://hpux.connect.org.uk/hppd/hpux/Gnu/coreutils-5.0.91/

Enjoy, Have FUN! H.Merijn

[f. halili]

HERE's my sample file:

# cat testfile
aaa
bbb
ccc
ddd

HERES' the command I use:

# cat testfile | sed '1!G;h;$!d'
ddd
ccc
bbb
aaa

- FHALILI

[Laurent Menase]

Hi Georges,

awk ' BEGIN { i=0 }
{ printf "%d %s\n",i,$0 }'
Happy new year
Laurent

[john korterman]

Hi,
with smaller files you can use an ed command in a script. This works directly in the file itself, e.g.:

ed $1 <g/^/m0
w
EOF

try running the above with you file as $1.

regards,
John K.

[Chris Watkins_1]

Thanks to the above posters, for new ideas.
I would have probably done it this way:


[start script code]
#!/bin/sh

echo "Enter the filename to reverse --> \c"
read filename
echo "Enter the desired new filename --> \c"
read newfilename

lines=`cat ${filename} |wc -l`
while [ ${lines} -gt 0 ]
do
echo `head -${lines} ${filename} |tail -n 1` >>${newfilename}
(( lines=${lines} - 1 ))
done
echo Finished!

[end script code]


So many more elegant (and short!) ways to do this.
Note to self... must learn perl.

[George C. Abraham_2]

Thanks to Everyone...

How many ways to do a single thing... Thank god for the ITRC..

Happy new year to all

[KapilRaj]

There are two more ways ....

1 : cat filename |awk '{ s[NR] = $0} END { for ( i = NR ; i >= 1 ; i-- ) {print s[i] } }'

2: KEEP THE MONITOR (Display Unit) REVERSE !!

Happy New Year

Kaps

[John Poff]

Hi,

Those are all fine software methods for reversing the file. You can also do it using hardware.

1. Turn your monitor upside down.

2. cat filename

;)

JP

[Alex Ostapenko]

...and another method using "pr" command:

cat testfile | pr -t -n | sort -nrk1,1 | cut -f2-

- where pr "-t" suppresses page title, "-n" adds line numbers
- where sort "-k1,1" specifies first field for sorting, "-n" specifies numeric sort, and "-r" specifies in reverse
- where cut "-f2-" means cut 2nd field and on

=:-) Alex

[Elmar P. Kolkman]

I just thought of the same solution:
grep -n '^.' | sort -rn | cut -d: -f2-

The same way, but using some different commands...

As for the up-side-down monitor: you'll have to alter your character set to up-side-down characters and do a rev instead of cat, otherwise the text is a bit hard to read... (rev prints the line right-to-left instead of left-to-right, for the ones who don't know the command)... ;-)

[H.Merijn Brand (procura]

Elmar, that won't work :)

grepping for ^. will skip the empty lines in a file.

Enjoy, Have FUN! H.Merijn [ Who wonders about the point assignment in this thread. I just cannot grasp what the value diff per post is for the reader ... ]

[Elmar P. Kolkman]

Ok. You're right. I looked further and found a command I forgot... nl will do the trick. And it is even shorter...

nl | sort -rn | cut -f2-

Happy new year and the best wishes for 2004 (and beyond, of course).

[Graham Cameron_1]

Elmar

You need to add the "-ba" switches to nl, else again it loses blanks.


George

This came up a week earier, most of the solutions are similar to above.
http://forums1.itrc.hp.com/service/forums/questionanswer.do?threadId=9248

-- Graham