Problem with Bash output redirection [duplicate]

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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

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