On 3/10/20 3:28 AM, Laurent Vivier wrote:
Le 09/03/2020 à 20:19, Unai Martinez-Corral a écrit :
All the tests are prefixed with 'x', in order to avoid risky comparisons
(i.e. a user deliberately trying to provoke a syntax error).
With the quotes I don't see how we can provoke a syntax error.
Could you provide an example?
Historically, in some shells:
foo=\(
bar=\)
if [ "$foo" = "$bar" ]; then echo hello world; fi
could output 'hello world' (by parsing a parenthesized one-argument
test, and the string '=' is non-empty), but:
if [ "x$foo" = "x$bar" ]; then echo goodbye; fi
did not (since no operator begins with 'x', you have guaranteed the
syntax that [ will parse). Similarly, if foo=! or foo=-a, you could get
syntax errors (if [ tried to treat the expansion of $foo as an operator
and got thrown off by the remaining arguments not matching an expected
pattern).
These days, POSIX says that with three arguments when the 2nd is a
binary operator, there is no ambiguity (the binary operator takes
precedence over the ( and ) around the non-empty string test), and
modern bash obeys the POSIX rule without needing the x prefix. But it
is still better to prefix with x for copy-paste portability to older
shells that do not match current POSIX rules.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
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