Shell Parameters

The shell has many predefined parameters that may be accessed. Here are some examples:

% sleep 10 &
[1] 3820
% echo $!
3820
% set a b c
% echo $#
3
% echo $ARGC
3
% ( exit 20 ) ; echo $?
20
% false; echo $status
1

($? and $status are equivalent.)

% echo $HOST $HOSTTYPE
dendrite sun4
% echo $UID $GID
701 60
% cd /tmp
% cd /home
% echo $PWD $OLDPWD
/home /tmp
% ls $OLDPWD/.getwd 
/tmp/.getwd

~+ and ~- are short for $PWD and $OLDPWD, respectively.

% ls ~-/.getwd
/tmp/.getwd
% ls -d ~+/learning
/home/learning
% echo $RANDOM
4880
% echo $RANDOM
11785
% echo $RANDOM
2062
% echo $TTY
/dev/ttyp4
% echo $VERSION
zsh v2.00.03
% echo $USERNAME
pf

The cdpath variable sets the search path for the cd command. If you do not specify . somewhere in the path, it is assumed to be the first component.

% cdpath=( /usr ~ ~/zsh )
% ls /usr
5bin         dict         lang         net          sccs         sys
5include     etc          lector       nserve       services     tmp
5lib         export       lib          oed          share        ucb
adm          games        local        old          skel         ucbinclude
bin          geac         lost+found   openwin      spool        ucblib
boot         hosts        macsyma_417  pat          src          xpg2bin
demo         include      man          princeton    stand        xpg2include
diag         kvm          mdec         pub          swap         xpg2lib
% cd spool
/usr/spool
% cd bin
/usr/bin
% cd func
~/func
% cd 
% cd pub
% pwd
/u/pfalstad/pub
% ls -d /usr/pub
/usr/pub

PATH and path both set the search path for commands. These two variables are equivalent, except that one is a string and one is an array. If the user modifies PATH, the shell changes path as well, and vice versa.

% PATH=/bin:/usr/bin:/tmp:.
% echo $path
/bin /usr/bin /tmp .
% path=( /usr/bin . /usr/local/bin /usr/ucb )
% echo $PATH
/usr/bin:.:/usr/local/bin:/usr/ucb

The same is true of CDPATH and cdpath:

% echo $CDPATH
/usr:/u/pfalstad:/u/pfalstad/zsh
% CDPATH=/u/subbarao:/usr/src:/tmp
% echo $cdpath
/u/subbarao /usr/src /tmp

In general, parameters with names in all lowercase are arrays; assignments to them take the form:

name=(elem ...)

Parameters with names in all uppercase are strings. If there is both an array and a string version of the same parameter, the string version is a colon-separated list, like PATH.

HISTFILE is the name of the history file, where the history is saved when a shell exits.

% zsh
phoenix% HISTFILE=/tmp/history
phoenix% SAVEHIST=20
phoenix% echo foo
foo
phoenix% date
Fri May 24 05:39:35 EDT 1991
phoenix% uptime
  5:39am  up 4 days, 20:02,  40 users,  load average: 2.30, 2.20, 2.00
phoenix% exit
% cat /tmp/history
HISTFILE=/tmp/history
SAVEHIST=20
echo foo
date
uptime
exit
% HISTSIZE=3
% history
   28  rm /tmp/history
   29  HISTSIZE=3
   30  history

In zsh, if you say

% >file

the command cat is normally assumed:

% >file
foo!    
^D
% cat file
foo!

Thus, you can view a file simply by typing:

% <file
foo!

However, this is not csh or sh compatible. To correct this, change the value of the parameter NULLCMD, which is cat by default.

% NULLCMD=:
% >file
% ls -l file
-rw-r--r--  1 pfalstad        0 May 24 05:41 file

If NULLCMD is unset, the shell reports an error if no command is specified (like csh).

% unset NULLCMD
% >file
zsh: redirection with no command

Actually, READNULLCMD is used whenever you have a null command reading input from a single file. Thus, you can set READNULLCMD to more or less rather than cat. Also, if you set NULLCMD to : for sh compatibility, you can still read files with < file if you leave READNULLCMD set to more.