asked on

Linux: Loop through file BACKWARDS

This loops through a file starting at the beginning:

#!/bin/bash
while read LINE
do
      if [ $(( i )) -lt 5 ];
      then 
       i=$((i+1))
       echo $LINE
      else break
      fi
done <"$1"

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How can I loop through the file starting at the end and going backward one line at a time? Can -tail be used for this? I do not want to read only the last 5 lines because I want to use a more complicated condition than the one above. Instead, I want to read the file backwards, starting at the last line then moving up one line at a time.

duncanb7

You want to display the file in reverse order , Right ?

if so , using tac command
tac youfile.txt>output,txt

some Linux distrubtion, tail -r yourfile.txt that also works

hankknight

ASKER

How can I do this with the code I posted? This does NOT work:

#!/bin/bash
while read LINE
do
      if [ $(( i )) -lt 5 ];
      then 
       i=$((i+1))
       echo $LINE
      else break
      fi
done < tac "$1"

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duncanb7

what command you input and what error is ?
Please send it out

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duncanb7

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hankknight

ASKER

That does what I want. Is there a way to do that without creating a temporary junk file?

duncanb7

I think it already asnwered your question
you can add rm -rf junk.txt in your script to delete the temp file .
creating a temp file is not harmful

duncanb7

Thanks

Have a nice day

Duncan

woolmilkporc

>> Is there a way to do that without creating a temporary junk file <<

Yes, and please see my last comments to your thread https://www.experts-exchange.com/questions/28295367/Linux-Count-lines-in-file-that-begin-with.html where I already provided this solution.

#!/bin/bash
while read LINE
do
if [ $(( i )) -lt 5 ];
then
i=$((i+1))
echo $LINE
else break
fi
done <<< $(tac $1)

duncanb7

woolmilkporc,

Yes, it is correct no need for creating temp file from your code.
I tried your code at my side, but the output of result is different from my result.

hankknight,

could you also try it if you have time ?

Duncan

hankknight

ASKER

<<< $(tac $1)
That does not seem to be working on my OS. I have posted a question about that here:
https://www.experts-exchange.com/questions/28297339/Linux-Process-file-in-reverse.html

woolmilkporc

To make this thread complete:

"$(tac $1)" must be enclosed in double quotes in order to keep the newline characters from being stripped off by the shell.

wmp