diff --git a/t/chainlint.sed b/t/chainlint.sed
new file mode 100644
index 00000000000000..a0de8a38823a18
--- /dev/null
+++ b/t/chainlint.sed
@@ -0,0 +1,346 @@
+#------------------------------------------------------------------------------
+# Detect broken &&-chains in tests.
+#
+# At present, only &&-chains in subshells are examined by this linter;
+# top-level &&-chains are instead checked directly by the test framework. Like
+# the top-level &&-chain linter, the subshell linter (intentionally) does not
+# check &&-chains within {...} blocks.
+#
+# Checking for &&-chain breakage is done line-by-line by pure textual
+# inspection.
+#
+# Incomplete lines (those ending with "\") are stitched together with following
+# lines to simplify processing, particularly of "one-liner" statements.
+# Top-level here-docs are swallowed to avoid false positives within the
+# here-doc body, although the statement to which the here-doc is attached is
+# retained.
+#
+# Heuristics are used to detect end-of-subshell when the closing ")" is cuddled
+# with the final subshell statement on the same line:
+#
+#    (cd foo &&
+#        bar)
+#
+# in order to avoid misinterpreting the ")" in constructs such as "x=$(...)"
+# and "case $x in *)" as ending the subshell.
+#
+# Lines missing a final "&&" are flagged with "?!AMP?!", and lines which chain
+# commands with ";" internally rather than "&&" are flagged "?!SEMI?!". A line
+# may be flagged for both violations.
+#
+# Detection of a missing &&-link in a multi-line subshell is complicated by the
+# fact that the last statement before the closing ")" must not end with "&&".
+# Since processing is line-by-line, it is not known whether a missing "&&" is
+# legitimate or not until the _next_ line is seen. To accommodate this, within
+# multi-line subshells, each line is stored in sed's "hold" area until after
+# the next line is seen and processed. If the next line is a stand-alone ")",
+# then a missing "&&" on the previous line is legitimate; otherwise a missing
+# "&&" is a break in the &&-chain.
+#
+#    (
+#         cd foo &&
+#         bar
+#    )
+#
+# In practical terms, when "bar" is encountered, it is flagged with "?!AMP?!",
+# but when the stand-alone ")" line is seen which closes the subshell, the
+# "?!AMP?!" violation is removed from the "bar" line (retrieved from the "hold"
+# area) since the final statement of a subshell must not end with "&&". The
+# final line of a subshell may still break the &&-chain by using ";" internally
+# to chain commands together rather than "&&", so "?!SEMI?!" is never removed
+# from a line (even though "?!AMP?!" might be).
+#
+# Care is taken to recognize the last _statement_ of a multi-line subshell, not
+# necessarily the last textual _line_ within the subshell, since &&-chaining
+# applies to statements, not to lines. Consequently, blank lines, comment
+# lines, and here-docs are swallowed (but not the command to which the here-doc
+# is attached), leaving the last statement in the "hold" area, not the last
+# line, thus simplifying &&-link checking.
+#
+# The final statement before "done" in for- and while-loops, and before "elif",
+# "else", and "fi" in if-then-else likewise must not end with "&&", thus
+# receives similar treatment.
+#
+# To facilitate regression testing (and manual debugging), a ">" annotation is
+# applied to the line containing ")" which closes a subshell, ">>" to a line
+# closing a nested subshell, and ">>>" to a line closing both at once. This
+# makes it easy to detect whether the heuristics correctly identify
+# end-of-subshell.
+#------------------------------------------------------------------------------
+
+# incomplete line -- slurp up next line
+:squash
+/\\$/ {
+      N
+      s/\\\n//
+      bsquash
+}
+
+# here-doc -- swallow it to avoid false hits within its body (but keep the
+# command to which it was attached)
+/<<[ 	]*[-\\]*EOF[ 	]*/ {
+	s/[ 	]*<<[ 	]*[-\\]*EOF//
+	h
+	:hereslurp
+	N
+	s/.*\n//
+	/^[ 	]*EOF[ 	]*$/!bhereslurp
+	x
+}
+
+# one-liner "(...) &&"
+/^[ 	]*!*[ 	]*(..*)[ 	]*&&[ 	]*$/boneline
+
+# same as above but without trailing "&&"
+/^[ 	]*!*[ 	]*(..*)[ 	]*$/boneline
+
+# one-liner "(...) >x" (or "2>x" or "<x" or "|x" or "&"
+/^[ 	]*!*[ 	]*(..*)[ 	]*[0-9]*[<>|&]/boneline
+
+# multi-line "(...\n...)"
+/^[ 	]*(/bsubshell
+
+# innocuous line -- print it and advance to next line
+b
+
+# found one-liner "(...)" -- mark suspect if it uses ";" internally rather than
+# "&&" (but not ";" in a string)
+:oneline
+/;/{
+	/"[^"]*;[^"]*"/!s/^/?!SEMI?!/
+}
+b
+
+:subshell
+# bare "(" line?
