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I want to compare particular line of two text file and update one of the file if they are not same.

Shell_Magic
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    "Append"? Or "update"? What do the files look like? What have you tried? – Benjamin W. Feb 01 '16 at 17:05
  • Please see this for information on how to better ask a question http://stackoverflow.com/help/mcve – Adam B Feb 01 '16 at 23:43
  • I want to compare the root device details(specifically not the whole file ) of one vfstab file with other and update the later if the details are not same – Shell_Magic Feb 02 '16 at 10:26

1 Answers1

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Updating a line in a text file is technically not possible (unless the replacing line is of exactly the same length). You have to create a new file, which you can, in the end, move to the old one.

From your tags, I assume that you are looking for a shell solution, which is maybe not a good idea. It's probably more convenient to do it in, for instance, Perl or Ruby or Python.

One possibility is to use the commands head and tail, which allow you to dissect a file into parts. You can split your file into three parts: The part before the line in question, the line itself, and the lines which come after.

Another possibility is to use a loop and the read command of the shell to process a file line by line, like this:

while read line           
do           
  ... # Decide here, whether to write $line or the replacement line          
done <your_file
user1934428
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  • Hi thanks for quick response !! How to replace the line if they are of same length – Shell_Magic Feb 02 '16 at 11:00
  • If you use zsh as a shell, have a look at the commands `sysopen`,`sysread` `sysseek` and `syswrite` in the *zshall* man-page. However, in this case a **real** programming language would be easier to use. – user1934428 Feb 02 '16 at 12:23