Plan9 Environment Variables -- Notes of Introduction to OS Abstractions Using Plan 9 from Bell Labs(III)

The
interface for using environment variables in Plan 9 is a file
interface.
All interfaces of environment variables can be found
under /env.

To
obtain the value for a environment variable, from a C program, we can
use the getenv
system call. If the variable is not defined, getenv
returns a null string. A related call is putenv,
which accepts a name and a value, and set the corresponding
environment variable accordingly.

#include <u.h>
#include <libc.h>
void main()
{
char * path;
path=getenv("path");
if(path==nil)
sysfatal("path not defined!");
print("PATH is %s /n", path);
exits(nil);
}

In
some cases it is convenient to define an environment variable just
for a command. This can be
done by defining it in the same command
line, before the command, like in the following exam-ple:

; temp=/tmp/foobar echo $temp
/tmp/foobar
; echo $temp
;

Useful
Environment Variables:

status
is updated by the shell once it finds out how it went to the last
command it executed.

path
is a list of paths where the shell should
look for executable files
to run the user commands.

user
contains the user name
.

sysname
contains the machine name.

The
file /dev/text represents
the
text shown in the window (when used within that window). To make
a copy of your shell session,
you already know what to do:

;
cp /dev/text $home/saved

The
same can be done for the image shown in the display for your window,
which is also represented as a file, /dev/window.
This is what we did to capture screen images .