UNIX for Dummies Questions & Answers

I never understood that, how on earth would a cd scriptwork? cd needs to change the current context's dir, running a script generally creates a new context..

Are you ready for this? In my mind this can only work if the Shell executes the script within the current Shell.

Code:

#!/usr/bin/sh
# @(#)B.11.11_LR 
# This is the executable version of cd implemented using the
# posix shell built-in cd command.
cd "$@"
exit $?

Just tried it. From the command line doesn't work. From a Shell script it doesn't work.
Good catch Corona688.

It's a long story. Some vendor added echo as a builtin command to sh and csh and then dropped the echo binary. This broke lots of stuff. It was easily fixed by installing an echo binary but it generated a lot of interest. Then Posix convened. They said that builtins are great but you must have executable binaries for all builtin commands. They did not really mean "cd" but a strict reading of the spec implied that they did. And an executable version of cd is not tough to write. Solaris 10 has a ksh script. I would post it, but it's copyrighted!

The external cd command is largely useless although HP did concoct a convoluted example of where it could be used it conjuction with "find -exec" to test if it's possible to cd to a directory or not.

BTW I often run /bin/pwd to be sure that I get my physical location.

Code:

$
$ cd /usr/local
$ pwd
/usr/local
$ /bin/pwd
/opt/sfw
$ ls -l /usr/local
lrwxrwxrwx   1 root     root           8 Nov 25  2008 /usr/local -> /opt/sfw
$

Methyl: That must be the fruit of crazed and draconic source control procedures. Or someone in management deciding builtins are evil and bad.
Per: Ah... not too far off, then.

If we can roll the clock back I prefer the binaries which behave exactly as defined in the "man" pages whether or not you specify the path. The various POSIX and BSD extensions are useful in a single platform environment but a real problem when writing portable Scripts. To be fair, there is a similar issue on M$ platforms.
Never had this issue with Novell Netware.


Apologies @gelv2005 for hijacking your thread. Interesting subject with real impact in the commerical world.

It does actually, at least as designed. There is a similar cd executable in Solaris and probably most Unixes, at least those POSIX compliant as the standard mandate that all utilities can be exec-ed, regardless of the usefulness: http://pubs.opengroup.org/onlinepubs...l#tag_23_01_07

/usr/bin/cd works as documented (quoted from Solaris cd manual page Man Page for cd (OpenSolaris Section 1) - The UNIX and Linux Forums ):

DESCRIPTION
/usr/bin/cd
The /usr/bin/cd utility changes the current directory in the
context of the cd utility only. This is in contrast to the
version built into the shell. /usr/bin/cd has no effect on
the invoking process but can be used to determine whether or
not a given directory can be set as the current directory.

Edit: Sorry, I overlook Perderabo had already answered to methyl's posting.
---------- Post updated 21-01-11 at 00:14 ---------- Previous update was 20-01-11 at 23:38 ----------

OpenSolaris has an interesting code that implements all of these utilities:
Cross Reference: /onnv/onnv-gate/usr/src/cmd/ksh/builtins/alias.c

No problem. The fact remains that when invoked as /usr/bin/cd it does not behave the same as a plain "cd" because it starts a new Shell. I am well aware that the many and various POSIX "standards" over the years contain anomalies.


The HP-UX implementation of the binary "pwd" is more interesting because I have used the fact that it behaves correctly (as distinct from the POSIX Shell version) to produce consistent portable scripts.