Re: bug (feature?) in zsh

["William H. Magill" <magill@mcgillsociety.org>]

On 16 Jan, 2005, at 01:33, Bart Schaefer wrote:

> On Jan 16,  2:16am, Toshiro wrote:
> } Subject: bug (feature?) in zsh
> }
> } I'm using Debian unstable, zsh version 4.2.2. I've noticed that, 
> when I
> } install a new package that puts some program in my path (for
> } example, /usr/bin) I can't invoke the program using completion in 
> any zsh
> } shell that I have opened before installing the program.
>
> This is almost reaching FAQ status, but isn't yet in the FAQ.
>
> The completion system (including compctl) uses the command hash table 
> to
> look up names for completion in command position, because it's 
> expensive
> to search the path for them.  That means that if you install some new
> piece of software, you need to run 'rehash' before the completion 
> system
> will see it, as you discovered.
>
> See the zsh-users thread with subject "HASH_LIST_ALL?" from October 
> 2004
> for more discussion; in particular, article 8059 presents a technique
> for working around this behavior

The use of a hash table for command execution/completion was 
implemented with csh -- many, many years ago. Those shells which have 
descended from csh (tcsh, zsh) behave in this same fashion. It is the 
expected behavior.

Sh behavior was always different. It did not use a hash table. 
Consequently shells which descended from sh (new sh, bash, ksh) do not 
have a hash table.

This difference was mainly because the "csh" was intended as an 
"interactive" shell, and therefore included features (like command 
completion) which made usage on what at the time were the standard 
input devices -- 110 baud ttys -- much easier on the operator. By 
comparison, sh, was deemed a "batch" shell where the full command paths 
were expected to be utilized, both as "documentation" in the script and 
to assure that the "environment" used by the shell was explicitly what 
the script writer wanted, and not that of the entity using the shell.  
(Note that this is still considered "Best Practice." And that it is 
required for most implementations of "cron" in use today -- the 
environment available to the cron job is that of process 1, NOT that of 
the User.)

Note also that I use the term "descended from" quite liberally. In the 
beginning there was only Bell Labs and it was called "sh." "sh" was 
replaced by "new sh" (frequently called sh5) when it became System V. 
Meanwhile BSD created csh to run on top of sh. All Unix variants and 
Linux variants boot into some form of "sh." Today, I believe all 
variants of Unix and Linux run "new sh" for process 0, which in turn 
spawns init. Only after User processes are spawned do the /etc/shells 
options come into play.

That this is not in the FAQ is not really surprising. Interesting, but 
not surprising.

For some strange reason, throughout the history of Unix, the concept of 
"rehash" has always been a deeply guarded secret of the Unix Wizards 
... only taught to the truly deserving; neophytes who sought to follow 
the ways of the guru. Others were simply told, "Logout and log back in, 
and it will work."

For the record, the "strange reason" is the difference between the 
"System Administrator" (aka root) and the "User" -- everybody else. The 
System Administrator is technically the only one who installs (or can 
install) software available to anyone other than the person installing 
it. "Technically," the System Administrator only does such a thing on a 
"closed" system -- i.e. one not permitting user logins (/etc/nologin). 
Therefore, the SysAdmin needs to "rehash" for testing purposes, but 
everyone else will "automagically" see the new information when they 
login.

Today, there are probably far more "SysAdmins" supporting "single-user" 
boxes, especially under Linux, than there are SysAdmins supporting 
multi-user, time-sharing systems.  This is especially true with the 
widespread use of sudo as opposed to actually logging in as the root 
user.

T.T.F.N.
William H. Magill
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