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Re: [Help-bash] inconsistency in expansions

From: John Kearney
Subject: Re: [Help-bash] inconsistency in expansions
Date: Mon, 07 May 2012 15:04:12 +0200
User-agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:12.0) Gecko/20120428 Thunderbird/12.0.1

Am 07.05.2012 13:35, schrieb Dan Douglas:
On Monday, May 07, 2012 12:15:16 PM John Kearney wrote:
Am 07.05.2012 02:07, schrieb Dan Douglas:
${$x} isn't a valid expansion, so it throws an error, the pattern
isn't matched, and nothing happens. ${$#} expands to the shell's PID,
and tries to chop off a pattern matching the empty string at the
beginning, which doesn't match the pattern at the end of "$*", so
nothing happens. "$3" expands to "canary", which does match the
pattern, so the string is modified.

Dan Douglas
Actually I'm a bit confused now.
Why this doesn't work

Anyway as a work around these work.

or for your example
echo "${*%${@:(-1):1}}"
echo "${*%${@:${#}:1}}"

You're mistaken about the form of a parameter expansion. A parameter expansion 
in Bash is either `$name', or consists of: `${' followed by potentially either 
a `!' or `#' followed by a "parameter name", followed by an optional 
"operator", followed by a context whose evaluation depends upon the preceeding 
operator, followed by a closing brace.

I can't guess how you were expecting ${${#}} to be interpreted. Perhaps You 
meant ${!#} ? (which is sort of a hack I wouldn't recommend for clarity.) The 
"parameter name" I mentioned above MUST be of the form `var' or `arr[expr or @ 
or *]`. No "nesting" of expansions can ever take place before the "operator" 
except within an array index specification. If the prefix is !, then the value 
of the given parameter must also be of the aforementioned form. Typically, 
expansions may be nested to the right of the operator.



is a valid parameter expansion.

Some fun with the '+' substitution:

 $ rev() { ${2+rev "${@:2}"}; printf '%s ' "$1"; }; rev {1..5}
5 4 3 2 1

Firstly I never said i didn't understand why ${$#} didn't work that was clear enough.

The how I got there is
address@hidden   equiv to address@hidden or $@
${array[1]}  equiv to $1 or ${1}
${array[10]}  equiv to ${10}
so ....
${array[-1]}   not equiv to ${-1}
address@hidden  not  equiv to ${${#}}
I can understand why this is tricky for the parser.
 I think i keep forgetting that this doesn't work. but it does seem a kinda natural step.

$1 to ${1} to ${$#} or even ${-1}
because ${value:(-1)} works, I even gave ${($#})} a go out of curiosity.
the confusing part is that $1 ...$9 are handled special.
a lot of noob also think this should work address@hidden
so instead we use the split functionality and do ${@:$#:1}

Just trying to point out why it confuses me every now and again.

          John Kearney's profile on LinkedIn John

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