`write` 0.1.0

Replace a file atomically, only if it holds what the caller says it holds (--if-sha256) or does not exist (--create); idempotent, locked, with --dry-run.

write [--root DIR] (--create | --if-sha256 HEX) [--content-sha256 HEX] [--dry-run] [--max-bytes N] [--max-diff-lines N] (--stdin | --content-file PATH) [--format json|text] PATH

output: one JSON document
schema: write.v1
reversibility: irreversible-bounded
guarantees: deterministic, idempotent, atomic, requires_precondition, dry_run
concurrency: locked: of writers racing with the same precondition, exactly one wins and the others are refused (M7)
confinement: beneath: with --root, paths are checked lexically against it and then opened beneath it one component at a time, following no symbolic link (path.symlink), so nothing outside the root is reachable through one; without it, a reader opens the path as given, links and all, and a writer still follows no link in the file's own name
reads the environment: no
reads the clock: no
compiler: de4e52fd8180
for an agent: SKILL.md (write skill)

Authority

Derived by cancho authority from the source, embedded in the binary, and checked against tools.toml's ceiling in CI. Every label is bounded.

args
dir_read
dir_write
err_write
file_read
file_write
fs_read("")
heap
io_read
io_write

What the row cannot say:

path extent: a path from the command line is checked at run time, so the row says fs_read("")
what a symlink reaches
what standard output carries
resource use beyond the tool's own caps (memory, wall time)
per-invocation behaviour such as --dry-run, which the row cannot see

Operands

Operand	Role	How many	Meaning
`PATH`	`path-write`	1	the file to create or replace

Flags

Flag	Kind	Role	Default	Meaning
`--root`	`path`	`root`		resolve PATH and --content-file relative to this directory and refuse paths outside it
`--create`	`bool`	`guard`		the path must not exist yet (if it holds exactly the new content, nothing is written and changed is false)
`--if-sha256`	`hex64`	`guard`		the file's current content must hash to this SHA-256
`--content-sha256`	`hex64`	`none`		the new content must hash to this SHA-256, checked before anything is written
`--dry-run`	`bool`	`guard`		check everything and report planned_actions, exit 9, write nothing
`--stdin`	`bool`	`none`		read the new content from standard input
`--content-file`	`path`	`path-read`		read the new content from this file
`--max-bytes`	`nat`	`none`	`67108864`	the largest new content accepted, and the largest old content diffed (ceiling 1073741824)
`--max-diff-lines`	`nat`	`none`	`200`	the most changed lines the diff shows (ceiling 100000)
`--format`	`choice:json/text`	`none`	`json`	json for a program, text for a person

Limits

Limit	Default	Ceiling
`max-bytes`	67,108,864	1,073,741,824
`max-diff-lines`	200	100,000

Exit codes

Code	Name	Meaning
0	`SUCCESS`	what was asked was done; zero matches is success
1	`GENERAL_ERROR`	the operating system failed mid-operation, or the tool caught a bug in itself
2	`INVALID_ARGS`	the invocation is malformed or names something of the wrong kind; nothing was read
3	`NOT_FOUND`	a named input does not exist
4	`PERMISSION_DENIED`	the operating system refused, or the tool's own confinement did
5	`CONFLICT`	state is not what the caller said it would be
8	`PRECONDITION_FAILED`	the answer is no, or a limit was reached
9	`DRY_RUN`	a --dry-run completed; planned_actions lists what an apply would do

Rules (28)

Match on rule, not on the message. A retry repair is an argv that can be run as it stands, and never widens what the tool may touch.

Rule	Exit	Repair	When
`args.unknown-flag`	2	sometimes	a flag that is not in the tool's table
`args.missing-value`	2	never	a flag that takes a value was given none
`args.bad-value`	2	never	a flag's value is not of the flag's kind
`args.unexpected-value`	2	never	a flag that takes no value was given one
`args.duplicate-flag`	2	never	the same flag was given twice
`args.conflict`	2	never	two flags that exclude each other were both given
`args.required-flag`	2	never	a flag the tool needs was not given
`args.missing-operand`	2	never	an operand the tool needs was not given
`args.too-many-operands`	2	never	more operands than the tool takes
`path.empty`	2	never	an empty path
`path.dotdot`	2	sometimes	a path with a `..` component
`path.absolute`	2	always	an absolute path where one relative to --root is wanted
`path.outside-root`	4	never	an absolute path that is not under --root
`path.too-long`	2	never	a path longer than 4096 bytes
`path.symlink`	4	never	a path that runs through a symbolic link, which the tool does not follow
`io.not-found`	3	never	a named input does not exist
`io.not-a-directory`	3	never	a path runs through something that is not a directory
`io.is-a-directory`	2	never	a directory where a file is wanted
`io.permission-denied`	4	never	the operating system refused access
`io.read-failed`	1	never	the operating system failed a read
`io.write-failed`	1	never	the operating system failed a write, or a write was short
`io.rename-unsupported`	8	never	the filesystem cannot refuse an existing name atomically, so move did not rename
`limit.input-too-large`	8	sometimes	an input longer than --max-bytes
`precondition.required`	2	never	a write that states no belief about the file
`precondition.hash-mismatch`	5	never	the file's hash is not the one --if-sha256 named
`precondition.content-mismatch`	5	never	the new content's hash is not the one --content-sha256 named
`conflict.exists`	5	never	--create was given and the path exists
`conflict.locked`	5	never	another writer holds the path's lock

Evidence

each gate is a test in cancho-tools/tests/conformance, run by CI on the commit this binary was built from:

M1 schema conformance
M2 determinism
M3 rule coverage and hint soundness
M4 fault injection
M5 differential against GNU
M6 authority
M7 mutation behaviour
M8 confinement
M9 memory flatness

Agent in the loop: not run: no claim that this tool is more agent-friendly than the incumbents is made (cancho docs/agent-toolbox.md section 7.2).

write 0.1.0