I wanted to tail the last 100 lines of a file to the same file, but the command
tail -n 100 file > file doesn't work, I assume because the stdout gets written to the file 'live', before everything was read from the original file.
Is there some way to pipe the output to something, that then keeps it until all 100 lines are there, and then outputs it to the file? Or just another way to shorten the file in this way?
sponge from moreutils is good for this. It will:
soak up standard input and write to a file
You use it like this:
tail -n 100 file | sponge file
to get exactly the effect you want.
Of course, the moment I finally ask the question on SE, the answer comes to me. I think less does what I need, so I just write:
tail -n 100 file | less > file
>) that truncates the file, rather than the command you run - this won't (or at least shouldn't!) work. The shell opens and empties the destination before it even starts the commands you give. See the (slightly obtuse) description of output redirection in POSIX.
– Michael Homer
Jul 31 '14 at 09:28
In:
tail -n 100 file > file
The shell forks a process, opens file for writing in it, with truncation (that is making it an empty file) and then executes tail in that process. To open file without truncation, you can use the <> redirection operator instead:
tail -n 100 file 1<> file
The problem though is that there will be no truncation at all. That is, file will be overridden with its last 100 lines, but after those 100 lines, what was originally in the file will still be there. So you'd need to call another command to do the truncating after tail has finished.
{ tail -n 100 file; perl -e 'truncate STDOUT, tell STDOUT'; } 1<> file
You can do it this way:
printf '%s\n' "$(tail -n 100 file)" > file
