Problem with Bash output redirection

Question

I was trying to remove all the lines of a file except the last line but the following command did not work, although file.txt is not empty.

$cat file.txt |ta         


        

Answer 1

Before 'cat' gets executed, Bash has already opened 'file.txt' for writing, clearing out its contents.

In general, don't write to files you're reading from in the same statement. This can be worked around by writing to a different file, as above:

$cat file.txt | tail -1 >anotherfile.txt
$mv anotherfile.txt file.txt

or by using a utility like sponge from moreutils:

$cat file.txt | tail -1 | sponge file.txt

This works because sponge waits until its input stream has ended before opening its output file.