A script is supposed to delete one run’s output, out/run1, and leave the rest of out/ alone:

#!/bin/bash
# clear-run.sh -- delete one run's folder: bash clear-run.sh run1
rm -rf "out/$1"

Run it without an argument, or with an empty one from a variable that didn’t get filled in, and "out/$1" becomes "out/". rm then deletes the whole out folder, every run in it, and the script exits 0. No root folder, no glob, nothing that looks dangerous in the source.

A related trap: cd "$1" || exit 1, the usual guard for a failed cd, doesn’t fire for an empty $1. On this Mac, cd "" succeeds and stays where you are, so whatever the script deletes next is deleted in the folder you ran it from.

The earlier post on empty variables in rm covers "$DIR"/* expanding to the top of the disk and why set -u misses empty values. This one is about the quieter cases: a path with a fixed prefix, and cd with an empty argument.

The rm calls below are real. They run only inside a new, empty demo folder. If you try them, do the same.

Setup

$ mkdir demo && cd demo
$ mkdir -p out/run1 out/run2 photos fakehome
$ touch out/run1/a.png out/run2/b.png out/notes.txt cover.png photos/p1.png fakehome/avatar.png

rm -rf "out/$1" with no argument deletes out/

With an argument, it does what it should:

$ bash clear-run.sh run1; echo "exit $?"
exit 0
$ find out | sort
out
out/notes.txt
out/run2
out/run2/b.png

Without one:

$ bash clear-run.sh; echo "exit $?"
exit 0
$ ls out
ls: out: No such file or directory

out/run2 and out/notes.txt are gone with it, and the exit status is 0. bash clear-run.sh "" does the same, which is what happens when the caller passes "$RUN" and RUN is empty.

A / as the argument gives out//, which is also just out:

$ mkdir -p out/run2 && touch out/run2/b.png
$ bash clear-run.sh /; echo "exit $?"
exit 0
$ ls out
ls: out: No such file or directory

. and .. are the cases macOS rm does refuse, even with a prefix in front:

$ mkdir -p out/run2 && touch out/run2/b.png
$ bash clear-run.sh .; echo "exit $?"
rm: "." and ".." may not be removed
exit 1

(I didn’t run bash clear-run.sh .. in this post’s replay, since out/.. is the demo folder itself. In my first test, in another scratch folder, it was refused with the same message.)

[ -d "out/$1" ] doesn’t help

A check that the folder exists passes, because out/ is a folder:

$ bash -c '[ -d "out/$1" ] && echo "out/$1 is a folder"' _ ""
out/ is a folder

set -u only half helps. With no argument, $1 is unset and set -u stops the script. With "", $1 is set, just empty, and it goes through:

$ bash -c 'set -u; echo "out/$1"; echo after' _ ""; echo "exit $?"
out/
after
exit 0

${1:?} catches both.

cd "" succeeds, so cd "$1" || exit 1 carries on

On macOS, every shell I tried treats an empty cd argument as “stay here” and returns 0:

$ for sh in bash zsh sh dash; do $sh -c 'cd ""; echo "$0: exit $?"' $sh; done
bash: exit 0
zsh: exit 0
sh: exit 0
dash: exit 0

So this script, which looks guarded, deletes in the wrong folder:

#!/bin/bash
# clean-in.sh -- delete the PNGs in the folder given as $1
cd "$1" || exit 1
rm -f *.png

$ find . -name '*.png' | sort
./cover.png
./fakehome/avatar.png
./out/run2/b.png
./photos/p1.png
$ bash clean-in.sh ""; echo "exit $?"
exit 0
$ find . -name '*.png' | sort
./fakehome/avatar.png
./out/run2/b.png
./photos/p1.png

cover.png, in the folder I ran the script from, is gone. Exit 0, no message.

