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#!/bin/sh
# Print a random line from input
self=rndl
# If there are no arguments, we're checking stdin; this is more complicated
# than checking file arguments because we have to count the lines in order to
# correctly choose a random one, and two passes means we require a temporary
# file if we don't want to read all of the input into memory (!)
if [ "$#" -eq 0 ] ; then
# Create a temporary directory with name in $td, and handle POSIX-ish traps to
# remove it when the script exits.
td=
cleanup() {
[ -n "$td" ] && rm -fr -- "$td"
if [ "$1" != EXIT ] ; then
trap - "$1"
kill "-$1" "$$"
fi
}
for sig in EXIT HUP INT TERM ; do
# shellcheck disable=SC2064
trap "cleanup $sig" "$sig"
done
td=$(mktd "$self") || exit
# We'll operate on stdin in the temp directory; write the script's stdin to
# it with cat(1)
set -- "$td"/stdin
cat >"$td"/stdin
fi
# Count the number of lines in the input
lc=$(sed -- '$=;d' "$@") || exit
# If there were none, bail
case $lc in
''|0)
printf 2>&1 'rndl: No lines found on input\n'
exit 2
;;
esac
# Try to get a random seed from rnds(1) for rndi(1)
seed=$(rnds)
# Get a random line number from rndi(1)
ri=$(rndi 1 "$lc" "$seed") || exit
# Print the line using sed(1)
sed -- "$ri"'!d' "$@"
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