cc: [email protected] Subject: Re: Re: [ast-developers] Reverse redirection / assignment order --------
On 10 January 2013 23:06, Cedric Blancher <[email protected]> wrote: > David, do you have any idea why ksh93 doesn't use POSIX order in the > example below? > > Ced > > ---------- Forwarded message ---------- > From: Dan Douglas <[email protected]> > Date: 9 January 2013 20:00 > Subject: Reverse redirection / assignment order > To: [email protected] > > > When expanding simple commands, steps 3 and 4 are reversed unconditionally for > all command types and number of words expanded, even in POSIX mode. > http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_1 8_09_01 > > The exceptions allowed by POSIX appear to only apply to ksh93. Other shells > always use the POSIX order, except Bash, which never uses the POSIX order, > though the manpage description is the same as POSIX. > > #!/usr/bin/env bash > > # 1) no command expanded, 2) special builtin, 3) regular builtin. > tst() { > "$sh" -c 'x=$(printf 2 >&2) ${1+"$1"} <&0$(printf 1 >&2)' _ "$@" > } 2>&1 > > for sh in {,{b,d}a,po,{,m}k,z}sh bb; do > printf '%-4s: %s %s %s\n' "$sh" "$(tst)" "$(tst :)" "$(tst true)" > done > > Out: > sh : 21 21 21 # bash posix mode > bash: 21 21 21 # normal mode > ksh : 21 21 12 # ksh93 is the other oddball shell > dash: 12 12 12 # ... > ... # Everything else same as dash > > I don't know why this order was chosen or what the advantages to one over the > other might be. > -- > Dan Douglas > > > -- > Cedric Blancher <[email protected]> > Institute Pasteur According to the standard, the normal order or expansion is command arguments, redirections, and then assignments. Thus the normal order is 12. However, for special builtins, the standard says that the order may be reversed. ksh93 does this for special builtins for backwards compatibility with the Bourne shell so that old scripts continue to run. Since : is a special builtin, and true is not, : does variable assignments before redirection and true does redirections before variable assignments. David Korn [email protected] _______________________________________________ ast-developers mailing list [email protected] http://lists.research.att.com/mailman/listinfo/ast-developers
