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Re: How to install a custom permanent postinstall handler?

Am 03.04.2016 um 08:24 schrieb Achim Gratz:
Yaakov Selkowitz writes:
I am not in favour of /bin/sh being alternatives-able.
I'd posit that it should not be bash then and somwone else might
reasonably want a different /bin/sh, perhaps even bash.  Which is
exactly why the alternatives system exists.

The *proper* course of action is to use the shebang for the script
interpreter you require, i.e. /bin/dash.
It's perfectly OK to use /bin/sh (I'd even recommend it) if all you want
is a POSIX shell.  You shouldn't be trying to use local then, but that's
another story (*).



please have in mind that the /bin/sh shell is the system default shell.
On Unix systems many initialisation scripts are executed during start up with the /bin/sh in a way that a shebang line is not used. So these scripts are executed with the standard /bin/sh every time. Because AIX and HP-UX are using a KSH (version 88), SOLARIS is using a Bourne shell version and Linux systems can use different shell types ( mostly DASH, BASH to my knowledge) it is difficult to write scripts to use during system initialization in a portable way.

This comes from the way how the different shells behave then invoked as /bin/sh. See the following section from the bash man page.

If*bash*is invoked with the name*sh*, it tries to mimic the startup behavior of historical versions of*sh*as closely as possible, while conforming to the POSIX standard as well. When invoked as an interactive login shell, or a non-interactive shell with the*--login*option, it first attempts to read and execute commands from//etc/profile/ <file://etc/profile>and/~/.profile/ <file://%7E/.profile>, in that order. The*--noprofile*option may be used to inhibit this behavior. When invoked as an interactive shell with the name*sh*,*bash*looks for the variable*ENV*,expands its value if it is defined, and uses the expanded value as the name of a file to read and execute. Since a shell invoked as*sh*does not attempt to read and execute commands from any other startup files, the*--rcfile*option has no effect. A non-interactive shell invoked with the name*sh*does not attempt to read any other startup files. When invoked as*sh*,*bash*enters/posix/mode
   after the startup files are read.

KSH behave in a similar way.

So it is not easy to write portable scripts for the standard shell /bin/sh between different systems and the exchange of the standard shell needs a check for all scripts, that uses a shebang line /bin/sh or are use during system initialisation.

I personally prefer to use a shebang line set to the shell type used (ksh, bash, dash). This is more portable for different system types.

For putting the /bin/sh under alternatives control on cygwin there must be some preparations. There are several postinstall and preremove scripts without a shebang line or with a shebang line /bin/sh or /usr/bin/sh. Are all of these scripts written to run correctly with a bash and dash invoked ?

Dirk Fassbender

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