How do I change the subshell to something other than /bin/sh?

How do I change the subshell to something other than /bin/sh?

[Julio Garcia]

Hello,

I am sorry if this has been answered already. I could not find and answer 
with google.

My problem is this. I am trying to run some scripts that I cannot change 
which assume capabilities found in bash. I know that zsh will execute them 
correctly as well. However, when I just invoke them from the zsh command 
line, they run under /bin/sh and fail. For example, I have a test script 
that looks like this:

logsol10-1 ~ > cat x.sh

echo $0

ps

The output when I run it is this:

logsol10-1 ~ > ./x.sh
./x.sh
   PID TTY         TIME CMD
24296 pts/8       0:00 zsh
  5205 pts/8       0:00 ps
  5204 pts/8       0:00 sh

Note that it execs sh. How can I tell it to use another shell? zsh or bash, 
for example?

Thanks,

Julio

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Re: How do I change the subshell to something other than /bin/sh?

[Stephane Chazelas]

On Thu, Oct 04, 2007 at 10:29:49AM -0600, Julio Garcia wrote:
[...]
> logsol10-1 ~ > ./x.sh
> ./x.sh
>   PID TTY         TIME CMD
> 24296 pts/8       0:00 zsh
>  5205 pts/8       0:00 ps
>  5204 pts/8       0:00 sh
>
> Note that it execs sh. How can I tell it to use another shell? zsh or bash, 
> for example?
[...]

You probably shouldn't do that. It's probably safer to trust zsh
for the current setting. Normally, if you write POSIX scripts,
you should *not* use the "#! /bin/sh -" lines, and trust the
"system" (if you're in a POSIX environment) for calling the
right shell (that is a POSIX one, which zsh is not by default)
to interpret the file. In the case of zsh, it's not actually the
"system" that decides what shell to call, but zsh. But zsh
should be setup to try and use the same shell as the "system"
would use.

Hmmm, looking at the code, looks like zsh has "/bin/sh"
hardcoded as the path of that shell. That makes it not POSIX on
systems where the POSIX shell is not in /bin like on Solaris.
(on Solaris, execvp(), system()... may call /bin/sh or a
standard shell depending on how you link the executable that
uses those functions).

Given that it's hardcoded, I don't think there's a way you can
change the shell zsh uses in this case.

But you can run call the interpreter directly: bash ./x.sh or
/usr/xpg4/bin/sh ./x.sh.

Or, if you want to call bash as "sh" for it to behave more
POSIXly:

ARGV0=sh bash ./x.sh

-- 
Stéphane

Re: How do I change the subshell to something other than /bin/sh?

[Peter Stephenson]

"Julio Garcia" wrote:
> My problem is this. I am trying to run some scripts that I cannot change 
> which assume capabilities found in bash. I know that zsh will execute them 
> correctly as well. However, when I just invoke them from the zsh command 
> line, they run under /bin/sh and fail. For example, I have a test script 
> that looks like this:
> 
> logsol10-1 ~ > cat x.sh
> 
> echo $0
> 
> ps

If you really can't change the script (and hence can't add a #! line),
you are stuck with running "zsh ./x.sh" wherever you call the script, or
some equivalent of that.  The decision about which interpreter to run
for a file that isn't executable is made by the operating system, so
the's no way of changing it at the user level.  So the question is
really about how you're calling the script and how much you can change
at that point.

-- 
Peter Stephenson <pws@csr.com>                  Software Engineer
CSR PLC, Churchill House, Cambridge Business Park, Cowley Road
Cambridge, CB4 0WZ, UK                          Tel: +44 (0)1223 692070

Re: How do I change the subshell to something other than /bin/sh?

[Stephane Chazelas]

On Thu, Oct 04, 2007 at 05:45:07PM +0100, Peter Stephenson wrote:
> "Julio Garcia" wrote:
> > My problem is this. I am trying to run some scripts that I cannot change 
> > which assume capabilities found in bash. I know that zsh will execute them 
> > correctly as well. However, when I just invoke them from the zsh command 
> > line, they run under /bin/sh and fail. For example, I have a test script 
> > that looks like this:
> > 
> > logsol10-1 ~ > cat x.sh
> > 
> > echo $0
> > 
> > ps
> 
> If you really can't change the script (and hence can't add a #! line),
> you are stuck with running "zsh ./x.sh" wherever you call the script, or
> some equivalent of that.  The decision about which interpreter to run
> for a file that isn't executable is made by the operating system, so
> the's no way of changing it at the user level.  So the question is
> really about how you're calling the script and how much you can change
> at that point.
[...]

On most system, the execve() of a script that doesn't have a #!
line will return with ENOEXEC. In which case, it will be zsh
that will run /bin/sh (if it doesn't find a NUL character in the
first 64 bytes of the file).

That's also what the exec*p() type libc functions do, except that
those may not run /bin/sh but whichever shell is required by the
standard they implement and don't search for NUL bytes.

Maybe in sh emulation, zsh could call itself in sh emulation in
that case?

-- 
Stéphane