On 10/4/17 6:10 PM, Martijn Dekker wrote:
> Op 04-10-17 om 17:52 schreef Chet Ramey:
>> It's interesting that other shells treat ${a:=b} as kind of like an
>> assignment statement (word splitting) but not quite (tilde expansion).
>
> Hmm...
>
> v=~/bla
> printf '%s\n' "$v"
>
> outputs /Users/martijn/bla on all shells, so tilde expansion applies for
> a simple assignment anyway.
>
> And
>
> unset -v v
> v=${v:=~/bla}
> printf '%s\n' "$v"
>
> acts exactly the same.
>
> This is all POSIXly correct, IIRC.
That's not quite what I meant. The difference between an assignment context
and a non-assignment word is that Posix specifies that tilde expansion is
performed following `:'.
"In an assignment (see XBD Variable Assignment), multiple tilde-prefixes
can be used: at the beginning of the word (that is, following the
<equals-sign> of the assignment), following any unquoted <colon>, or both."
Given the following script:
HOME=/usr/xyz
XPATH=/bin:/usr/bin:.
ADDPATH=${ADDPATH:-~/bin:~/bin2:$XPATH}
echo "$ADDPATH"
unset ADDPATH
: ${ADDPATH:=~/bin:~/bin2:$XPATH}
echo "$ADDPATH"
unset ADDPATH
Different shells will treat the `:~' tilde prefix differently between the
two statements, since they don't consider the := expansion a variable
assignment.
It would be nice if the shell were consistent in this regard.
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU c...@case.edu http://cnswww.cns.cwru.edu/~chet/
