How to maintain only four lines in my file in shell scripting?

Question

Input file:

01.05.2016 87893938
02.05.2016 35435345  
03.05.2016 35435345  
04.05.2016 12345678

I want to keep only four lines in my file. that means if date 05.05.2016 6905698 is inserted then first line of my file will be removed. I only want to keep last four days data in my file.

Output file:

02.05.2016 35435345  
03.05.2016 35435345  
04.05.2016 12345678  
05.05.2016 89459678

What process will add lines to the end of your file? (How will you know that the file has been updated, or - how often do you want to check?) — Jeff Schaller, May 05 '16 at 11:06
just a note that '6905698' does not show up in your sample output file... — Jeff Schaller, May 05 '16 at 11:12

score 2 · Accepted Answer · edited Apr 13 '17 at 12:36

2

cp -p file file.orig && \
tail -n 4 file.orig > file && \
rm file.orig

This will copy the original file to a backup copy, retrieve only the last 4 lines from the backup copy, put those 4 lines into the original file name, then remove the backup copy.

edited Apr 13 '17 at 12:36

Community

1

answered May 05 '16 at 11:10

Jeff Schaller

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`tail -n4 file > file.orig && cat file.orig > file && rm -f file.orig` will be more efficient for large files. It also fails "safely" if someone tried `mkdir file.orig` first. – roaima May 05 '16 at 13:25
true! it could also be improved with `mktemp` – Jeff Schaller May 05 '16 at 13:28
cat your_filename | tail -n 4 > newfilename,,,,writes the last 4 lines to your new file, put this in a script and your can call the script as per your convenience ! – RajuBhai May 05 '16 at 19:16

How to maintain only four lines in my file in shell scripting?

1 Answers1