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 .