Problem with Bash output redirection [duplicate]

Question

This question already has answers here: Why doesnt "tail" work to truncate log files? (6 answers) Closed 5 months ago. 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 |tail -1 > file.txt $cat file.txt Why is it so? JavaScript questions and answers, JavaScript questions pdf, JavaScript question bank, JavaScript questions and answers pdf, mcq on JavaScript pdf, JavaScript questions and solutions, JavaScript mcq Test , Interview JavaScript questions, JavaScript Questions for Interview, JavaScript MCQ (Multiple Choice Questions)

Answer 1

Redirecting from a file through a pipeline back to the same file is unsafe; if file.txt is overwritten by the shell when setting up the last stage of the pipeline before tail starts reading off the first stage, you end up with empty output. Do the following instead: tail -1 file.txt >file.txt.new && mv file.txt.new file.txt ...well, actually, don't do that in production code; particularly if you're in a security-sensitive environment and running as root, the following is more appropriate: tempfile="$(mktemp file.txt.XXXXXX)" chown --reference=file.txt -- "$tempfile" chmod --reference=file.txt -- "$tempfile" tail -1 file.txt >"$tempfile" && mv -- "$tempfile" file.txt Another approach (avoiding temporary files, unless <<< implicitly creates them on your platform) is the following: lastline="$(tail -1 file.txt)"; cat >file.txt <<<"$lastline" (The above implementation is bash-specific, but works in cases where echo does not -- such as when the last line contains "--version", for instance). Finally, one can use sponge from moreutils: tail -1 file.txt | sponge file.txt