Re: [Bug-tar] --auto-compress command line option

[Helmut Waitzmann]

Denis Excoffier <address@hidden> writes:

>I'm not sure i understand all the implications of what Helmut has
>written. I can add:
>- of course the configure options would not be mandatory, and if not
>  set, the behaviour is like now

If the system administrator has configured tar to use a fixed location of
the compressing programs rather than to search using the PATH environment
variable, then there is nothing, a user could do against that, except:

>- at individual level, to use a compress program of your own, you always
>  can take benefit of the --use-compress-program command line option

thus the options '-z', '-Z', '-j' won't work anymore for that user.

>- at site level, the selection of which versions of the
>  compress/gzip/bzip2/lzma programs to be used is done by the same
>  people as those that select (and configure, build, install) the
>  version of tar

Yes, it's the administrator's choice.  But the users should be given the
chance to override it.

>- with the --auto-compress option, you don't know in advance which
>  compress program will be used, therefore in my opinion you cannot use
>  --use-compress-program in this case

I don't know the '--auto-compress' option (it's not listed yet in
"http://www.gnu.org/software/tar/manual/html_node/tar_42.html#SEC42";),
but probably it's the same problem as with  '-z', '-Z' or '-j'.

>- yes perhaps my proposal would also need the addition of a bypass
>  option, in order to the PATH to be used instead of the full path
>  selected at configure time

Yes.  But than I would rather like to see an option which tells tar to
use the full path selected at configure time, thereby let tar by default
do a PATH search:  This would keep tar backward compatible with existing
versions of tar.  Someone, who'd like to use the new behavior, could put
that option into the TAR_OPTIONS environment variable.

>- i only wish to get the same mechanism as with the tar program
>  itself. You can say:
>  (PATH=/site/local/bin; tar etc.)
>  but you can also say
>  (/site/local/bin/tar etc.)
>  that is: not relying on the PATH if you don't want to
>  If you need to enforce a specific set of compress programs, you  have to
>  wrap tar
>  into a subshell like the one Helmut proposes, this is too heavy in  my
>  opinion, or...

... make sure, that the location of that specific set of compress
programs is the first component of the PATH environment variable.

>  please Helmut show us your login shell!!!

When you (interactively) log in to your account, then your shell will be
invoked as a login shell by default (so it's quite normal to have a login
shell).

A login shell first reads commands from several startup files before it
executes the given shell script or command line (supplied as a parameter
to the option "-c") or prompts for commands to be executed.  (For details
of what a login shell does, see the manual page of the shell.)

For example, a POSIX shell, when invoked as a login shell, first reads
commands from the files "/etc/profile" and "$HOME"/.profile, if they
exist, prior to do anything else.

Thus a system administrator can (and really should) set the PATH
environment variable by writing an appropriate command in the file
"/etc/profile".

Then users may change that variable in their "$HOME"/.profile files, if
they want to, thereby being able to "configure" their own versions of
"gzip", "compress", ...  programs.

A common pitfall is to use "su" without its option "-" when changing to
a different user, because "su" invokes the new user's startup shell as a
non-login shell unless given the option "-".

As a non-login startup shell is almost never a good idea (not just
because of the missing initialization of the PATH environment variable,
but also for further parts of a process's environment, e.g. the umask),
specifying the compress programs for tar at configure time wouldn't help
much anyway.

To make a long story short:

If you want GNU tar to always use some special compress programs, you may
put them in a directory of their own and then name the path of that
directory as the first component of the PATH environment variable:

If you want to do that as the system administrator, change the "export
PATH=..." command in the file "/etc/profile", for example:

export PATH=/site/local/bin:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin

If you want to do that as an ordinary user, you may put the command

export \
PATH=/site/local/bin:"${PATH:-/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin}"

into the file "$HOME"/.profile.

Kind regards,

Helmut
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