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Add documentation about indirect variable expansion

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axel 2006-03-02 21:51:08 +10:00
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@ -450,22 +450,22 @@ personalized Japanese greeting.
The {$USER}san syntax might need a bit of an elaboration. Posix
shells allow you to specify a variable name using '$VARNAME' or
'${VARNAME}'. Fish only supports the former, but has no support
whatsoever for the latter or anything remotely like it. So what is
'{$VARNAME}' then? Well, '{WHATEVER}' is <a href='#brace'>brace
expansion</a>, the same as supported by Posix shells, i.e. 'a{b,c}d'
-> 'abd acd' works both in bash and on fish. So '{$VARNAME}' is a
bracket-expansion with only a single element, i.e. it becomes
expanded to '$VARNAME', which will be variable expanded to the value
of the variable 'VARNAME'. So you might think that the brackets don't
actually do anything, and that is nearly the truth. The snag is that
there once along the way was a '}' in there somewhere, and } is not a
valid character in a variable name. So anything after the otherwise
pointless bracket expansion becomes NOT a part of the variable name,
even if it happens to be a legal variable name character. That's why
'{$USER}san' looks for the variable '$USER' and not for the variable
'$USERsan'. It's a case of one syntax lending itself nicely to
solving an unrelated problem in it's spare time.
'${VARNAME}'. Fish supports the former, and has no support whatsoever
for the latter or anything like it. So what is '{$VARNAME}' then?
Well, '{WHATEVER}' is <a href='#brace'>brace expansion</a>, identical
to that supported by Posix shells, i.e. 'a{b,c}d' -> 'abd acd' works
both in bash and on fish. So '{$VARNAME}' is a bracket-expansion with
only a single element, i.e. it becomes expanded to '$VARNAME', which
will be variable expanded to the value of the variable 'VARNAME'. So
you might think that the brackets don't actually do anything, and that
is nearly the truth. The snag is that there once along the way was a
'}' in there somewhere, and } is not a valid character in a variable
name. So anything after the otherwise pointless bracket expansion
becomes explicitly NOT a part of the variable name, even if it happens
to be a legal variable name character. That's why '{$USER}san' looks
for the variable '$USER' and not for the variable '$USERsan'. It's
simply a case of one syntax lending itself nicely to solving an
unrelated problem in it's spare time.
Variable expansion is the only type of expansion performed on double
quoted strings. There is, however, an important difference in how
@ -479,6 +479,29 @@ result in exactly one argument. Undefined variables will expand to the
empty string, and array variables will be concatenated using the space
character.
There is one further notable feature of fish variable
expansion. Consider the following code snippet:
<pre>
set foo a b c
set a 10; set b 20; set c 30
for i in (seq (count $$foo))
echo $$foo[$i]
end
# Output is:
# 10
# 20
# 30
</pre>
The above code demonstrates how to use multiple '$' symbols to expand
the value of a variable as a variable name. One can simply think of
the $-symbol as a variable dereference operator. When using this
feature together with array brackets, the brackets will always match
the innermost $ dereference. Thus, $$foo[5] will always mean the fift
element of the foo variable should be dereferenced and never that the fift
element of the doubly dereferenced variable foo. The latter can
instead be expressed as $$foo[1][5].
\subsection expand-home Home directory expansion