Most shells 'exec' the last command in -c scripts, e.g.:
$ bash -c 'printf "$$ "; sh -c "echo \$\$"'
66303 66303
$ dash -c 'printf "$$ "; sh -c "echo \$\$"'
66304 66304
$ ksh93 -c 'printf "$$ "; sh -c "echo \$\$"'
66305 66305
$ yash -c 'printf "$$ "; sh -c "echo \$\$"'
66309 66309
$ zsh -c 'printf "$$ "; sh -c "echo \$\$"'
66310 66310
However, no shell seems to do this for scripts loaded from a file:
$ echo 'printf "$$ "; sh -c "echo \$\$"' >/tmp/test.sh
$ chmod +x /tmp/test.sh
$ bash /tmp/test.sh
66325 66326
$ dash /tmp/test.sh
66327 66328
$ /bin/ksh /tmp/test.sh
66329 66330
$ yash /tmp/test.sh
66331 66332
$ zsh /tmp/test.sh
66333 66334
My question: why is this? I would have thought that a script is a script
and that it should make no essential difference whether a script is
taken from a -c argument or loaded from a file. What makes the
optimisation appropriate for one but not the other?
ksh93 used to use this exec-the-last-command optimisation also for
scripts loaded from a regular file, but disabled this (keeping it only
for -c scripts) in version 2011-12-24. The corresponding changelog entry
is characteristically uninformative, so doesn't help answer my question.
Now that a couple of redirection-related bugs have been fixed with that
optimisation, we've re-enabled it for regular scripts in ksh 93u+m ahead
of releasing the first beta of this reboot (Very Soon Now(tm)), but if
there is a good reason that shouldn't be done, we should revert this.
--
|| modernish -- harness the shell
|| https://github.com/modernish/modernish
||
|| KornShell lives!
|| https://github.com/ksh93/ksh
