Date: Mon, 15 Jan 2024 00:13:47 -0600
From: "Daniel Santos via austin-group-l at The Open Group"
<[email protected]>
Message-ID: <[email protected]>
I agree with what you say, but beware:
| Otherwise, I myfn() { local shell_restore=$(set +o | grep 'pipefail$');
| set -o pipefail; <body>; eval "$shell_restore"; }
that needless optimisation attempt is n9t guaranteed to work.
'set +o' generates an implementation defined string which when
executed will restore any options altered between the set, and
executing the output from it, to their values at the time the
set was executed. That's what you want there.
However nothing guarantees that you can extract a line from
that output string, and execute that - in fact the shell might
just output one long set command with lots of +o and -o options
in it (and -x or +x for any which have n0 long names, or just
any which have a 1 letter equiv, just to make the string shorter.
Or all kinds of other techniques. The NetBSD shell does it
like this...
$ set +o
set -o default -o promptcmds -o vi -o xlock -o xtrace
$ set -o pipefail
$ set +o
set -o default -o pipefail -o promptcmds -o vi -o xlock -o xtrace
eval'ing tbe output from the first set +o would restore
things to how they were before, that's the magic "set -o default"
which returns all options to their shell startup values.
(There's a spec for what that means, but it isn't relevant
here).
But if you grep for pipefail you won't find it, so your eval
would end up executing nothing. So forget that attemmpted
optimisation and just save, and then eval, the entire output
from set +o - thhat's what is specified to work, what isn't
specified is how.
If your plan is to allow some other option to be changed by
<body> and persist to the caller, then yoou simply have to
change that option after the eval (perhaps again, if the <body>
also needs the effect of the change). That's rare however.
If you don't care about portability, then some shells offer
simpler mechanisms that are much easier to use, and have
the same effect. But definit;ly not standard mechanisms.
kre
