In Chapter 16 “Starting and Stopping the Shell” the
ENV variable was discussed. The
ENV variable
specifies a file, usually .kshrc,
which is executed whenever you spawn a new, interactive POSIX or
Korn Shell. An interactive shell, is a shell
that has input and output tied directly to the terminal. Therefore,
you can access standard input, standard output, and standard error.
To determine whether or not your shell is interactive, type:
and look for:
This .kshrc file normally
contains commands to set up the POSIX or Korn Shell's environment.
However, if this file is exceedingly long, spawning the new shell
can be a long process. This complicated ENV
variable prevents .kshrc
from being read when not in interactive mode.
You can turn off the processing of the ENV
file for noninteractive shells with the following in your ~/.profile:
export ENV='${FILE[(_$-=1)+(_=0)-(_$-!=_${-%%*i*})]}'
export FILE=$HOME/.envfile |
The idea behind this scheme is to set up an array (FILE)
whose first element is the file we want executed at startup, and
whose second element is null.
$ export FILE=$HOME/.envfile
$ echo $FILE[0]
/users/pbm/.envfile
$ echo $FILE[1]
$ |
We then want to set ENV to
FILE[0] for interactive shells
and set ENV to FILE[1]
for noninteractive shells. To do this we need an expression that
will evaluate to "0" for an interactive shell and will evaluate
to "1" for a noninteractive shell.
ENV='${FILE[magic_expression]}' |
The flags variable ($-) is
the key to forming our "magic_expression".
If the shell is interactive, the flags will contain an 'i'.
The expression used as the index consists of three parts that are
combined to form the final index:
(_$-=1) (_=0) (_$- != _${-%%*i*}) |
Let's look at each part of this expression:
- Part
1: (_$-=1)
This creates a parameter named _$-
and assigns it a value of 1. The value of this expression is the
value of the assignment, namely 1. Note: Since parameter substitution
is performed on this expression, the name of the variable will look
something like "_ism". To see this,
try:
$ echo $- # see what flags are
ism
$ echo _$-
_ism
$ echo $_ism
# no variable _ism defined yet
$ ((_$-=1)) # create and assign to _ism
$ echo $_ism # now _ism has a value
1
$ |
- Part 2: (_=0)
Set parameter named _
to 0 (we need this since we may reference $_
in the next step). The value of this expression is 0.
- Part 3: (_$- != _${-%%*i*})
This expression checks to see if a parameter named
_$- (remember
this is _ism
in our example, and we have set it to 1 in the first expression)
has a different value than the parameter named on the right hand
side of the != operator. That parameter
name will be either "_" or "_$-"
(_ism) depending on the expansion
of "${-%%*i*}". The latter will
evaluate to null for an interactive shell and to "_$-"
for a noninteractive shell.
So, for an interactive shell we have (_$- != _)
which is true (value arithmetically speaking is 1) and for a noninteractive
shell we have (_$- != _$-_) which
is false (value for the expression is 0).
Adding it all up we get the following for an interactive shell:
(_$-=1) + (_=0) - (_$- != _)
1 + 0 - 1 = 0
${FILE[0]} = "$HOME/.envfile" |
And for a noninteractive shell:
(_$-=1) + (_=0) - (_$- != _$-)
1 + 0 - 0 = 1
${FILE[1]} = "" |
Printable version
