15 Jul
2016
15 Jul
'16
2:27 p.m.
Random832:
The existence of cd as a real command is a bit silly (Ubuntu doesn't
seem to bother with it), but it is technically required by the standard.
===
Just for the record, Fedora 21 supplies /bin/cd, as part
of package bash-4.3.42-1. Interestingly, it is a shell
script:
lu$ cat /bin/cd
#!/bin/sh
builtin cd "$@"
lu$
As has been said here, it's hard to see the functional point.
Others have remarked on the continued life of /bin/true and
/bin/false. There are some who use those as shells in /etc/passwd
for logins that should never actually be allowed to do anything
directly. I have no strong personal feeling about that, I'm just
reporting.
And to be fair (as has also already been displayed here), the
copyright notice inserted in the once-empty /bin/true was hundreds
of bytes long, not thousands. Let us call out silliness, but let
us not make it out as any sillier than it actually is.
Norman Wilson
Toronto ON
UNIX old fart and amateur pedant
10 Sep
10 Sep
9:45 a.m.
New subject: Shell control through external commands
Il 15/07/2016 alle 14:27, Norman Wilson ha scritto:
Just for the record, Fedora 21 supplies /bin/cd, as
part
of package bash-4.3.42-1. Interestingly, it is a shell
script:
lu$ cat /bin/cd
#!/bin/sh
builtin cd "$@"
lu$
But doesn't this change the current dir only in the child shell?
Which then exits right after the second line, parent shell's $PWD
unaffected. I really don't see how this script is useful.
11:13 a.m.
New subject: Shell control through external commands
On 10 Sep 2016 09:45 +0200, from dnied(a)tiscali.it (Dario Niedermann):
Il 15/07/2016 alle 14:27, Norman Wilson ha scritto:
It does appear rather useless. Curiously, Debian (checked on Wheezy =
bash 4.2+dfsg-0.1+deb7u3 and Jessie = bash 4.3-11+b1) seems to not
supply anything like that, so it would appear to be some kind of
Fedora-ism rather than a part of anything upstream; that, or the
Debian folks are actually paying attention to what they ship onto
users' systems.
Even more curious, check this out from Debian Wheezy (I suspect the
Jessie system would behave identically):
$ cat > /michael/home/bin/cd
#!/bin/sh
builtin cd "$@"
^D
$ chmod +x bin/cd
$ echo $PATH
/michael/home/bin:/usr/local/bin:/usr/bin:/bin:/usr/local/games:/usr/games
$ type cd
cd is a shell builtin
$
so not even the shell will use the script in the first place!
If I force the shell to use the script I created (by saying ~/bin/cd
instead of just cd), after fixing /bin/sh to /bin/bash because when
run as sh the shell has no concept of "builtin", it behaves like you'd
expect: the working directory changes _within_ the script, but $PWD is
unchanged in the parent shell once the script finishes, turning it
into a somewhat expensive no-op.
Unless Fedora's shell has been specifically modified to pay attention
to the child process' $PWD, of course...
--
Michael Kjörling • https://michael.kjorling.se • michael(a)kjorling.se
“People who think they know everything really annoy
those of us who know we don’t.” (Bjarne Stroustrup)
lu$ cat /bin/cd
#!/bin/sh
builtin cd "$@"
lu$
But doesn't this change the current dir only in the child shell?
Which then exits right after the second line, parent shell's $PWD
unaffected. I really don't see how this script is useful. 
11:41 a.m.
New subject: Shell control through external commands
Michael Kjörling <michael(a)kjorling.se> wrote:
On 10 Sep 2016 09:45 +0200, from dnied(a)tiscali.it
(Dario Niedermann):
POSIX requires some commands to be callable via exec().
Jörg
--
EMail:joerg@schily.net (home) Jörg Schilling D-13353 Berlin
joerg.schilling(a)fokus.fraunhofer.de (work) Blog: http://schily.blogspot.com/
URL: http://cdrecord.org/private/
http://sourceforge.net/projects/schilytools/files/'
Il 15/07/2016 alle 14:27, Norman Wilson ha
scritto:
It does appear rather useless. Curiously, Debian (checked on Wheezy =
bash 4.2+dfsg-0.1+deb7u3 and Jessie = bash 4.3-11+b1) seems to not
supply anything like that, so it would appear to be some kind of
Fedora-ism rather than a part of anything upstream; that, or the
Debian folks are actually paying attention to what they ship onto
users' systems. lu$ cat /bin/cd
#!/bin/sh
builtin cd "$@"
lu$
But doesn't this change the current dir only in the child shell?
