Copyright © 1998 IEEE. All rights reserved.
Interpretation Number: #157
Topic: shell
Relevant Clauses: 3.9.1
Interpretation Request:
-----------------------
A) 3.9.1 (Simple Commands) has the text (page 135, lines 725 to 727):
When a given simple command is required to be executed (i.e. when any
conditional construct such as an AND-OR list or a case statement has
not bypassed the simple command) ...
Does this present a requirement on the time of execution of such a command
in relation to tokenization and parsing, i.e. that it happens as soon as a
simple command has been identified and it is known that it is the next
simple command to be executed? Specifically, consider a script containing
alias foo=bar; foo
where foo is not previously defined as a function or alias. Which (if
either) of the following behaviours are conforming?
(a) The command foo is executed, the alias command not having been
executed by the time the token foo was delimited;
(b) The command bar is executed, the alias command having been executed by
the time the token foo was delimited?
B) 3.2.3 (Double Quotes) (page 118, lines 55 to 59) says, concerning the
$( ... ) construct
The tokenizing rules in 3.3 shall be applied recursively to find the
matching ).
Does the reference to the tokenizing rules in 3.3 include alias
substitution (3.3.1)?
If so, then:
(a) Is the expansion of an alias encountered included in the text of
the command to be substituted (in place of the original text), despite
the provision to the contrary in 3.3(5) (page 120, line 125)?
(b) May a ) in the value of the alias terminate the $( ... )
construct?
(c) Is alias expansion also applied when and if command substitution
is applied?
(d) Since alias substitution requires recognition of reserved words
in correct grammatical context, its application here requires parsing
of the command whose output shall be substituted to occur in the
process of finding the matching ). Is the behaviour in the event of
a grammatical error defined (since recovery is necessary to continue
to determine whether aliases should be substituted)? For example, may
errors or diagnostics occur from the following?
true || $(if; foo)
If not, then:
Is the reference to parsing of the command whose output is to be
substituted in E.3.2 (page 823, line 2975) in error?
C) 3.3 (Token Recognition) allows the formation of a token that is
delimited by the end of input.
(a) This allows a token with an opening quote (single or double) but no
closing quote. Must the end of input be considered to end the quoted
text, or may an implementation consider this a syntax error.
Specifically, must
sh -c "echo '\$foo"
output the four characters $foo (quote removal having been applied to the
single ') without error, and may a conforming application use this
construct?
(b) If the end of input leaves a ${, $(, $(( or ` construct unterminates,
is the effect defined?
D) 3.2.3 (page 119, lines 71 to 74) provides that the constructs
"`echo hello"
and
`echo "foo`
produce undefined results. Are the results undefined if these constructs
are encountered during token recognition, or only if expansions need to be
performed on them?
E) 3.3.1 (Alias substitution) does not clearly state what is to be done
with the value of an alias that is substituted. Is it required that the
resulting text itself be tokenized? If so, is the value of the alias
treated as a separate input source, so that any final token is delimited
by the end of the value of the alias; if not, what are the effects of the
following definitions and uses of aliases?
alias foo="'"
foo bar'
alias bar="echo >"
bar>baz
Interpretation for IEEE Std 1003.2-1992:
----------------------------------------
A. The standard clearly states (page 120 line 149-150) that
tokenization occurs before any grammatical rules are applied.
All compound commands are tokenized in their entirety before they
are executed. The grammar rules show that the whole line shall be
read before tokenization. Therefore the command shall be "foo"
and never "bar". This also means that a function
f() {
alias foo=bar
foo
}
will execute "foo" and not "bar", since the entire function
is tokenized before alias substition takes place.
B. The standard appears to mandate that alias substituion should occur,
however, concerns have been raised about this which are being
referred to the sponsor.
Historically shells have not done this, and the standard would
appear to be in error.
C. a. The standard does not speak to this issue
of whether the single-quoted (or double-quoted, or back-quoted)
string is correctly terminated by this end of token, and
therefore it is acceptable for an implementation to treat this as an error.
As such no conformance distinction can be made
between alternative implementations based on this.
b. For ${, $( or $(( the standard clearly states tha the
effect is defined as a syntax error --
the tokenization completes, but the resulting grammar is in
error. For ', the effect is as above (C subsection a). For `, the effect should
also be as above (the back-quoted string is a single token).
D. The standard clearly states that the constructs are undefined in all cases.
E. The standard clearly states in 3.3.1 alias substitution along
with 5.1, the definition for the alias utility, that the
alias provides a string not a set of tokens that are interpreted/parsed
when the alias substitution occurs. There is nothing in the standard that
states that there is an end of token at the end of an alias.
IEEE Rationale:
---------------
None.
Back to IEEE Standards Interpretations for IEEE Std 1003.2-1992