+/^[ 	]*([	]*$/ {
+	# stash for later printing
+	h
+	bnextline
+}
+# "(..." line -- split off and stash "(", then process "..." as its own line
+x
+s/.*/(/
+x
+s/(//
+bslurp
+
+:nextline
+N
+s/.*\n//
+
+:slurp
+# incomplete line "...\"
+/\\$/bincomplete
+# multi-line quoted string "...\n..."
+/^[^"]*"[^"]*$/bdqstring
+# multi-line quoted string '...\n...' (but not contraction in string "it's so")
+/^[^']*'[^']*$/{
+	/"[^'"]*'[^'"]*"/!bsqstring
+}
+# here-doc -- swallow it
+/<<[ 	]*[-\\]*EOF/bheredoc
+/<<[ 	]*[-\\]*EOT/bheredoc
+/<<[ 	]*[-\\]*INPUT_END/bheredoc
+# comment or empty line -- discard since final non-comment, non-empty line
+# before closing ")", "done", "elsif", "else", or "fi" will need to be
+# re-visited to drop "suspect" marking since final line of those constructs
+# legitimately lacks "&&", so "suspect" mark must be removed
+/^[ 	]*#/bnextline
+/^[ 	]*$/bnextline
+# in-line comment -- strip it (but not "#" in a string, Bash ${#...} array
+# length, or Perforce "//depot/path#42" revision in filespec)
+/[ 	]#/{
+	/"[^"]*#[^"]*"/!s/[ 	]#.*$//
+}
+# one-liner "case ... esac"
+/^[ 	]*case[ 	]*..*esac/bcheckchain
+# multi-line "case ... esac"
+/^[ 	]*case[ 	]..*[ 	]in/bcase
+# multi-line "for ... done" or "while ... done"
+/^[ 	]*for[ 	]..*[ 	]in/bcontinue
+/^[ 	]*while[ 	]/bcontinue
+/^[ 	]*do[ 	]/bcontinue
+/^[ 	]*do[ 	]*$/bcontinue
+/;[ 	]*do/bcontinue
+/^[ 	]*done[ 	]*&&[ 	]*$/bdone
+/^[ 	]*done[ 	]*$/bdone
+/^[ 	]*done[ 	]*[<>|]/bdone
+/^[ 	]*done[ 	]*)/bdone
+/||[ 	]*exit[ 	]/bcontinue
+/||[ 	]*exit[ 	]*$/bcontinue
+# multi-line "if...elsif...else...fi"
+/^[ 	]*if[ 	]/bcontinue
+/^[ 	]*then[ 	]/bcontinue
+/^[ 	]*then[ 	]*$/bcontinue
+/;[ 	]*then/bcontinue
+/^[ 	]*elif[ 	]/belse
+/^[ 	]*elif[ 	]*$/belse
+/^[ 	]*else[ 	]/belse
+/^[ 	]*else[ 	]*$/belse
+/^[ 	]*fi[ 	]*&&[ 	]*$/bdone
+/^[ 	]*fi[ 	]*$/bdone
+/^[ 	]*fi[ 	]*[<>|]/bdone
+/^[ 	]*fi[ 	]*)/bdone
+# nested one-liner "(...) &&"
+/^[ 	]*(.*)[ 	]*&&[ 	]*$/bcheckchain
+# nested one-liner "(...)"
+/^[ 	]*(.*)[ 	]*$/bcheckchain
+# nested one-liner "(...) >x" (or "2>x" or "<x" or "|x")
+/^[ 	]*(.*)[ 	]*[0-9]*[<>|]/bcheckchain
+# nested multi-line "(...\n...)"
+/^[ 	]*(/bnest
+# multi-line "{...\n...}"
+/^[ 	]*{/bblock
+# closing ")" on own line -- exit subshell
+/^[ 	]*)/bclosesolo
+# "$((...))" -- arithmetic expansion; not closing ")"
+/\$(([^)][^)]*))[^)]*$/bcheckchain
+# "$(...)" -- command substitution; not closing ")"
+/\$([^)][^)]*)[^)]*$/bcheckchain
+# multi-line "$(...\n...)" -- command substitution; treat as nested subshell
+/\$([ 	     ]*$/bnest
+# "=(...)" -- Bash array assignment; not closing ")"
+/=(/bcheckchain
+# closing "...) &&"
+/)[ 	]*&&[ 	]*$/bclose
+# closing "...)"