Which then exits right after the second line, parent shell's $PWD
unaffected. I really don't see how this script is useful. 
7:06 p.m.
New subject: Shell control through external commands
On Sat, Sep 10, 2016, at 05:13, Michael Kjörling wrote:
On 10 Sep 2016 09:45 +0200, from dnied(a)tiscali.it
(Dario Niedermann):
> Il 15/07/2016 alle 14:27, Norman Wilson ha scritto:
>> lu$ cat /bin/cd
>> #!/bin/sh
>> builtin cd "$@"
>> lu$
>
> But doesn't this change the current dir only in the child shell?
> Which then exits right after the second line, parent shell's $PWD
> unaffected. I really don't see how this script is useful.
I've wondered about this in the past, the conclusion was that it is
something that is technically required by POSIX. There is a general
requirement that "regular builtin" commands [with no exception for cd]
"shall be implemented in a manner so that they can be accessed via the
exec family of functions as defined in the System Interfaces volume of
POSIX.1-2008 and can be invoked directly by those standard utilities
that require it (env, find, nice, nohup, time, xargs)."
7:45 p.m.
New subject: Shell control through external commands
Il 10/09/2016 alle 11:41, Joerg Schilling ha scritto:
POSIX requires some commands to be callable via
exec().
Il 10/09/2016 alle 19:06, Random832 ha scritto:
something that is technically required by POSIX. There
is a general
requirement that "regular builtin" commands [with no exception for cd]
"shall be implemented in a manner so that they can be accessed via the
exec family of functions as defined in the System Interfaces volume of
POSIX.1-2008 and can be invoked directly by those standard utilities
that require it (env, find, nice, nohup, time, xargs)."
TIL! Still, it seems to me that there is no way the `cd' script can
actually accomplish anything, even when called by exec().
9:53 p.m.
New subject: Shell control through external commands
On Sat, Sep 10, 2016 at 1:45 PM, Dario Niedermann <dnied(a)tiscali.it> wrote:
Il 10/09/2016 alle 11:41, Joerg Schilling ha scritto:
It can return an exit status: this makes it mostly equivalent to something
like 'test -d /foo && test -x /foo'.
- Dan C.
POSIX requires some commands to be callable via
exec().
Il 10/09/2016 alle 19:06, Random832 ha scritto:
something that is technically required by POSIX.
There is a general
requirement that "regular builtin" commands [with no exception for cd]
"shall be implemented in a manner so that they can be accessed via the
exec family of functions as defined in the System Interfaces volume of
POSIX.1-2008 and can be invoked directly by those standard utilities
that require it (env, find, nice, nohup, time, xargs)."
TIL! Still, it seems to me that there is no way the `cd' script can
actually accomplish anything, even when called by exec().
10:02 p.m.
New subject: Shell control through external commands
On 9/10/16 1:45 PM, Dario Niedermann wrote:
TIL! Still, it seems to me that there is no way the `cd' script can
actually accomplish anything, even when called by exec().
Use it as a permissions check.
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU chet(a)case.edu http://cnswww.cns.cwru.edu/~chet/
10:27 p.m.
New subject: Shell control through external commands
Il 10/09/2016 alle 22:02, Chet Ramey ha scritto:
Use it as a permissions check.
Il 10/09/2016 alle 21:53, Dan Cross ha scritto:
It can return an exit status: this makes it mostly
equivalent to something
like 'test -d /foo && test -x /foo'.
Yeah, good point.
11 Sep
11 Sep
12:03 p.m.
New subject: Shell control through external commands
On 10/09/2016 20:45, Dario Niedermann wrote:
TIL! Still, it seems to me that there is no way the
`cd' script can
actually accomplish anything, even when called by exec().
# Time directory access
/usr/bin/time cd /mounts/foo
# Verify that directories can be cd'd
find /mounts/foo -type d -exec cd '{}' \; && echo OK
2988
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10 comments
9 participants
participants (9)
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Chet Ramey -
Dan Cross -
Dario Niedermann -
Diomidis Spinellis -
Jacob Goense -
Michael Kjörling -
Norman Wilson -
Random832 -
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