chmod args (was Re: Need a "watching" program)

[david.w.weatherford]

In article <8605 at chinet.chi.il.us> ignatz at chinet.chi.il.us (Dave Ihnat) writes:
>>In article <12743 at ihlpy.ATT.COM> bdavies at ihlpy.UUCP (55314-Davies,B.) writes:
>>Really.  *I* know what you all mean, but why does everyone teach the
>>octal way when these mnemonic ways exist that are so nice and easy to
>>understand for everyone?  Don't you all believe in abstraction?
>
>Actually, they still have two different and useful functions.  The mnemonic
>method is a mask, while using the octal is a total reset.  So, the mnemonic
>is simpler if I just want to toggle a particular permission; I don't need
>to be sure to not destroy other bits:
>
>Old permission	Desired Permission	Octal arg.	Mnemonic arg.
>777		774			774		o-wx,o+r
>
>Notice that if I didn't know the old permission--say, in a shell script--
>the mnemonic, while still a bit verbose, is still simpler than the shellish
>to extract the old values, mask in only those I want to change, and then
>execute the chmod.
>
>On the other hand, if I'm installing a program and have definite ideas
>about the permissions I want to set, then the octal form is cleaner:
>
>Desired Permission	Octal arg.	Mnemonic arg.
>2710			2710		u+rwx,g+xs,g-rw,o-rwx
>
>At this point, the octal mask is cleaner, since you still have to do the mental
>dance to interpret the now rather long mnemonic string.
>
>Finally, I firmly believe that it's an absolute necessity for even casual
>Unix users to fully understand file permissions; most security violations
>on Unix can be traced to user carelessness, either due to misunderstanding or
>simple lack of care when dealing with file permissions.  There are some quite
>decent interactive tools in the PD to allow naieve users to manipulate their
>permissions without resort to either bitmasks or the rather dense mnemonics
>of chmod, if this is a problem.

Newsgroups: alt.sources
Subject: Re: chmod args (was Re: Need a "watching" program)
Summary: You can used "chmod =rw foo" as easily as "chmod 666"
Expires: 
References: <8923 at csli.Stanford.EDU> <11680 at s.ms.uky.edu> <8928 at csli.Stanford.EDU> <12743 at ihlpy.ATT.COM> <1953 at ur-cc.UUCP> <2126 at amelia.nas.nasa.gov> <8605 at chinet.chi.il.us>
Sender: 
Reply-To: dww at cbnewsl.ATT.COM (david.w.weatherford)
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Distribution: usa
Organization: AT&T Bell Laboratories
Keywords: chmod

In article <8605 at chinet.chi.il.us> ignatz at chinet.chi.il.us (Dave Ihnat) writes:
:>In article <12743 at ihlpy.ATT.COM> bdavies at ihlpy.UUCP (55314-Davies,B.) writes:
:>Really.  *I* know what you all mean, but why does everyone teach the
:>octal way when these mnemonic ways exist that are so nice and easy to
:>understand for everyone?  Don't you all believe in abstraction?
:
:Actually, they still have two different and useful functions.  The mnemonic
:method is a mask, while using the octal is a total reset.

This is not so.  Mnemonic mode has the "=" operator to do "total reset".

:							    So, the mnemonic
:is simpler if I just want to toggle a particular permission; I don't need
:to be sure to not destroy other bits:

:Old permission	Desired Permission	Octal arg.	Mnemonic arg.
:777		774			774		o-wx,o+r

The above example could be expressed "o=r".  Same number of characters,
no danger of munging other bits.

:Notice that if I didn't know the old permission--say, in a shell script--
:the mnemonic, while still a bit verbose, is still simpler than the shellish
:to extract the old values, mask in only those I want to change, and then
:execute the chmod.

:On the other hand, if I'm installing a program and have definite ideas
:about the permissions I want to set, then the octal form is cleaner:

:Desired Permission	Octal arg.	Mnemonic arg.
:2710			2710		u+rwx,g+xs,g-rw,o-rwx

:At this point, the octal mask is cleaner, since you still have to do the mental
:dance to interpret the now rather long mnemonic string.

Use the "=" operator; in most cases it is (almost) as succinct.  Such as,
=rw for 666, =rwx for 777, =r for 444.  If you want different permissions
for each of u, g, and o, then the string can be a bit verbose, as in
"u=rwx,g=rx,o=r" for 751.  Even so, the mnemonic form is more, uh, well,
mnemonic!  The 2710 above could be expressed as "u=rwx,g=xs,o=" much more
succinctly.  Still not as short as the octal form, but clearer, and no
mental gymnastics required.

:Finally, I firmly believe that it's an absolute necessity for even casual
:Unix users to fully understand file permissions; most security violations
:on Unix can be traced to user carelessness, either due to misunderstanding or
:simple lack of care when dealing with file permissions.

No argument here.

:							  There are some quite
:decent interactive tools in the PD to allow naieve users to manipulate their
:permissions without resort to either bitmasks or the rather dense mnemonics
:of chmod, if this is a problem.

I still think that the mnemonics are not "dense" -- I rather like them, but
each to his own.

Dave Weatherford			AT&T Bell Laboratories
attunix!dww				Summit, NJ