+/)[ 	]*$/bclose
+# closing "...) >x" (or "2>x" or "<x" or "|x")
+/)[ 	]*[<>|]/bclose
+:checkchain
+# mark suspect if line uses ";" internally rather than "&&" (but not ";" in a
+# string and not ";;" in one-liner "case...esac")
+/;/{
+	/;;/!{
+		/"[^"]*;[^"]*"/!s/^/?!SEMI?!/
+	}
+}
+# line ends with pipe "...|" -- valid; not missing "&&"
+/|[ 	]*$/bcontinue
+# missing end-of-line "&&" -- mark suspect
+/&&[ 	]*$/!s/^/?!AMP?!/
+:continue
+# retrieve and print previous line
+x
+n
+bslurp
+
+# found incomplete line "...\" -- slurp up next line
+:incomplete
+N
+s/\\\n//
+bslurp
+
+# found multi-line double-quoted string "...\n..." -- slurp until end of string
+:dqstring
+s/"//g
+N
+s/\n//
+/"/!bdqstring
+bcheckchain
+
+# found multi-line single-quoted string '...\n...' -- slurp until end of string
+:sqstring
+s/'//g
+N
+s/\n//
+/'/!bsqstring
+bcheckchain
+
+# found here-doc -- swallow it to avoid false hits within its body (but keep
+# the command to which it was attached); take care to handle here-docs nested
+# within here-docs by only recognizing closing tag matching outer here-doc
+# opening tag
+:heredoc
+/EOF/{ s/[ 	]*<<[ 	]*[-\\]*EOF//; s/^/EOF/; }
+/EOT/{ s/[ 	]*<<[ 	]*[-\\]*EOT//; s/^/EOT/; }
+/INPUT_END/{ s/[ 	]*<<[ 	]*[-\\]*INPUT_END//; s/^/INPUT_END/; }
+:hereslurpsub
+N
+/^EOF.*\n[ 	]*EOF[ 	]*$/bhereclose
+/^EOT.*\n[ 	]*EOT[ 	]*$/bhereclose
+/^INPUT_END.*\n[ 	]*INPUT_END[ 	]*$/bhereclose
+bhereslurpsub
+:hereclose
+s/^EOF//
+s/^EOT//
+s/^INPUT_END//
+s/\n.*$//
+bcheckchain
+
+# found "case ... in" -- pass through untouched
+:case
+x
+n
+/^[ 	]*esac/bslurp
+bcase
+
+# found "else" or "elif" -- drop "suspect" from final line before "else" since
+# that line legitimately lacks "&&"
+:else
+x
+s/?!AMP?!//
+x
+bcontinue
+
+# found "done" closing for-loop or while-loop, or "fi" closing if-then -- drop
+# "suspect" from final contained line since that line legitimately lacks "&&"
+:done
+x
+s/?!AMP?!//
+x
+# is 'done' or 'fi' cuddled with ")" to close subshell?
+/done.*)/bclose
+/fi.*)/bclose
+bcheckchain
+
+# found nested multi-line "(...\n...)" -- pass through untouched
+:nest
+x
+:nestslurp
+n
+# closing ")" on own line -- stop nested slurp
+/^[ 	]*)/bnestclose
+# comment -- not closing ")" if in comment
+/^[ 	]*#/bnestcontinue
+# "$((...))" -- arithmetic expansion; not closing ")"
+/\$(([^)][^)]*))[^)]*$/bnestcontinue
+# "$(...)" -- command substitution; not closing ")"
+/\$([^)][^)]*)[^)]*$/bnestcontinue
+# closing "...)" -- stop nested slurp
+/)/bnestclose
+:nestcontinue
+x
+bnestslurp
+:nestclose
+s/^/>>/
+# is it "))" which closes nested and parent subshells?
+/)[ 	]*)/bslurp
+bcheckchain
+
+# found multi-line "{...\n...}" block -- pass through untouched
+:block
+x
+n
+# closing "}" -- stop block slurp
+/}/bcheckchain
+bblock
+
+# found closing ")" on own line -- drop "suspect" from final line of subshell
+# since that line legitimately lacks "&&" and exit subshell loop
+:closesolo
+x
+s/?!AMP?!//
+p
+x
+s/^/>/
+b
+
+# found closing "...)" -- exit subshell loop
+:close
+x
+p
+x
+s/^/>/
+b
diff --git a/t/test-lib.sh b/t/test-lib.sh
index 28315706be709d..78f7097746fc88 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -675,7 +675,8 @@ test_run_ () {
 		trace=
 		# 117 is magic because it is unlikely to match the exit
 		# code of other programs
-		if test "OK-117" != "$(test_eval_ "(exit 117) && $1${LF}${LF}echo OK-\$?" 3>&1)"
+		if $(printf '%s\n' "$1" | sed -f "$GIT_BUILD_DIR/t/chainlint.sed" | grep -q '?![A-Z][A-Z]*?!') ||
+			test "OK-117" != "$(test_eval_ "(exit 117) && $1${LF}${LF}echo OK-\$?" 3>&1)"
 		then
 			error "bug in the test script: broken &&-chain or run-away HERE-DOC: $1"
 		fi