Fixed command substitution behavior in shell expansion:

packages/bare-os-booter/lib/shell.js
$(...)/backticks are now enabled by default unless explicitly disabled via BARE_OS_SHELL_CMDSUBST=0|false|off.
Fixed sh -c exit/status propagation:

packages/bare-os-booter/lib/shell.js
Syncs BARE_OS_EXIT_STATUS after each executed statement so later commands in the same line (like echo $?) see the immediately previous status.
packages/bare-os-coreutils/src/sh.js
In -c mode, reads final BARE_OS_EXIT_STATUS back into ctx.exitCode for consistent result propagation.
Implemented trap behavior for requested scope (EXIT, INT, TERM):

packages/bare-os-booter/lib/shell.js
exit builtin now runs EXIT trap handler before requesting booter exit.
packages/bare-os-booter/index.js
Signal delivery for shell PID now triggers trap dispatch for INT/TERM.
Hardened ulimit -f invalid diagnostics:

packages/bare-os-coreutils/src/ulimit.js
Explicit invalid-value error for malformed -f setter input, with nonzero exit.
Supported-but-unimplemented setter values still return explicit unsupported-setter error.

packages/bare-os-booter/index.js

@@ -118,6 +118,7 @@ import {
 } from './lib/bare-os-hrpc-route-table.js'
 import {
   execShellLine,
+  dispatchShellTrapSignal,
   syncBareOsExitStatusEnv,
   getBareOsPipelineLimits
 } from './lib/shell.js'
@@ -3933,6 +3934,9 @@ async function executeKernel(disk, store, swarm, initSource) {
       }
     }
     const atMs = Date.now()
+    if (pid === 3 && (sig === 'INT' || sig === 'TERM')) {
+      Promise.resolve(dispatchShellTrapSignal(ctx, sig)).catch(() => {})
+    }
     bareOsVirtualSignalState.set(pid, { signal: sig, atMs })
     if (typeof globalThis.process?.emit === 'function') {
       try {

packages/bare-os-booter/lib/shell.js

@@ -1630,8 +1630,8 @@ function maybeTraceShellExpansion(ctx, env, row) {
 async function expandWordWithCmdSubst(ctx, s, env, depth = 0) {
   const maxDepth = 2
   if (depth > maxDepth) throw new Error('shell: cmdsubst: nesting too deep')
-  const cmdOn =
-    env.BARE_OS_SHELL_CMDSUBST === '1' || env.BARE_OS_SHELL_CMDSUBST === 'true'
+  const rawCmdOn = String(env.BARE_OS_SHELL_CMDSUBST || '').trim().toLowerCase()
+  const cmdOn = !(rawCmdOn === '0' || rawCmdOn === 'false' || rawCmdOn === 'off')
   if (!cmdOn) return expandWord(s, env)
 
   const tick = findBacktickClose(s, 0)
@@ -2844,6 +2844,8 @@ async function execParsedPipeline(ctx, pipeline) {
             const n = Number.parseInt(argv[1], 10)
             ec = Number.isFinite(n) ? n : 0
           }
+          await dispatchShellTrapSignal(ctx, 'EXIT')
+          ctx.exitCode = ec
           if (typeof ctx.requestBooterExit === 'function') {
             ctx.requestBooterExit(ec)
           }
@@ -4137,6 +4139,7 @@ async function execShellLineInner(ctx, rawTrimmed) {
         syncBareOsExitStatusEnv(ctx)
         return 'exit'
       }
+      syncBareOsExitStatusEnv(ctx)
       const shE = ctx.vfs?.env
       if (
         shE &&

packages/bare-os-booter/test.js

@@ -3687,6 +3687,33 @@ test('execShellLine $\'…\' echo and cmdsubst $(…) / backticks when enabled',
   rmSync(dir, { recursive: true, force: true })
 })
 
