How to Prevent a Bash Script From Overwriting Files With noclobber

“Clobbering” a file has a very specific meaning in Bash — it’s when you use the > redirect operator to overwrite an existing regular file. Setting noclobber results in a non–zero exit code when a script tries to clobber a file, leaving the file untouched. See for example this script, intended to get 32 bytes of random binary data and convert it to hexadecimal so that it can be stored in a configuration file:

noclobber also applies to symbolic links to regular files. Some special files like /dev/null are not actually changed when you attempt to clobber them. noclobber is smart enough to allow such redirects.

xxxxxxxxxx

#!/usr/bin/env bash

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set -o errexit -o noclobber

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working_directory="$(mktemp --directory)"

bin_path="${working_directory}/secret.bin"

hex_path="${working_directory}/secret.hex"

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dd bs=32 count=1 if=/dev/urandom of="$bin_path" status=none

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# shellcheck disable=SC2094

xxd -cols 32 -plain "$bin_path" > "$bin_path"

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logger "New secret available in ${hex_path}."

There’s a simple typo: the xxd output should be redirected to $hex_path, but instead it’s redirected back to the already existing file.