puzzle

[U.SivaKumar_2]

Hi,
I have three files a.txt b.txt c.txt and a shell script run.sh .
The file c.txt is empty. My requirement - When
I run run.sh once the cotents of file a.txt should be in c.txt. And If run run.sh second time the contents of b.txt should be in c.txt.
The order of file content of c.txt should be in round robin (a.txt->c.txt,b.txt->c.txt,a.txt->c.txt.....so on) every time I execute the run.sh.

So what can be the shell script for run.sh ?.


regards,
U.SivaKumar

[John Palmer · Accepted solution]

Something like this...

#!/usr/bin/sh

if diff b.txt c.txt 2>/dev/null
then cp a.txt c.txt
else cp b.txt c.txt
fi

exit

If b.txt is the same as c.txt then a.txt->c.txt otherwise b.txt->c.txt.

Regards,
John

[U.SivaKumar_2]

Hi,
Well done Sir.
What happens if my a.txt and b.txt are of same size ?.

regards,
U.SivaKumar

[Donald Kok]

Hi,
I would maintain a checkfile which only holds the last copied file. Then in the script you can check on this content.

Another option is to change a bit on the c.txt file. Like the sticky bit or the x-bit. And in the script check on this bit.

Good luck
Donald

[Dietmar Konermann]

Hi!

You need to store the laste state somewhere... or you could use diff to get the last cp state (would be inefficient with large files!).

Idea: hold a state file.

#!/usr/bin/sh

if [[ "$(cat nextfile 2>/dev/null)" = b.txt ]]; then
cp b.txt c.txt
echo a.txt >nextfile
else
cp a.txt c.txt
echo b.txt >nextfile
fi

[Simon Abbott]

Hello,

For your script to know which file to write from, it will have to know which file was written from last time. I would set this in a variable or a file. If the script needs to work for differnent users or sessions it would have to be in a file.

Therefore I'd do somthing like this:

if [ ! -f last_written_from ] ; then
echo b > last_written_from
fi

if [ "$(cat last_written_from)" = "b" ] ; then
cp a.txt c.txt
echo a > last_written_from
else
cp b.txt c.txt
echo b > last_written_from
fi

[Carlos Fernandez Riera]

if [ -s c.txt ]; then
echo "CHANGING"
file=$(ls -1rt a.txt b.txt|head -1)
cat $file >c.txt
touch $file
else
cat a.txt >c.txt
touch a.txt
fi
sleep 1 #

[H.Merijn Brand (procura]

1. cmp is faster than diff
2. use MD5 or sum + timestamp to identify which file is in c.txt
3. use tar or cpio instead of cp to maintan access and modification times, so you can use these to check
4. use a stampfile in which you state what the next file will be (or what the last file was)
5. use symlinks :)

[Trond Haugen]

King Johns answer is short and beautiful.
diff compares the contents of the files.

Regards,
Trond

[Jordan Bean]

If it isn't necessary to copy the contents of the files, why not alternate links?

#!/usr/bin/sh
if [ -e a.txt -a -e b.txt ]
then
if [ a.txt -ef c.txt ]
then
ln -f b.txt c.txt
else
ln -f a.txt c.txt
fi
fi

[Yogeeraj_1]

hi,


if diff b.txt c.txt 2>/dev/null
then cp a.txt c.txt
else cp b.txt c.txt
fi


this is beautiful scripting indeed ;)

Thanks King John.

Best Regards
Yogeeraj

[John Palmer]

Actually, on further reflection, using cmp -s is more efficient. I sort of assumed from the original post that files called ?.txt weren't likely to be huge.

if cmp -s b.txt c.txt
then cp a.txt c.txt
else cp b.txt c.txt
fi

Linking the files may be OK but if there is any intention to alter the contents of c.txt then the linked original file gets changed as well.

Regards,
John

[H.Merijn Brand (procura]

Don't check content at all :)

l1:/u/usr/merijn 126 > perl -we '$a=(readlink($c="c.txt"))=~/^a/?"b":"a";unlink$c;symlink"$a.txt",$c' ; ll *.txt
1363 -rw-rw-rw- 1 merijn softwr 64 Aug 14 10:48 a.txt
1420 -rw-rw-rw- 1 merijn softwr 64 Aug 14 10:49 b.txt
1423 lrwxrwxrwx 1 merijn softwr 5 Aug 14 11:10 c.txt -> a.txt
l1:/u/usr/merijn 127 > perl -we '$a=(readlink($c="c.txt"))=~/^a/?"b":"a";unlink$c;symlink"$a.txt",$c' ; ll *.txt
1363 -rw-rw-rw- 1 merijn softwr 64 Aug 14 10:48 a.txt
1420 -rw-rw-rw- 1 merijn softwr 64 Aug 14 10:49 b.txt
1423 lrwxrwxrwx 1 merijn softwr 5 Aug 14 11:10 c.txt -> b.txt
l1:/u/usr/merijn 128 > perl -we '$a=(readlink($c="c.txt"))=~/^a/?"b":"a";unlink$c;symlink"$a.txt",$c' ; ll *.txt
1363 -rw-rw-rw- 1 merijn softwr 64 Aug 14 10:48 a.txt
1420 -rw-rw-rw- 1 merijn softwr 64 Aug 14 10:49 b.txt
1423 lrwxrwxrwx 1 merijn softwr 5 Aug 14 11:10 c.txt -> a.txt
l1:/u/usr/merijn 129 >