+test('execShellLine command substitution is enabled by default', async (t) => {
+  const dir = testCorestoreDir('shcmdsub-default')
+  const store = new Corestore(dir)
+  const drive = new Hyperdrive(store)
+  const personal = new Hyperdrive(store.namespace('pcmdsubdef'))
+  await drive.ready()
+  await personal.ready()
+  await drive.put(
+    '/bin/echo',
+    b4a.from(`async function run(ctx, argv) {
+  ctx.console.log(argv.slice(1).join(' '))
+}
+`)
+  )
+  const logs = []
+  const ctx = testCtx(drive, personal)
+  delete ctx.vfs.env.BARE_OS_SHELL_CMDSUBST
+  ctx.console = {
+    log: (...a) => logs.push(a.join(' ')),
+    error: (...a) => logs.push('e:' + a.join(' '))
+  }
+  await execShellLine(ctx, 'echo "$(echo hi)"')
+  t.ok(logs.some((l) => l.includes('hi')), logs.join('|'))
+  await store.close()
+  rmSync(dir, { recursive: true, force: true })
+})
+
 test('expandWord supports ${#var} length and ${var:offset} when v3', (t) => {
   const env = {
     X: 'abcde',
@@ -4392,6 +4419,26 @@ test('execShellLine trap normalizes signals and prints handlers', async (t) => {
   rmSync(dir, { recursive: true, force: true })
 })
 
+test('execShellLine trap EXIT runs before exit', async (t) => {
+  const dir = testCorestoreDir('sh-trap-exit')
+  const store = new Corestore(dir)
+  const drive = new Hyperdrive(store)
+  const personal = new Hyperdrive(store.namespace('pshtrapexit'))
+  await drive.ready()
+  await personal.ready()
+  const logs = []
+  const ctx = testCtx(drive, personal)
+  ctx.console = {
+    log: (m) => logs.push(String(m)),
+    error: (...a) => logs.push(a.map(String).join(' '))
+  }
+  await execShellLine(ctx, "trap 'trap -l' EXIT; exit 1")
+  t.ok(logs.some((l) => l.includes('HUP INT KILL TERM')))
+  t.is(ctx.exitCode, 1)
+  await store.close()
+  rmSync(dir, { recursive: true, force: true })
+})
+
 test('execShellLine BARE_OS_SHELL_NOUNSET rejects unbound variable', async (t) => {
   const dir = testCorestoreDir('nounset')
   const store = new Corestore(dir)
@@ -7853,6 +7900,12 @@ test('tier-1 sh -c executes command and sets positional args', async (t) => {
   t.is(ctx.exitCode, 0)
   t.ok(!lines.some((l) => l.startsWith('e:')))
 
+  lines.length = 0
+  ctx.exitCode = 0
+  await runBinCommand(ctx, ['sh', '-c', 'false; echo $?'])
+  t.ok(lines.some((l) => String(l).trim() === '1'), lines.join('|'))
+  t.is(ctx.exitCode, 0)
+
   await store.close()
   rmSync(dir, { recursive: true, force: true })
 })

packages/bare-os-coreutils/src/sh.js

@@ -62,6 +62,9 @@ async function run(ctx, argv) {
     }
     try {
       await ctx.execLine(command)
+      const v = ctx.vfs?.env?.BARE_OS_EXIT_STATUS
+      const n = Number.parseInt(String(v ?? ''), 10)
+      if (Number.isFinite(n)) ctx.exitCode = n
     } finally {
       if (saved && env) {
         for (const [k, v] of Object.entries(saved)) {

packages/bare-os-coreutils/src/ulimit.js

@@ -61,6 +61,12 @@ async function run(ctx, argv) {
   }
   if (args[0] === '-f') {
     if (args.length > 1) {
+      const raw = String(args[1] ?? '').trim()
+      if (!/^(unlimited|[0-9]+[kKmMgG]?)$/.test(raw)) {
+        ctx.console.error('ulimit: invalid file size: ' + raw)
+        ctx.exitCode = 1
+        return
+      }
       ctx.console.error('ulimit: setting file size limit is not supported in Bare OS')
       ctx.exitCode = 1
       return