Unquoted cd $1 goes to your home folder

Without quotes, an empty $1 disappears from the command line, and a bare cd goes to $HOME. This script only prints where each cd lands. I set HOME to the scratch fakehome folder for the run, so nothing real is involved:

#!/bin/bash
# where.sh -- only prints where cd ends up
cd "$1" || exit 1
echo "quoted:   ${PWD##*/}"
cd "$OLDPWD"
cd $1 || exit 1
echo "unquoted: ${PWD##*/}"

$ HOME=$PWD/fakehome bash where.sh ""; echo "exit $?"
quoted:   demo
unquoted: fakehome
exit 0

With a real HOME, the rm -f *.png after an unquoted cd $1 would run in your home folder. I didn’t run that.

The fixed scripts

#!/bin/bash
# clear-run-fixed.sh -- delete one run's folder: bash clear-run-fixed.sh run1
set -u
run=${1:?usage: clear-run-fixed.sh RUN_NAME}
case $run in
  */*|.|..) echo "bad run name: $run" >&2; exit 1 ;;
esac
rm -rf -- "out/$run"
  • ${1:?...} stops on a missing or empty argument, before rm.
  • The case refuses anything with a slash (/, ../demo, run1/../..) and . / .., so the path can only name something directly inside out/.
  • -- keeps a name starting with - from being read as an rm option.
$ mkdir -p out/run1 out/run2 && touch out/run1/a.png out/run2/b.png out/notes.txt
$ bash clear-run-fixed.sh; echo "exit $?"
clear-run-fixed.sh: line 4: 1: usage: clear-run-fixed.sh RUN_NAME
exit 1
$ bash clear-run-fixed.sh ""; echo "exit $?"
clear-run-fixed.sh: line 4: 1: usage: clear-run-fixed.sh RUN_NAME
exit 1
$ bash clear-run-fixed.sh /; echo "exit $?"
bad run name: /
exit 1
$ bash clear-run-fixed.sh ..; echo "exit $?"
bad run name: ..
exit 1
$ bash clear-run-fixed.sh run1/../..; echo "exit $?"
bad run name: run1/../..
exit 1
$ bash clear-run-fixed.sh run1; echo "exit $?"
exit 0
$ find out | sort
out
out/notes.txt
out/run2
out/run2/b.png

For the cd script, refuse the empty argument before cd:

#!/bin/bash
# clean-in-fixed.sh -- delete the PNGs in the folder given as $1
dir=${1:?usage: clean-in-fixed.sh DIR}
cd -- "$dir" || exit 1
rm -f -- *.png

$ touch cover.png
$ bash clean-in-fixed.sh ""; echo "exit $?"
clean-in-fixed.sh: line 3: 1: usage: clean-in-fixed.sh DIR
exit 1
$ bash clean-in-fixed.sh photoz; echo "exit $?"
clean-in-fixed.sh: line 4: cd: photoz: No such file or directory
exit 1
$ bash clean-in-fixed.sh photos; echo "exit $?"
exit 0
$ find . -name '*.png' | sort
./cover.png
./fakehome/avatar.png
./out/run2/b.png

Only photos/p1.png was deleted. (The 1: in bash’s message is the name of the parameter, $1.)

Checklist

  • rm -rf "prefix/$1" with an empty or missing $1 deletes all of prefix/, exit 0. So does $1=/.
  • macOS rm refuses prefix/. and prefix/.., but not prefix/ or prefix//.
  • [ -d "prefix/$1" ] passes when $1 is empty, because prefix/ is a folder.
  • cd "" returns 0 and stays in the current folder (bash, zsh, sh, dash on macOS), so cd "$1" || exit 1 doesn’t catch an empty $1.
  • Unquoted cd $1 with an empty $1 goes to $HOME.
  • Use ${1:?message} before building the path or calling cd, and refuse names with /, . and .. if the argument should name one entry.
  • Test destructive scripts with no argument and with "" in a scratch folder before pointing them at real data.

Tested on macOS 26.5.2 (Apple silicon) with /bin/bash (GNU bash, version 3.2.57(1)-release (arm64-apple-darwin25)), /bin/zsh (zsh 5.9 (arm64-apple-darwin25.0)), /bin/sh and /bin/dash, macOS /bin/rm. All real rm calls ran in a scratch folder under /tmp; the home-folder case was run with HOME pointed at a scratch folder and only printed the folder. Linux, newer bash versions and other cd implementations were not tested.