Re: Why does a shell script need current directory?

[Chet Ramey]

On 2/13/21 9:17 PM, Peng Yu wrote:
> > The part of the paragraph that you omitted makes it pretty clear
> > that a value of PWD inherited in the environment must correspond
> > to the current working directory (which necessarily has to exist)
> > to be considered.  Even then, there are many cases in which the
> > shell has license to ignore it.
>
> I have to say the POSIX manual is confusing. According to the
> following text, if PWD is specified from the environment, does shell
> have to check the absolute pathname of the current working directory?

Yes. It doesn't have to check if the value from the environment is not
an absolute pathname.

> """
> If a value for PWD is passed to the shell in the environment when it
> is executed, the value is an absolute pathname of   the current
> working directory that is no longer than {PATH_MAX} bytes including
> the terminating null byte, and the value does not contain any
> components that are dot or dot-dot, then the shell shall set PWD to
> the value from the environment.
> """
>
> If so, this doesn't make too much sense. If the shell needs to check
> the absolute path of the current working directory anyway, why does it
> bother whether PWD is specified in the environment. It seems that this
> double requirement is either unnecessary, or I misunderstood
> something.

It's easier to stat "." and the value passed in the environment and check
whether they refer to the same directory. Most of the time they do. Then,
since you have to have the right value of PWD to pass in the environment,
you call getcwd only if that check fails.

--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/