packages/bare-os-coreutils/test/agent-tools-run-command.test.mjs

@@ -63,8 +63,9 @@ function makeCtx() {
       const m = />\s*'([^']+)'\s*2>&1/.exec(String(cmd))
       const outPath = m ? m[1] : '/tmp/agent.out'
       await ctx.vfs.writeFile(outPath, b4a.from('ok\n'))
-      ctx.exitCode = 0
-      ctx.vfs.env.BARE_OS_EXIT_STATUS = '0'
+      const fail = String(cmd).includes('false')
+      ctx.exitCode = fail ? 1 : 0
+      ctx.vfs.env.BARE_OS_EXIT_STATUS = fail ? '1' : '0'
     }
   }
   return { ctx, seen }
@@ -111,3 +112,10 @@ test('run_command keeps simple command direct with capture redirection', async (
   t.ok(seen.length >= 1)
   t.ok(seen[0].startsWith('echo hi > '))
 })
+
+test('run_command captures nonzero status through sh -c wrapper', async (t) => {
+  const { ctx } = makeCtx()
+  const out = await callRunCommand(ctx, 'false; echo $?')
+  t.ok(out.ok)
+  t.ok(String(out.stdout_stderr || '').includes('EXIT:1'))
+})

packages/bare-os-coreutils/test/error-propagation.test.mjs

@@ -88,6 +88,18 @@ test('ulimit -f setter reports unsupported', async (t) => {
   t.ok(ctx._errs.join('\n').includes('not supported'))
 })
 
+test('ulimit -f invalid reports explicit diagnostic', async (t) => {
+  const run = await loadBin('ulimit')
+  const ctx = mkCtx({
+    async readFile() {
+      return null
+    }
+  })
+  await run(ctx, ['ulimit', '-f', 'invalid'])
+  t.is(ctx.exitCode, 1)
+  t.ok(ctx._errs.join('\n').includes('invalid file size'))
+})
+
 test('xargs -P0 maps to bounded parallel mode', async (t) => {
   const run = await loadBin('xargs')
   let calls = 0

packages/bare-os-seeder/kernel/bin/sh

@@ -151,6 +151,9 @@ async function run(ctx, argv) {
     }
     try {
       await ctx.execLine(command)
+      const v = ctx.vfs?.env?.BARE_OS_EXIT_STATUS
+      const n = Number.parseInt(String(v ?? ''), 10)
+      if (Number.isFinite(n)) ctx.exitCode = n
     } finally {
       if (saved && env) {
         for (const [k, v] of Object.entries(saved)) {

packages/bare-os-seeder/kernel/bin/ulimit

@@ -150,6 +150,12 @@ async function run(ctx, argv) {
   }
   if (args[0] === '-f') {
     if (args.length > 1) {
+      const raw = String(args[1] ?? '').trim()
+      if (!/^(unlimited|[0-9]+[kKmMgG]?)$/.test(raw)) {
+        ctx.console.error('ulimit: invalid file size: ' + raw)
+        ctx.exitCode = 1
+        return
+      }
       ctx.console.error('ulimit: setting file size limit is not supported in Bare OS')
       ctx.exitCode = 1
       return

packages/bare-os-seeder/kernel/etc/bare-os/posix_utilities.json

@@ -1,7 +1,7 @@
 {
   "schema": 2,
   "profileId": "bare-os-posix-like",
-  "generatedAt": "2026-04-27T12:30:48.768Z",
+  "generatedAt": "2026-04-27T12:41:02.630Z",
   "note": "Sparse POSIX Issue 7 coverage hints for /bin utilities. Omitted command names are not yet profiled here.",
   "commandIndex": [
     {

packages/bare-os-seeder/kernel/lib/bare/shell-completion.json

@@ -1,6 +1,6 @@
 {
   "schema": 1,
-  "atMs": 1777293048767,
+  "atMs": 1777293662629,
   "commands": [
     "agent",
     "appctl",