[Discussion] POSIX, the Love of Backticks and All That Jazz Post: 302990689

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Top Forums UNIX for Advanced & Expert Users [Discussion] POSIX, the Love of Backticks and All That Jazz Post 302990689 by bakunin on Monday 30th of January 2017 06:56:04 PM

01-30-2017

Registered User

Quote:

Originally Posted by Peasant

So if you have small, efficient script with no nested subshells, are backquotes still the lowest denominator e.g will it run without magic cookie on any unix OS ever produced ?

I would never run the risk of having whatever shell executing my code. Out of pure paranoia you won't see any script without a shebang line stating the shell it is intended to run in explicitly.

You see, i once wrote shell scripts for an installed base of ~22k systems worldwide, ranging from IBM 42Ts and 43Ps (desktop systems) up to big SP/2s and everything in between. Believe me, whatever administrative horror you are able to think of - 5 systems somewhere had exactly this and then some. root not allowed to write a file in /tmp? Filesystems with no more free inodes? A user named "root" but with a non-0 UID? I have seen all this and more.

Most of my scripts are not "small" therefore. Not, because they do so much, but because they take extreme care of error handling and take nothing i can think of for granted.

Quote:

Originally Posted by Peasant

Example would be a simple mv script using basename command in subshell.

basename is IMHO a bad example. I would do that with parameter expansion as (in the interest of speed) anything else that could be done with it.

bakunin

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LEARN ABOUT LINUX

alien::package::rpm

Alien::Package::Rpm(3pm)				User Contributed Perl Documentation				  Alien::Package::Rpm(3pm)

NAME

       Alien::Package::Rpm - an object that represents a rpm package

DESCRIPTION

       This is an object class that represents a rpm package. It is derived from Alien::Package.

FIELDS

       prefixes
	   Relocatable rpm packages have a prefixes field.

METHODS

       checkfile
	   Detect rpm files by their extention.

       install
	   Install a rpm. If RPMINSTALLOPT is set in the environement, the options in it are passed to rpm on its command line.

       scan
	   Implement the scan method to read a rpm file.

       unpack
	   Implement the unpack method to unpack a rpm file. This is a little nasty because it has to handle relocatable rpms and has to do a bit
	   of permissions fixing as well.

       prep
	   Prepare for package building by generating the spec file.

       cleantree
	   Delete the spec file.

       build
	   Build a rpm. If RPMBUILDOPT is set in the environement, the options in it are passed to rpm on its command line.

	   An optional parameter, if passed, can be used to specify the program to use to build the rpm. It defaults to rpmbuild.

       version
	   Set/get version.

	   When retreiving the version, remove any dashes in it.

       postinst
       postrm
       preinst
       prerm
	   Set/get script fields.

	   When retrieving a value, we have to do some truely sick mangling. Since debian/slackware scripts can be anything -- perl programs or
	   binary files -- and rpm is limited to only shell scripts, we need to encode the files and add a scrap of shell script to make it
	   unextract and run on the fly.

	   When setting a value, we do some mangling too. Rpm maintainer scripts are typically shell scripts, but often lack the leading shebang
	   line.  This can confuse dpkg, so add the shebang if it looks like there is no shebang magic already in place.

	   Additionally, it's not uncommon for rpm maintainer scripts to contain bashisms, which can be triggered when they are ran on systems
	   where /bin/sh is not bash. To work around this, the shebang line of the scripts is changed to use bash.

	   Also, if the rpm is relocatable, the script could refer to RPM_INSTALL_PREFIX, which is set by rpm at run time. Deal with this by
	   adding code to the script to set RPM_INSTALL_PREFIX.

       arch
	   Set/get arch field. When the arch field is set, some sanitizing is done first to convert it to the debian format used internally. When
	   it's retreived it's converted back to rpm form from the internal form.

AUTHOR

       Joey Hess <joey@kitenet.net>

perl v5.12.3							    2011-06-11						  Alien::Package::Rpm(3pm)