Date: Sun, 13 Mar 2011 19:19:14 -0700 From: Devin Teske <dteske@vicor.com> To: Maxim Khitrov <max@mxcrypt.com> Cc: FreeBSD <freebsd-questions@freebsd.org> Subject: Re: Shell script termination with exit function in backquotes Message-ID: <759A467E-407A-4DB8-9756-08011B5405F0@vicor.com> In-Reply-To: <AANLkTi=-CFmxRicGcosvzhBbM3DMjbWwQNirMrJ1_KP=@mail.gmail.com> References: <AANLkTi=-CFmxRicGcosvzhBbM3DMjbWwQNirMrJ1_KP=@mail.gmail.com>
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On Mar 13, 2011, at 5:26 PM, Maxim Khitrov wrote:
> Hello everyone,
>=20
> I might be doing something dumb here, but this doesn't make sense to
> me. When I run the following script, I would expect to see no output:
>=20
> ----
> #!/bin/sh
>=20
> exit_prog()
> {
> echo -n 'before'
> exit 0
> echo -n 'after'
> }
>=20
> echo line 1: `exit_prog`
> echo line 2:
> echo line 3: `exit 1`
Replace with:
echo line 3: `( exit 1 )`
This executes the command within a sub-shell (preventing the exit call =
from effecting the current shell -- achieving what you desire).
> echo line 4:
> ----
>=20
> The reason I expect to see no output is because 'exit 0' should be
> called before any of the echo lines are allowed to execute. Instead,
> what I get on FreeBSD 7 & 8 is:
>=20
> ----
> line 1: before
> line 2:
> ----
>=20
> I don't understand this because 'exit 0' seems to terminate the call
> to 'exit_prog',
The function call is performed in a sub-shell (internally).
> but the execution of the script continues. However,
> when 'exit 1' is called, the script terminates before printing out the
> last 2 lines.
>=20
> It seems that 'exit' inside a function doesn't work when that function
> is called with backquotes. I assume it has something to do with the
> fact that commands in backquotes are executed in a sub-shell
That is incorrect. Here is a unit-test (to be performed on FreeBSD):
dteske@localhost ~ $ cat << "EOF" | /bin/sh -
> echo PS1=3D$PS1; : `unset PS1`; echo PS1=3D$PS1
> EOF
PS1=3D\[\e[32;1m\]\u@\H \[\e[34m\]\W \$\[\e[0m\]
PS1=3D
The above scriptlet does three things:
1. echo's the contents of the PS1 environment variable.
2. Executes "unset PS1" within back-ticks.
3. re-echo's the contents of the PS1 environment variable.
Upon execution, we see that despite being executed within back-ticks, =
the PS1 variable was wiped-out. If commands on FreeBSD under /bin/sh =
were executed within a sub-shell, said sub-shell would have a separate =
environment that wouldn't effect its parent. Contrast this behavior with =
the following scriptlet:
dteske@localhost ~ $ cat << "EOF" | /bin/sh -
> echo PS1=3D$PS1; : `(unset PS1)`; echo PS1=3D$PS1
> EOF
PS1=3D\[\e[32;1m\]\u@\H \[\e[34m\]\W \$\[\e[0m\]
PS1=3D\[\e[32;1m\]\u@\H \[\e[34m\]\W \$\[\e[0m\]
In the above, we've re-executed the same script but with one minor =
change -- we've placed the "unset PS1" command within parentheses, which =
under FreeBSD's /bin/sh causes the creation of a sub-shell with separate =
environment and exit code, etc.
You can read more on FreeBSD by searching sh(1) for "Grouping Commands =
Together".
> , but the
> behavior is inconsistent.
>=20
> When I run the same script on RHEL using bash, all 4 lines are =
printed:
If you make the changes that I've suggested, you'll have consistent =
execution. The reason you're having inconsistent behavior is because =
Linux has /bin/sh symbolically linked to /bin/bash while FreeBSD has a =
more traditional shell (we'll call it bourne shell "plus").
>=20
> ----
> line 1: before
> line 2:
> line 3:
> line 4:
> ----
>=20
> What's going on here?
>=20
> - Max
> _______________________________________________
> freebsd-questions@freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-questions
> To unsubscribe, send any mail to =
"freebsd-questions-unsubscribe@freebsd.org"
--=20
Cheers,
Devin Teske
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