How to copy the contents of a file to the end of the same file?-CodePudding

I need to copy the contents of a file to the end of the same file.

I wrote the following code.

#!/bin/bash
cat file.txt >> file1.txt
cat file1.txt >> file.txt
rm file1.txt

But it creates an additional file. How can this be done without creating an additional file?

CodePudding user response：

You could do something like:

 input=file1.txt
 dd bs="$(wc -c < "$input")" count=1 if="$input" >> "$input"

but....why? There's nothing wrong with using auxiliary files, and if you remove it shortly after it is created there is almost zero chance it will ever cause any actual disk operations. Just create the temp file and remove it. Note that this has an inherent race condition, since the computation of the file size (and wc -c is a terrible way to do that!) may produce a value that is wrong if any other process is mutating the file, but this issue is inherent in your problem description.

CodePudding user response：

You can create a variable that as the value of you file and put it to the end :)

CodePudding user response：

Or use sponge - that's exactly what it's there for.

cat file1.txt file1.txt | sponge file1.txt