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|ID||Category||Severity||Type||Date Submitted||Last Update|
|0001235||[1003.1(2016/18)/Issue7+TC2] Shell and Utilities||Objection||Enhancement Request||2019-03-09 00:46||2019-11-14 14:31|
|Priority||normal||Resolution||Accepted As Marked|
|Section||22.214.171.124 Case Conditional Construct|
|Final Accepted Text||Note: 0004434|
|Summary||0001235: explicitly prohibit strcmp fallback in case statement|
(another follow up to bug:1190)
In the Bourne shell, ksh88 and ksh93:
case [ab] in
[ab]) echo match
outputs "match" which is quite surprising and dangerous as it could bypass input validations like:
case $1 in
) : OK;;
*) echo >&2 not a decimal digit; exit 1;;
Possibly the rationale was to align with another (mis)feature introduced by the Bourne shell, where:
would remove the [ab] file if no file matched the pattern (instead of cancelling the command in earlier sh implementations (and csh, tcsh, fish zsh)).
POSIX currently doesn't allow that ksh88/ksh93 behaviour. But since it is a deviation from the reference implementation and since most certified systems whose shell is based on AT&T ksh still have that non-conformance, it would be nice to make it explicit that that behaviour is not allowed.
Add a conformance test case that rejects that behaviour.
Add a rationale section stating something like:
The Bourne and Korn shells used to revert to a byte to byte comparison when wildcard patterns didn't match in a "case" statement, that behaviour was considered undesirable and is not allowed by this specification.
In case it is not obvious from other notes (attached to other issues) and
from messages on the mailing list, I completely agree with this - any shell
which allows a strcmp() of the pattern and word to be considered a match
is simply abhorrent (regardless of how ancient this practice was). There is
no need for it - one can always simply do
case word in
( pattern | "pattern" ) ... ;;
if it is intended to match a pattern as either a pattern or a string.
I might go a little further, and actually add text to the normative part
of the standard to explicitly outlaw this practice, something like
The shell shall not treat a pattern as a string and apply an additional
match against the pattern treated as if it contained no wildcard characters.
except with better wording.
In any case, we need to make it quite clear that shells which do this (whatever
their heritage) are non-conforming, and that it is not required of an
application (script) to attempt to defeat this behaviour, with code like
case word in
( "pattern" ) the non-match code here;;
( pattern ) the matching code here;;
( * ) the non-match code here ... again;;
as that's revolting, and not always easy to accomplish. Even ;& and
such are no help at all for this.
The only thing in the description I disagree with is labelling of the
glob behaviour of returning an unmatched pattern as a literal string.
Dealing with that is much easier than dealing with the consequences of
producing an error in the case of an unmatched pattern.
Note that in ksh93, the strcmp() seems to be done after backslash removal:
$ a='[a]' ksh -c 'case $a in $a) echo match; esac' match $ a='\a' ksh -c 'case $a in $a) echo match; esac' $ a='[a]' b='[\a]' ksh -c 'case $a in $b) echo match; esac' match
On page 3744 line 128516 section C.2.9.4 Compound Commands, add a new paragraph:
Some historical shells would fall back to doing a byte to byte comparison with each pattern if the pattern matching rules did not produce a match. That behavior is not allowed by this standard because it allows user input to bypass input validations like:case $1 in ) : OK;; *) echo >&2 not a decimal digit; exit 1;; esac
|2019-03-09 00:46||stephane||New Issue|
|2019-03-09 00:46||stephane||Name||=> Stephane Chazelas|
|2019-03-09 00:46||stephane||Section||=> 126.96.36.199 Case Conditional Construct|
|2019-03-11 02:17||kre||Note Added: 0004294|
|2019-03-11 07:05||stephane||Note Added: 0004299|
|2019-03-11 15:53||eblake||Interp Status||=> ---|
|2019-03-11 15:53||eblake||Summary||explicitely prohibit strcmp fallback in case statement => explicitly prohibit strcmp fallback in case statement|
|2019-06-20 10:54||geoffclare||Note Added: 0004434|
|2019-06-20 15:28||geoffclare||Final Accepted Text||=> Note: 0004434|
|2019-06-20 15:28||geoffclare||Status||New => Resolved|
|2019-06-20 15:28||geoffclare||Resolution||Open => Accepted As Marked|
|2019-06-20 15:28||geoffclare||Tag Attached: tc3-2008|
|2019-11-14 14:31||geoffclare||Status||Resolved => Applied|
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