Development record
rm-rf-root-or-home reads a default expansion by its variable only: rm -rf ${DIR:-$HOME} and rm -rf ${DIR:-/} delete the home or the root when DIR is unset and allow, because a word's written spelling holds one text and the default's own word is the other value it can print. An alternative nested more than three deep (${X:+${X:+${X:+${X:+$HOME}}}}) and ${PWD:0:1}, which prints the root and warns as $PWD, are the same class. Named in 17-guard.md's residuals.
rm-rf-root-or-home reads a default expansion by its variable only: rm -rf ${DIR:-$HOME} and rm -rf ${DIR:-/} delete the home or the root when DIR is unset and allow, because a word's written spelling holds one text and the default's own word is the other value it can print. An alternative nested more than three deep (${X:+${X:+${X:+${X:+$HOME}}}}) and ${PWD:0:1}, which prints the root and warns as $PWD, are the same class. Named in 17-guard.md's residuals.
Deferral 2026-09-29
Deferred past v0.11.1: Reading a default's word needs a written spelling that holds more than one text (the variable's value or the default's word), which changes segment.spelled from one string per word to a set and the payload pairing that copies it (spellPayload); owed: that representation, then the default word, deep alternatives and a substring's root read through it, test first.
Evidence 2026-09-29: a default after a subscript
The class includes a default the bash 3.2 of macOS reads at the first operator after a subscript's ]. With X unset, bash 3.2 and /bin/sh print the word for ${X[0]]-$HOME}, ${X[0]]:-$HOME}, ${X[0]]=$HOME}, ${X[0]]:=$HOME} and ${X[0]]x-$HOME} (the home), and for ${X[0]]-/} (the root); bash 5 refuses each as a bad substitution. With X set each prints X's value. The guard reads the subscript's operator (unknown.go subscriptOperators) and spells a - or = there as the variable, as it spells ${X:-$HOME}, so each allows. The pin ${X[0]]-$HOME} in homeresiduals_test.go is this residual, not a claim that the form stays off the home. The same owed representation, a spelling that holds both texts, reads them.
Grounds
- pursued: rm -rf ${DIR:-$HOME}, ${DIR:-/}, ${X[0]]-$HOME}, a four-deep alternative and ${PWD:0:1} block bare and in sh -c and bash -c while ${DIR:-./build} allows; a default, alternative or substring form that prints the root or home and still allows would show it wrong.
Frontmatter
| id | iss-2609290426544292 |
|---|---|
| type | issue |
| lifecycle | resolved |
| severity | major |
.abcd/work/issues/resolved/iss-2609290426544292-rm-rf-root-or-home-reads-a-default.md
open on GitHub ↗ · commit history ↗
mentions3
- iss-2609292320015665rm-rf-root-or-home reads a trimmed expansion as its variable alone, but a suffix trim whose pattern is unknown text or begins with a glob can leave only the leading slash of an absolute path: with X=/a/b, bash 3.2 prints / for ${X%${X#?}} and ${X%%[!/]*}, so rm -rf ${X%${X#?}} deletes the root and allows. Found while fixing iss-2609290426544292, whose written spelling now holds a set of texts; a trim can add / to that set, but reading every trim as the root would refuse the everyday ${DIR%/} and ${f%.*}, so the rule needs the pattern's shape.
- iss-2609300009506126rm-rf-root-or-home reads an expansion that prints nothing whatever the value is as its variable, so the text around it is never read as the whole word: with X=/a/b, bash 3.2, /bin/sh and dash print nothing for ${X%%*}, ${X##*}, ${X%%/*} and ${X:0:0}, so rm -rf ${X%%*}/ deletes the root and rm -rf $HOME/${X%%*} the home, and both allow. bash 3.2, the /bin/bash and /bin/sh of macOS, also prints nothing for a trim, a replacement or a substring after a scalar's subscript (${X[0]%zzz}/ is /). Found while fixing iss-2609292320015665, the trim that leaves only the root; an alternative's empty text was added by iss-2609290426544292, and these are its siblings.
- iss-2609300009581165rm-rf-root-or-home reads a pattern replacement as its variable unless its pattern is only *, but a replacement whose pattern can match the whole of an absolute path prints its string in its place, and one whose pattern can match all of the path after its leading slash prints / and its string: with X=/a/b, bash 3.2 and /bin/sh print the home for ${X/\/*/$HOME}, ${X/?*/$HOME} and ${X/$X/~}, and / for ${X/${X#?}} and ${X//[!\/]*/}, and rm -rf of each allows. Found while fixing iss-2609292320015665; iss-2609290426544292 read only the *-only pattern and left the rest alone to keep ${DIR/#\~/$HOME} allowed.