Re: Prepending text to the beginning of a file

[Robert E. Griffith]

I suspect that the fundamental problem is that its less efficient to to 
shift the contents of a file to make room to prepend more data in the 
front than using a temp file so to my knowledge that shifting algorithm 
does not exist in any standard tool.

I think that the best you can do is either use a temporary file or, if 
you know the file is small enough, read it entirely into memory and then 
write the new data and then the old back out to the same file.

--BobG

On 6/16/22 16:48, Akbarkhon Variskhanov wrote:
> Yes, there is sed and ed that can do just that.
>
> But I was just wondering if it can be done with the shell's built-ins and
> maybe `cat`.
>
> We can concatenate files in the order they're given and output that to
> another file. Working as intended!
>
> But what if some data is stored in a variable and we want to prepend it to
> the beginning of a file without creating temporary ones? Sort of like a >>
> but before the buffer.
>
> I tried something like
> exec 3<>somefile
> cat - <&3 >&3 <<eof
> $(var)
> eof
>
> And it didn't truncate the file, but also overwrote the first couple of
> lines. To be honest, I can't quite grasp the <> operator's purpose.
>
> Then I stumbled upon this neat hack: https://stackoverflow.com/a/54715861
> echo "$(echo -n 'hello'; cat filename)" > filename
> But that only works if the file is small and fits into echo's arg list.
>
> Please, share your ideas and thoughts. I'm very curious to read them. Try
> not to resort to editors. :)
>
> Thank you.